Questo è un esempio di nota del relatore. Useremo questi per aggiungere ulteriori informazioni alle diapositive. Questi potrebbero essere punti chiave che l'istruttore dovrebbe copertina così come le risposte alle domande tipiche che sorgono in classe.

• On Debian/Ubuntu, you can also install Cargo, the Rust source and the Rust formatter via apt. However, this gets you an outdated rust version and may lead to unexpected behavior. The command would be:

  sudo apt install cargo rust-src rustfmt
  

Punti chiave:

• Rust ha un programma di rilascio rapido con una nuova versione in uscita ogni sei settimane. Le nuove versioni mantengono la retrocompatibilità con vecchie versioni --- in più abilitano nuove funzionalità.

• Esistono tre canali di rilascio: "stable", "beta" e "nightly".

• Le nuove funzionalità vengono testate su "nightly", "beta" è ciò che diventa "stabile" ogni sei settimane.

• Dependencies can also be resolved from alternative registries, git, folders, and more.

• Rust ha anche le [edizioni]: l'edizione attuale è Rust 2021. Le precedenti edizioni erano Rust 2015 e Rust 2018.

  • Le edizioni possono apportare modifiche incompatibili con le versioni precedenti del linguaggio.

  • Per evitare incompatibilità del codice, le edizioni sono opt-in: si può selezionare l'edizione di riferimento' attraverso il file Cargo.toml.

  • Per evitare di dividere l'ecosistema, i compilatori di Rust possono mescolare il codice scritto per diverse edizioni.

  • Nota che è abbastanza raro usare il compilatore direttamente non attraverso cargo (la maggior parte degli utenti non lo fa mai).

  • It might be worth alluding that Cargo itself is an extremely powerful and comprehensive tool. It is capable of many advanced features including but not limited to:

    • Struttura del progetto/pacchetto
    • [aree di lavoro]
    • Dipendenze di sviluppo e gestione/memorizzazione nella cache delle dipendenze di runtime
    • [costruzione script]
    • [installazione globale]
    • È anche estensibile con plug-in di comandi secondari (come cargo clippy).

  • Leggi di più dall'official Cargo Book

La maggior parte degli esempi di codice sono modificabili come mostrato sopra. Alcuni esempi di codice non sono modificabili per vari motivi:

• I playground incorporati non possono eseguire unit test. Copia e incolla il codice e aprilo nel Playground reale per dimostrare le 'unit test'.

• I playground incorporati perdono il loro stato nel momento in cui esci dalla pagina! Questo è il motivo per cui gli studenti dovrebbero risolvere gli esercizi utilizzando un'installazione locale di Rust o tramite Playground.

Cerca di incoraggiare i partecipanti alla classe a installare Cargo e utilizzare un editore locale. Semplificherà la loro vita poiché avranno un ambiente di sviluppo normale.

Si prega di ricordare agli studenti che:

• Dovrebbero fare domande quando ne sentono il bisogno, non trattenerle fino alla fine.
• La lezione è pensata per essere interattiva e le discussioni sono molto incoraggiate!
  • As an instructor, you should try to keep the discussions relevant, i.e., keep the discussions related to how Rust does things vs some other language. It can be hard to find the right balance, but err on the side of allowing discussions since they engage people much more than one-way communication.
• Le domande probabilmente significheranno che parleremo di cose prima delle diapositive.
  • Questo è perfettamente okay! La ripetizione è una parte importante dell'apprendimento. Ricordare che le diapositive sono solo un supporto e sei libero di saltarle come preferisci.

The idea for the first day is to show the "basic" things in Rust that should have immediate parallels in other languages. The more advanced parts of Rust come on the subsequent days.

If you're teaching this in a classroom, this is a good place to go over the schedule. Note that there is an exercise at the end of each segment, followed by a break. Plan to cover the exercise solution after the break. The times listed here are a suggestion in order to keep the course on schedule. Feel free to be flexible and adjust as necessary!

Rust è usato nelle stesse aree di C++:

• Elevata flessibilità.
• Alto livello di controllo.
• Can be scaled down to very constrained devices such as microcontrollers.
• Non ha runtime o garbage collection.
• Si concentra su affidabilità e sicurezza senza sacrificare le prestazioni.

This slide tries to make the students comfortable with Rust code. They will see a ton of it over the next four days so we start small with something familiar.

Punti chiave:

• Rust is very much like other languages in the C/C++/Java tradition. It is imperative and it doesn't try to reinvent things unless absolutely necessary.

• Rust è moderno con pieno supporto per cose come Unicode.

• Rust uses macros for situations where you want to have a variable number of arguments (no function overloading).

• Le macro definite "igieniche" significa che non catturano accidentalmente identificatori da l'ambito in cui vengono utilizzate. Le macro di Rust sono in realtà solo parzialmente igieniche.

• Rust is multi-paradigm. For example, it has powerful object-oriented programming features, and, while it is not a functional language, it includes a range of functional concepts.

Do not spend much time here. All of these points will be covered in more depth later.

Assicurati di chiedere alla classe con quali linguaggi hanno esperienza. In base alla risposta puoi evidenziare diverse caratteristiche di Rust:

• Esperienza con C o C++: Rust elimina un'intera classe di errori di runtime tramite il borrow checker. Ottiene prestazioni come in C e C++, ma non hai i problemi di sicurezza della memoria. Inoltre, si ha un linguaggio moderno con costrutti come il pattern matching e la gestione delle dipendenze incorporata.

• Esperienza con Java, Go, Python, JavaScript...: Ottiene la stessa sicurezza della memoria come in quei linguaggi, più una sensazione simile ad un linguaggio di alto livello. Inoltre raggiunge prestazioni veloci e prevedibili come C e C++ (nessun Garbage Collector) così come l'accesso all'hardware di basso livello (quando serve)

As students head into the break, encourage them to open up the playground and experiment a little. Encourage them to keep the tab open and try things out during the rest of the course. This is particularly helpful for advanced students who want to know more about Rust's optimizations or generated assembly.

• Uncomment the x = 20 to demonstrate that variables are immutable by default. Add the mut keyword to allow changes.

• The i32 here is the type of the variable. This must be known at compile time, but type inference (covered later) allows the programmer to omit it in many cases.

Ci sono alcune sintassi che non sono mostrate sopra:

• All underscores in numbers can be left out, they are for legibility only. So 1_000 can be written as 1000 (or 10_00), and 123_i64 can be written as 123i64.

This is the first time we've seen a function other than main, but the meaning should be clear: it takes three integers, and returns an integer. Functions will be covered in more detail later.

Arithmetic is very similar to other languages, with similar precedence.

What about integer overflow? In C and C++ overflow of signed integers is actually undefined, and might do different things on different platforms or compilers. In Rust, it's defined.

Change the i32's to i16 to see an integer overflow, which panics (checked) in a debug build and wraps in a release build. There are other options, such as overflowing, saturating, and carrying. These are accessed with method syntax, e.g., (a * b).saturating_add(b * c).saturating_add(c * a).

In fact, the compiler will detect overflow of constant expressions, which is why the example requires a separate function.

This slide introduces strings. Everything here will be covered in more depth later, but this is enough for subsequent slides and exercises to use strings.

• Invalid UTF-8 in a string is UB, and this not allowed in safe Rust.

• String is a user-defined type with a constructor (::new()) and methods like s.push_str(..).

• The & in &str indicates that this is a reference. We will cover references later, so for now just think of &str as a unit meaning "a read-only string".

• The commented-out line is indexing into the string by byte position. 12..13 does not end on a character boundary, so the program panics. Adjust it to a range that does, based on the error message.

• Le stringhe non elaborate (raw) consentono di creare un valore &str con i caratteri di escape disabilitati: r"\n" == "\\n". Puoi racchiudere le doppie virgolette usando una quantità uguale di # su entrambi i lati delle virgolette:

  fn main() {
      println!(r#"<a href="link.html">link</a>"#);
      println!("<a href=\"link.html\">link</a>");
  }

This slide demonstrates how the Rust compiler infers types based on constraints given by variable declarations and usages.

It is very important to emphasize that variables declared like this are not of some sort of dynamic "any type" that can hold any data. The machine code generated by such declaration is identical to the explicit declaration of a type. The compiler does the job for us and helps us write more concise code.

When nothing constrains the type of an integer literal, Rust defaults to i32. This sometimes appears as {integer} in error messages. Similarly, floating-point literals default to f64.

fn main() {
    let x = 3.14;
    let y = 20;
    assert_eq!(x, y);
    // ERROR: no implementation for `{float} == {integer}`
}

Because if is an expression and must have a particular type, both of its branch blocks must have the same type. Show what happens if you add ; after "small" in the second example.

When if is used in an expression, the expression must have a ; to separate it from the next statement. Remove the ; before println! to see the compiler error.

• We will discuss iteration later; for now, just stick to range expressions.
• Note that the for loop only iterates to 4. Show the 1..=5 syntax for an inclusive range.

• Note that loop is the only looping construct which returns a non-trivial value. This is because it's guaranteed to be entered at least once (unlike while and for loops).

• You can show how the value of the block changes by changing the last line in the block. For instance, adding/removing a semicolon or using a return.
• Show that a variable's scope is limited by adding a b in the inner block in the last example, and then trying to access it outside that block.
• Shadowing is different from mutation, because after shadowing both variable's memory locations exist at the same time. Both are available under the same name, depending where you use it in the code.
• A shadowing variable can have a different type.
• Shadowing looks obscure at first, but is convenient for holding on to values after .unwrap().

• I parametri di dichiarazione sono seguiti da un tipo (il contrario di alcuni linguaggi di programmazione) e quindi da un tipo restituito.
• The last expression in a function body (or any block) becomes the return value. Simply omit the ; at the end of the expression. The return keyword can be used for early return, but the "bare value" form is idiomatic at the end of a function (refactor gcd to use a return).
• Alcune funzioni non hanno alcun valore di ritorno e restituiscono il 'tipo di unità', (). Il compilatore lo dedurrà se il tipo restituito ->() viene omesso.
• Overloading is not supported -- each function has a single implementation.
  • Always takes a fixed number of parameters. Default arguments are not supported. Macros can be used to support variadic functions.
  • Always takes a single set of parameter types. These types can be generic, which will be covered later.

The takeaway from this section is that these common conveniences exist, and how to use them. Why they are defined as macros, and what they expand to, is not especially critical.

The course does not cover defining macros, but a later section will describe use of derive macros.

Punti chiave:

Array:

• A value of the array type [T; N] holds N (a compile-time constant) elements of the same type T. Note that the length of the array is part of its type, which means that [u8; 3] and [u8; 4] are considered two different types. Slices, which have a size determined at runtime, are covered later.

• Try accessing an out-of-bounds array element. Array accesses are checked at runtime. Rust can usually optimize these checks away, and they can be avoided using unsafe Rust.

• Si possono usare i letterali per assegnare i valori agli array.

• The println! macro asks for the debug implementation with the ? format parameter: {} gives the default output, {:?} gives the debug output. Types such as integers and strings implement the default output, but arrays only implement the debug output. This means that we must use debug output here.

• Aggiungendo #, eg {a:#?}, si invoca un formato "pretty printing", il quale può essere più facile da leggere.

Tuple:

• Come gli array, le tuple hanno una lunghezza fissa.

• Le tuple raggruppano valori di tipi diversi in un tipo composto.

• È possibile accedere ai campi di una tupla tramite il punto e l'indice del valore, ad es. t.0, t.1.

• The empty tuple () is also known as the "unit type". It is both a type, and the only valid value of that type --- that is to say both the type and its value are expressed as (). It is used to indicate, for example, that a function or expression has no return value, as we'll see in a future slide.

  • You can think of it as void that can be familiar to you from other programming languages.

This functionality uses the IntoIterator trait, but we haven't covered that yet.

The assert_ne! macro is new here. There are also assert_eq! and assert! macros. These are always checked while, debug-only variants like debug_assert! compile to nothing in release builds.

Punti chiave:

• You might point out how some specific characters are being used when in a pattern

  • | come un or
  • .. può espandersi fino a quanto necessario
  • 1..=5 rappresenta un intervallo inclusivo
  • _ è un carattere jolly

• Match guards as a separate syntax feature are important and necessary when we wish to concisely express more complex ideas than patterns alone would allow.

• They are not the same as separate if expression inside of the match arm. An if expression inside of the branch block (after =>) happens after the match arm is selected. Failing the if condition inside of that block won't result in other arms of the original match expression being considered.

• The condition defined in the guard applies to every expression in a pattern with an |.

• Create a new array pattern using _ to represent an element.
• Aggiungi più valori all’array.
• Point out that how .. will expand to account for different number of elements.
• Show matching against the tail with patterns [.., b] and [a@..,b]

• A reference is said to "borrow" the value it refers to, and this is a good model for students not familiar with pointers: code can use the reference to access the value, but is still "owned" by the original variable. The course will get into more detail on ownership in day 3.

• References are implemented as pointers, and a key advantage is that they can be much smaller than the thing they point to. Students familiar with C or C++ will recognize references as pointers. Later parts of the course will cover how Rust prevents the memory-safety bugs that come from using raw pointers.

• Rust does not automatically create references for you - the & is always required.

• Rust will auto-dereference in some cases, in particular when invoking methods (try r.count_ones()). There is no need for an -> operator like in C++.

• In this example, r is mutable so that it can be reassigned (r = &b). Note that this re-binds r, so that it refers to something else. This is different from C++, where assignment to a reference changes the referenced value.

• A shared reference does not allow modifying the value it refers to, even if that value was mutable. Try *r = 'X'.

• Rust is tracking the lifetimes of all references to ensure they live long enough. Dangling references cannot occur in safe Rust. x_axis would return a reference to point, but point will be deallocated when the function returns, so this will not compile.

• Parleremo di più del borrowing quando arriveremo alla proprietà (ownership).

Punti chiave:

• "Exclusive" means that only this reference can be used to access the value. No other references (shared or exclusive) can exist at the same time, and the referenced value cannot be accessed while the exclusive reference exists. Try making an &point.0 or changing point.0 while x_coord is alive.

• Be sure to note the difference between let mut x_coord: &i32 and let x_coord: &mut i32. The first one represents a shared reference which can be bound to different values, while the second represents an exclusive reference to a mutable value.

Punti chiave:

• Structs funzionano come in C o C++.
  • Like in C++, and unlike in C, no typedef is needed to define a type.
  • Unlike in C++, there is no inheritance between structs.
• This may be a good time to let people know there are different types of structs.
  • Zero-sized structs (e.g. struct Foo;) might be used when implementing a trait on some type but don’t have any data that you want to store in the value itself.
  • The next slide will introduce Tuple structs, used when the field names are not important.
• If you already have variables with the right names, then you can create the struct using a shorthand.
• The syntax ..avery allows us to copy the majority of the fields from the old struct without having to explicitly type it all out. It must always be the last element.

• Newtypes are a great way to encode additional information about the value in a primitive type, for example:
  • The number is measured in some units: Newtons in the example above.
  • The value passed some validation when it was created, so you no longer have to validate it again at every use: PhoneNumber(String) or OddNumber(u32).
• Demonstrate how to add a f64 value to a Newtons type by accessing the single field in the newtype.
  • Rust generally doesn’t like inexplicit things, like automatic unwrapping or for instance using booleans as integers.
  • Operator overloading is discussed on Day 3 (generics).
• The example is a subtle reference to the Mars Climate Orbiter failure.

Punti chiave:

• Enumerations allow you to collect a set of values under one type.
• Direction is a type with variants. There are two values of Direction: Direction::Left and Direction::Right.
• PlayerMove is a type with three variants. In addition to the payloads, Rust will store a discriminant so that it knows at runtime which variant is in a PlayerMove value.
• This might be a good time to compare structs and enums:
  • In both, you can have a simple version without fields (unit struct) or one with different types of fields (variant payloads).
  • You could even implement the different variants of an enum with separate structs but then they wouldn’t be the same type as they would if they were all defined in an enum.
• Rust uses minimal space to store the discriminant.

  • If necessary, it stores an integer of the smallest required size

  • If the allowed variant values do not cover all bit patterns, it will use invalid bit patterns to encode the discriminant (the "niche optimization"). For example, Option<&u8> stores either a pointer to an integer or NULL for the None variant.

  • You can control the discriminant if needed (e.g., for compatibility with C):

    #[repr(u32)]
    enum Bar {
        A, // 0
        B = 10000,
        C, // 10001
    }
    
    fn main() {
        println!("A: {}", Bar::A as u32);
        println!("B: {}", Bar::B as u32);
        println!("C: {}", Bar::C as u32);
    }

    Without repr, the discriminant type takes 2 bytes, because 10001 fits 2 bytes.

Ulteriori aspetti da Esplorare

Rust has several optimizations it can employ to make enums take up less space.

• Null pointer optimization: For some types, Rust guarantees that size_of::<T>() equals size_of::<Option<T>>().

  Example code if you want to show how the bitwise representation may look like in practice. It's important to note that the compiler provides no guarantees regarding this representation, therefore this is totally unsafe.

  use std::mem::transmute;
  
  macro_rules! dbg_bits {
      ($e:expr, $bit_type:ty) => {
          println!("- {}: {:#x}", stringify!($e), transmute::<_, $bit_type>($e));
      };
  }
  
  fn main() {
      unsafe {
          println!("bool:");
          dbg_bits!(false, u8);
          dbg_bits!(true, u8);
  
          println!("Option<bool>:");
          dbg_bits!(None::<bool>, u8);
          dbg_bits!(Some(false), u8);
          dbg_bits!(Some(true), u8);
  
          println!("Option<Option<bool>>:");
          dbg_bits!(Some(Some(false)), u8);
          dbg_bits!(Some(Some(true)), u8);
          dbg_bits!(Some(None::<bool>), u8);
          dbg_bits!(None::<Option<bool>>, u8);
  
          println!("Option<&i32>:");
          dbg_bits!(None::<&i32>, usize);
          dbg_bits!(Some(&0i32), usize);
      }
  }

• Mention that const behaves semantically similar to C++'s constexpr.
• static, on the other hand, is much more similar to a const or mutable global variable in C++.
• static provides object identity: an address in memory and state as required by types with interior mutability such as Mutex<T>.
• It isn't super common that one would need a runtime evaluated constant, but it is helpful and safer than using a static.

Tabella proprietà:

Ulteriori aspetti da Esplorare

Because static variables are accessible from any thread, they must be Sync. Interior mutability is possible through a Mutex, atomic or similar.

Thread-local data can be created with the macro std::thread_local.

C programmers will recognize this as similar to a typedef.

Strutture (Struct)

• Change the literal values in foo to match with the other patterns.
• Add a new field to Foo and make changes to the pattern as needed.
• The distinction between a capture and a constant expression can be hard to spot. Try changing the 2 in the second arm to a variable, and see that it subtly doesn't work. Change it to a const and see it working again.

Enumerazioni (Enums)

Punti chiave:

• The if/else expression is returning an enum that is later unpacked with a match.
• You can try adding a third variant to the enum definition and displaying the errors when running the code. Point out the places where your code is now inexhaustive and how the compiler tries to give you hints.
• The values in the enum variants can only be accessed after being pattern matched.
• Demonstrate what happens when the search is inexhaustive. Note the advantage the Rust compiler provides by confirming when all cases are handled.
• Save the result of divide_in_two in the result variable and match it in a loop. That won't compile because msg is consumed when matched. To fix it, match &result instead of result. That will make msg a reference so it won't be consumed. This "match ergonomics" appeared in Rust 2018. If you want to support older Rust, replace msg with ref msg in the pattern.

if-let

• Unlike match, if let does not have to cover all branches. This can make it more concise than match.
• A common usage is handling Some values when working with Option.
• Unlike match, if let does not support guard clauses for pattern matching.

let-else

if-lets can pile up, as shown. The let-else construct supports flattening this nested code. Rewrite the awkward version for students, so they can see the transformation.

The rewritten version is:

#![allow(unused)]
fn main() {
fn hex_or_die_trying(maybe_string: Option<String>) -> Result<u32, String> {
    let Some(s) = maybe_string else {
        return Err(String::from("got None"));
    };

    let Some(first_byte_char) = s.chars().next() else {
        return Err(String::from("got empty string"));
    };

    let Some(digit) = first_byte_char.to_digit(16) else {
        return Err(String::from("not a hex digit"));
    };

    return Ok(digit);
}
}

while-let

• Point out that the while let loop will keep going as long as the value matches the pattern.
• You could rewrite the while let loop as an infinite loop with an if statement that breaks when there is no value to unwrap for name.pop(). The while let provides syntactic sugar for the above scenario.

Punti chiave:

• It can be helpful to introduce methods by comparing them to functions.
  • Methods are called on an instance of a type (such as a struct or enum), the first parameter represents the instance as self.
  • Developers may choose to use methods to take advantage of method receiver syntax and to help keep them more organized. By using methods we can keep all the implementation code in one predictable place.
• Point out the use of the keyword self, a method receiver.
  • Show that it is an abbreviated term for self: Self and perhaps show how the struct name could also be used.
  • Explain that Self is a type alias for the type the impl block is in and can be used elsewhere in the block.
  • Note how self is used like other structs and dot notation can be used to refer to individual fields.
  • This might be a good time to demonstrate how the &self differs from self by trying to run finish twice.
  • Beyond variants on self, there are also special wrapper types allowed to be receiver types, such as Box<Self>.

• A trait defines a number of methods that types must have in order to implement the trait.

• Traits are implemented in an impl <trait> for <type> { .. } block.

• Traits may specify pre-implemented (provided) methods and methods that users are required to implement themselves. Provided methods can rely on required methods. In this case, greet is provided, and relies on talk.

Derivation is implemented with macros, and many crates provide useful derive macros to add useful functionality. For example, serde can derive serialization support for a struct using #[derive(Serialize)].

Oggetti che implementano Trait (Trait Objects)

Trait objects allow for values of different types, for instance in a collection:

struct Dog {
    name: String,
    age: i8,
}
struct Cat {
    lives: i8,
}

trait Pet {
    fn talk(&self) -> String;
}

impl Pet for Dog {
    fn talk(&self) -> String {
        format!("Woof, my name is {}!", self.name)
    }
}

impl Pet for Cat {
    fn talk(&self) -> String {
        String::from("Miau!")
    }
}

fn main() {
    let pets: Vec<Box<dyn Pet>> = vec![
        Box::new(Cat { lives: 9 }),
        Box::new(Dog { name: String::from("Fido"), age: 5 }),
    ];
    for pet in pets {
        println!("Hello, who are you? {}", pet.talk());
    }
}

Memory layout after allocating pets:

Exercise: Generic Logger

Let's design a simple logging utility, using a trait Logger with a log method. Code which might log its progress can then take an &impl Logger. In testing, this might put messages in the test logfile, while in a production build it would send messages to a log server.

However, the StderrLogger given below logs all messages, regardless of verbosity. Your task is to write a VerbosityFilter type that will ignore messages above a maximum verbosity.

This is a common pattern: a struct wrapping a trait implementation and implementing that same trait, adding behavior in the process. What other kinds of wrappers might be useful in a logging utility?

use std::fmt::Display;

pub trait Logger {
    /// Log a message at the given verbosity level.
    fn log(&self, verbosity: u8, message: impl Display);
}

struct StderrLogger;

impl Logger for StderrLogger {
    fn log(&self, verbosity: u8, message: impl Display) {
        eprintln!("verbosity={verbosity}: {message}");
    }
}

fn do_things(logger: &impl Logger) {
    logger.log(5, "FYI");
    logger.log(2, "Uhoh");
}

// TODO: Define and implement `VerbosityFilter`.

fn main() {
    let l = VerbosityFilter { max_verbosity: 3, inner: StderrLogger };
    do_things(&l);
}

Soluzioni

use std::fmt::Display;

pub trait Logger {
    /// Log a message at the given verbosity level.
    fn log(&self, verbosity: u8, message: impl Display);
}

struct StderrLogger;

impl Logger for StderrLogger {
    fn log(&self, verbosity: u8, message: impl Display) {
        eprintln!("verbosity={verbosity}: {message}");
    }
}

fn do_things(logger: &impl Logger) {
    logger.log(5, "FYI");
    logger.log(2, "Uhoh");
}

/// Only log messages up to the given verbosity level.
struct VerbosityFilter<L: Logger> {
    max_verbosity: u8,
    inner: L,
}

impl<L: Logger> Logger for VerbosityFilter<L> {
    fn log(&self, verbosity: u8, message: impl Display) {
        if verbosity <= self.max_verbosity {
            self.inner.log(verbosity, message);
        }
    }
}

fn main() {
    let l = VerbosityFilter { max_verbosity: 3, inner: StderrLogger };
    do_things(&l);
}

Generics

This segment should take about 45 minutes. It contains:

Funzioni Esterne (Extern)

Rust supports generics, which lets you abstract algorithms or data structures (such as sorting or a binary tree) over the types used or stored.

/// Pick `even` or `odd` depending on the value of `n`.
fn pick<T>(n: i32, even: T, odd: T) -> T {
    if n % 2 == 0 {
        even
    } else {
        odd
    }
}

fn main() {
    println!("picked a number: {:?}", pick(97, 222, 333));
    println!("picked a tuple: {:?}", pick(28, ("dog", 1), ("cat", 2)));
}

Tipi di dati Generic

You can use generics to abstract over the concrete field type:

#[derive(Debug)]
struct Point<T> {
    x: T,
    y: T,
}

impl<T> Point<T> {
    fn coords(&self) -> (&T, &T) {
        (&self.x, &self.y)
    }

    // fn set_x(&mut self, x: T)
}

fn main() {
    let integer = Point { x: 5, y: 10 };
    let float = Point { x: 1.0, y: 4.0 };
    println!("{integer:?} and {float:?}");
    println!("coords: {:?}", integer.coords());
}

Trait Bounds

When working with generics, you often want to require the types to implement some trait, so that you can call this trait's methods.

You can do this with T: Trait or impl Trait:

fn duplicate<T: Clone>(a: T) -> (T, T) {
    (a.clone(), a.clone())
}

// struct NotClonable;

fn main() {
    let foo = String::from("foo");
    let pair = duplicate(foo);
    println!("{pair:?}");
}

impl Trait

Similar to trait bounds, an impl Trait syntax can be used in function arguments and return values:

// Syntactic sugar for:
//   fn add_42_millions<T: Into<i32>>(x: T) -> i32 {
fn add_42_millions(x: impl Into<i32>) -> i32 {
    x.into() + 42_000_000
}

fn pair_of(x: u32) -> impl std::fmt::Debug {
    (x + 1, x - 1)
}

fn main() {
    let many = add_42_millions(42_i8);
    println!("{many}");
    let many_more = add_42_millions(10_000_000);
    println!("{many_more}");
    let debuggable = pair_of(27);
    println!("debuggable: {debuggable:?}");
}

Exercise: Generic min

In this short exercise, you will implement a generic min function that determines the minimum of two values, using a LessThan trait.

trait LessThan {
    /// Return true if self is less than other.
    fn less_than(&self, other: &Self) -> bool;
}

#[derive(Debug, PartialEq, Eq, Clone, Copy)]
struct Citation {
    author: &'static str,
    year: u32,
}

impl LessThan for Citation {
    fn less_than(&self, other: &Self) -> bool {
        if self.author < other.author {
            true
        } else if self.author > other.author {
            false
        } else {
            self.year < other.year
        }
    }
}

// TODO: implement the `min` function used in `main`.

fn main() {
    let cit1 = Citation { author: "Shapiro", year: 2011 };
    let cit2 = Citation { author: "Baumann", year: 2010 };
    let cit3 = Citation { author: "Baumann", year: 2019 };
    debug_assert_eq!(min(cit1, cit2), cit2);
    debug_assert_eq!(min(cit2, cit3), cit2);
    debug_assert_eq!(min(cit1, cit3), cit3);
}

Soluzioni

trait LessThan {
    /// Return true if self is less than other.
    fn less_than(&self, other: &Self) -> bool;
}

#[derive(Debug, PartialEq, Eq, Clone, Copy)]
struct Citation {
    author: &'static str,
    year: u32,
}

impl LessThan for Citation {
    fn less_than(&self, other: &Self) -> bool {
        if self.author < other.author {
            true
        } else if self.author > other.author {
            false
        } else {
            self.year < other.year
        }
    }
}

fn min<T: LessThan>(l: T, r: T) -> T {
    if l.less_than(&r) {
        l
    } else {
        r
    }
}

fn main() {
    let cit1 = Citation { author: "Shapiro", year: 2011 };
    let cit2 = Citation { author: "Baumann", year: 2010 };
    let cit3 = Citation { author: "Baumann", year: 2019 };
    debug_assert_eq!(min(cit1, cit2), cit2);
    debug_assert_eq!(min(cit2, cit3), cit2);
    debug_assert_eq!(min(cit1, cit3), cit3);
}

Welcome Back

Including 10 minute breaks, this session should take about 3 hours. It contains:

Libreria standard

This segment should take about 1 hour and 10 minutes. It contains:

Libreria standard

Rust comes with a standard library which helps establish a set of common types used by Rust libraries and programs. This way, two libraries can work together smoothly because they both use the same String type.

In fact, Rust contains several layers of the Standard Library: core, alloc and std.

• core includes the most basic types and functions that don't depend on libc, allocator or even the presence of an operating system.
• alloc includes types which require a global heap allocator, such as Vec, Box and Arc.
• Embedded Rust applications often only use core, and sometimes alloc.

Test nella documentazione

Rust comes with extensive documentation. For example:

• All of the details about loops.
• Primitive types like u8.
• Standard library types like Option or BinaryHeap.

In fact, you can document your own code:

/// Determine whether the first argument is divisible by the second argument.
///
/// If the second argument is zero, the result is false.
fn is_divisible_by(lhs: u32, rhs: u32) -> bool {
    if rhs == 0 {
        return false;
    }
    lhs % rhs == 0
}

The contents are treated as Markdown. All published Rust library crates are automatically documented at docs.rs using the rustdoc tool. It is idiomatic to document all public items in an API using this pattern.

To document an item from inside the item (such as inside a module), use //! or /*! .. */, called "inner doc comments":

//! This module contains functionality relating to divisibility of integers.

Durata

We have already seen some use of Option<T>. It stores either a value of type T or nothing. For example, String::find returns an Option<usize>.

fn main() {
    let name = "Löwe 老虎 Léopard Gepardi";
    let mut position: Option<usize> = name.find('é');
    println!("find returned {position:?}");
    assert_eq!(position.unwrap(), 14);
    position = name.find('Z');
    println!("find returned {position:?}");
    assert_eq!(position.expect("Character not found"), 0);
}

Opzione, Risultato

Result is similar to Option, but indicates the success or failure of an operation, each with a different type. This is similar to the Res defined in the expression exercise, but generic: Result<T, E> where T is used in the Ok variant and E appears in the Err variant.

use std::fs::File;
use std::io::Read;

fn main() {
    let file: Result<File, std::io::Error> = File::open("diary.txt");
    match file {
        Ok(mut file) => {
            let mut contents = String::new();
            if let Ok(bytes) = file.read_to_string(&mut contents) {
                println!("Dear diary: {contents} ({bytes} bytes)");
            } else {
                println!("Could not read file content");
            }
        }
        Err(err) => {
            println!("The diary could not be opened: {err}");
        }
    }
}

Stringa (String)

String is the standard heap-allocated growable UTF-8 string buffer:

fn main() {
    let mut s1 = String::new();
    s1.push_str("Hello");
    println!("s1: len = {}, capacity = {}", s1.len(), s1.capacity());

    let mut s2 = String::with_capacity(s1.len() + 1);
    s2.push_str(&s1);
    s2.push('!');
    println!("s2: len = {}, capacity = {}", s2.len(), s2.capacity());

    let s3 = String::from("🇨🇭");
    println!("s3: len = {}, number of chars = {}", s3.len(), s3.chars().count());
}

String implements Deref<Target = str>, which means that you can call all str methods on a String.

Vec

Vec is the standard resizable heap-allocated buffer:

fn main() {
    let mut v1 = Vec::new();
    v1.push(42);
    println!("v1: len = {}, capacity = {}", v1.len(), v1.capacity());

    let mut v2 = Vec::with_capacity(v1.len() + 1);
    v2.extend(v1.iter());
    v2.push(9999);
    println!("v2: len = {}, capacity = {}", v2.len(), v2.capacity());

    // Canonical macro to initialize a vector with elements.
    let mut v3 = vec![0, 0, 1, 2, 3, 4];

    // Retain only the even elements.
    v3.retain(|x| x % 2 == 0);
    println!("{v3:?}");

    // Remove consecutive duplicates.
    v3.dedup();
    println!("{v3:?}");
}

Vec implements Deref<Target = [T]>, which means that you can call slice methods on a Vec.

HashMap

Standard hash map with protection against HashDoS attacks:

use std::collections::HashMap;

fn main() {
    let mut page_counts = HashMap::new();
    page_counts.insert("Adventures of Huckleberry Finn".to_string(), 207);
    page_counts.insert("Grimms' Fairy Tales".to_string(), 751);
    page_counts.insert("Pride and Prejudice".to_string(), 303);

    if !page_counts.contains_key("Les Misérables") {
        println!(
            "We know about {} books, but not Les Misérables.",
            page_counts.len()
        );
    }

    for book in ["Pride and Prejudice", "Alice's Adventure in Wonderland"] {
        match page_counts.get(book) {
            Some(count) => println!("{book}: {count} pages"),
            None => println!("{book} is unknown."),
        }
    }

    // Use the .entry() method to insert a value if nothing is found.
    for book in ["Pride and Prejudice", "Alice's Adventure in Wonderland"] {
        let page_count: &mut i32 = page_counts.entry(book.to_string()).or_insert(0);
        *page_count += 1;
    }

    println!("{page_counts:#?}");
}

Esercizi

In this exercise you will take a very simple data structure and make it generic. It uses a std::collections::HashMap to keep track of which values have been seen and how many times each one has appeared.

The initial version of Counter is hard coded to only work for u32 values. Make the struct and its methods generic over the type of value being tracked, that way Counter can track any type of value.

If you finish early, try using the entry method to halve the number of hash lookups required to implement the count method.

use std::collections::HashMap;

/// Counter counts the number of times each value of type T has been seen.
struct Counter {
    values: HashMap<u32, u64>,
}

impl Counter {
    /// Create a new Counter.
    fn new() -> Self {
        Counter {
            values: HashMap::new(),
        }
    }

    /// Count an occurrence of the given value.
    fn count(&mut self, value: u32) {
        if self.values.contains_key(&value) {
            *self.values.get_mut(&value).unwrap() += 1;
        } else {
            self.values.insert(value, 1);
        }
    }

    /// Return the number of times the given value has been seen.
    fn times_seen(&self, value: u32) -> u64 {
        self.values.get(&value).copied().unwrap_or_default()
    }
}

fn main() {
    let mut ctr = Counter::new();
    ctr.count(13);
    ctr.count(14);
    ctr.count(16);
    ctr.count(14);
    ctr.count(14);
    ctr.count(11);

    for i in 10..20 {
        println!("saw {} values equal to {}", ctr.times_seen(i), i);
    }

    let mut strctr = Counter::new();
    strctr.count("apple");
    strctr.count("orange");
    strctr.count("apple");
    println!("got {} apples", strctr.times_seen("apple"));
}

Soluzioni

use std::collections::HashMap;
use std::hash::Hash;

/// Counter counts the number of times each value of type T has been seen.
struct Counter<T: Eq + Hash> {
    values: HashMap<T, u64>,
}

impl<T: Eq + Hash> Counter<T> {
    /// Create a new Counter.
    fn new() -> Self {
        Counter { values: HashMap::new() }
    }

    /// Count an occurrence of the given value.
    fn count(&mut self, value: T) {
        *self.values.entry(value).or_default() += 1;
    }

    /// Return the number of times the given value has been seen.
    fn times_seen(&self, value: T) -> u64 {
        self.values.get(&value).copied().unwrap_or_default()
    }
}

fn main() {
    let mut ctr = Counter::new();
    ctr.count(13);
    ctr.count(14);
    ctr.count(16);
    ctr.count(14);
    ctr.count(14);
    ctr.count(11);

    for i in 10..20 {
        println!("saw {} values equal to {}", ctr.times_seen(i), i);
    }

    let mut strctr = Counter::new();
    strctr.count("apple");
    strctr.count("orange");
    strctr.count("apple");
    println!("got {} apples", strctr.times_seen("apple"));
}

Libreria standard

This segment should take about 1 hour and 40 minutes. It contains:

Comparazione

These traits support comparisons between values. All traits can be derived for types containing fields that implement these traits.

PartialEq and Eq

PartialEq is a partial equivalence relation, with required method eq and provided method ne. The == and != operators will call these methods.

struct Key {
    id: u32,
    metadata: Option<String>,
}
impl PartialEq for Key {
    fn eq(&self, other: &Self) -> bool {
        self.id == other.id
    }
}

Eq is a full equivalence relation (reflexive, symmetric, and transitive) and implies PartialEq. Functions that require full equivalence will use Eq as a trait bound.

PartialOrd and Ord

PartialOrd defines a partial ordering, with a partial_cmp method. It is used to implement the <, <=, >=, and > operators.

use std::cmp::Ordering;
#[derive(Eq, PartialEq)]
struct Citation {
    author: String,
    year: u32,
}
impl PartialOrd for Citation {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        match self.author.partial_cmp(&other.author) {
            Some(Ordering::Equal) => self.year.partial_cmp(&other.year),
            author_ord => author_ord,
        }
    }
}

Ord is a total ordering, with cmp returning Ordering.

struct Key {
    id: u32,
    metadata: Option<String>,
}
impl PartialEq<u32> for Key {
    fn eq(&self, other: &u32) -> bool {
        self.id == *other
    }
}

Iteratori

Operator overloading is implemented via traits in std::ops:

#[derive(Debug, Copy, Clone)]
struct Point {
    x: i32,
    y: i32,
}

impl std::ops::Add for Point {
    type Output = Self;

    fn add(self, other: Self) -> Self {
        Self { x: self.x + other.x, y: self.y + other.y }
    }
}

fn main() {
    let p1 = Point { x: 10, y: 20 };
    let p2 = Point { x: 100, y: 200 };
    println!("{:?} + {:?} = {:?}", p1, p2, p1 + p2);
}

From and Into

Types implement From and Into to facilitate type conversions:

fn main() {
    let s = String::from("hello");
    let addr = std::net::Ipv4Addr::from([127, 0, 0, 1]);
    let one = i16::from(true);
    let bigger = i32::from(123_i16);
    println!("{s}, {addr}, {one}, {bigger}");
}

Into is automatically implemented when From is implemented:

fn main() {
    let s: String = "hello".into();
    let addr: std::net::Ipv4Addr = [127, 0, 0, 1].into();
    let one: i16 = true.into();
    let bigger: i32 = 123_i16.into();
    println!("{s}, {addr}, {one}, {bigger}");
}

Testare

Rust has no implicit type conversions, but does support explicit casts with as. These generally follow C semantics where those are defined.

fn main() {
    let value: i64 = 1000;
    println!("as u16: {}", value as u16);
    println!("as i16: {}", value as i16);
    println!("as u8: {}", value as u8);
}

The results of as are always defined in Rust and consistent across platforms. This might not match your intuition for changing sign or casting to a smaller type -- check the docs, and comment for clarity.

Casting with as is a relatively sharp tool that is easy to use incorrectly, and can be a source of subtle bugs as future maintenance work changes the types that are used or the ranges of values in types. Casts are best used only when the intent is to indicate unconditional truncation (e.g. selecting the bottom 32 bits of a u64 with as u32, regardless of what was in the high bits).

For infallible casts (e.g. u32 to u64), prefer using From or Into over as to confirm that the cast is in fact infallible. For fallible casts, TryFrom and TryInto are available when you want to handle casts that fit differently from those that don't.

Read and Write

Using Read and BufRead, you can abstract over u8 sources:

use std::io::{BufRead, BufReader, Read, Result};

fn count_lines<R: Read>(reader: R) -> usize {
    let buf_reader = BufReader::new(reader);
    buf_reader.lines().count()
}

fn main() -> Result<()> {
    let slice: &[u8] = b"foo\nbar\nbaz\n";
    println!("lines in slice: {}", count_lines(slice));

    let file = std::fs::File::open(std::env::current_exe()?)?;
    println!("lines in file: {}", count_lines(file));
    Ok(())
}

Similarly, Write lets you abstract over u8 sinks:

use std::io::{Result, Write};

fn log<W: Write>(writer: &mut W, msg: &str) -> Result<()> {
    writer.write_all(msg.as_bytes())?;
    writer.write_all("\n".as_bytes())
}

fn main() -> Result<()> {
    let mut buffer = Vec::new();
    log(&mut buffer, "Hello")?;
    log(&mut buffer, "World")?;
    println!("Logged: {:?}", buffer);
    Ok(())
}

Il trait Default

Il trait Default produce un valore predefinito per un tipo.

#[derive(Debug, Default)]
struct Derived {
    x: u32,
    y: String,
    z: Implemented,
}

#[derive(Debug)]
struct Implemented(String);

impl Default for Implemented {
    fn default() -> Self {
        Self("John Smith".into())
    }
}

fn main() {
    let default_struct = Derived::default();
    println!("{default_struct:#?}");

    let almost_default_struct =
        Derived { y: "Y is set!".into(), ..Derived::default() };
    println!("{almost_default_struct:#?}");

    let nothing: Option<Derived> = None;
    println!("{:#?}", nothing.unwrap_or_default());
}

Closures

Closures or lambda expressions have types which cannot be named. However, they implement special Fn, FnMut, and FnOnce traits:

fn apply_with_log(func: impl FnOnce(i32) -> i32, input: i32) -> i32 {
    println!("Calling function on {input}");
    func(input)
}

fn main() {
    let add_3 = |x| x + 3;
    println!("add_3: {}", apply_with_log(add_3, 10));
    println!("add_3: {}", apply_with_log(add_3, 20));

    let mut v = Vec::new();
    let mut accumulate = |x: i32| {
        v.push(x);
        v.iter().sum::<i32>()
    };
    println!("accumulate: {}", apply_with_log(&mut accumulate, 4));
    println!("accumulate: {}", apply_with_log(&mut accumulate, 5));

    let multiply_sum = |x| x * v.into_iter().sum::<i32>();
    println!("multiply_sum: {}", apply_with_log(multiply_sum, 3));
}

fn make_greeter(prefix: String) -> impl Fn(&str) {
    return move |name| println!("{} {}", prefix, name);
}

fn main() {
    let hi = make_greeter("Hi".to_string());
    hi("there");
}

Esercizi

In this example, you will implement the classic "ROT13" cipher. Copy this code to the playground, and implement the missing bits. Only rotate ASCII alphabetic characters, to ensure the result is still valid UTF-8.

use std::io::Read;

struct RotDecoder<R: Read> {
    input: R,
    rot: u8,
}

// Implement the `Read` trait for `RotDecoder`.

fn main() {
    let mut rot =
        RotDecoder { input: "Gb trg gb gur bgure fvqr!".as_bytes(), rot: 13 };
    let mut result = String::new();
    rot.read_to_string(&mut result).unwrap();
    println!("{}", result);
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn joke() {
        let mut rot =
            RotDecoder { input: "Gb trg gb gur bgure fvqr!".as_bytes(), rot: 13 };
        let mut result = String::new();
        rot.read_to_string(&mut result).unwrap();
        assert_eq!(&result, "To get to the other side!");
    }

    #[test]
    fn binary() {
        let input: Vec<u8> = (0..=255u8).collect();
        let mut rot = RotDecoder::<&[u8]> { input: input.as_ref(), rot: 13 };
        let mut buf = [0u8; 256];
        assert_eq!(rot.read(&mut buf).unwrap(), 256);
        for i in 0..=255 {
            if input[i] != buf[i] {
                assert!(input[i].is_ascii_alphabetic());
                assert!(buf[i].is_ascii_alphabetic());
            }
        }
    }
}

What happens if you chain two RotDecoder instances together, each rotating by 13 characters?

Soluzioni

use std::io::Read;

struct RotDecoder<R: Read> {
    input: R,
    rot: u8,
}

impl<R: Read> Read for RotDecoder<R> {
    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
        let size = self.input.read(buf)?;
        for b in &mut buf[..size] {
            if b.is_ascii_alphabetic() {
                let base = if b.is_ascii_uppercase() { 'A' } else { 'a' } as u8;
                *b = (*b - base + self.rot) % 26 + base;
            }
        }
        Ok(size)
    }
}

fn main() {
    let mut rot =
        RotDecoder { input: "Gb trg gb gur bgure fvqr!".as_bytes(), rot: 13 };
    let mut result = String::new();
    rot.read_to_string(&mut result).unwrap();
    println!("{}", result);
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn joke() {
        let mut rot =
            RotDecoder { input: "Gb trg gb gur bgure fvqr!".as_bytes(), rot: 13 };
        let mut result = String::new();
        rot.read_to_string(&mut result).unwrap();
        assert_eq!(&result, "To get to the other side!");
    }

    #[test]
    fn binary() {
        let input: Vec<u8> = (0..=255u8).collect();
        let mut rot = RotDecoder::<&[u8]> { input: input.as_ref(), rot: 13 };
        let mut buf = [0u8; 256];
        assert_eq!(rot.read(&mut buf).unwrap(), 256);
        for i in 0..=255 {
            if input[i] != buf[i] {
                assert!(input[i].is_ascii_alphabetic());
                assert!(buf[i].is_ascii_alphabetic());
            }
        }
    }
}

Welcome to Day 3

Today, we will cover:

• Memory management, lifetimes, and the borrow checker: how Rust ensures memory safety.
• Smart pointers: standard library pointer types.

Schedule

Including 10 minute breaks, this session should take about 2 hours and 15 minutes. It contains:

Gestione della Memoria

This segment should take about 1 hour and 10 minutes. It contains:

Review of Program Memory

Programs allocate memory in two ways:

• Stack: Continuous area of memory for local variables.

  • Values have fixed sizes known at compile time.
  • Extremely fast: just move a stack pointer.
  • Easy to manage: follows function calls.
  • Ottima località di memoria.

• Heap: Storage of values outside of function calls.

  • Values have dynamic sizes determined at runtime.
  • Slightly slower than the stack: some book-keeping needed.
  • Nessuna garanzia di località di memoria.

Esempio

Creating a String puts fixed-sized metadata on the stack and dynamically sized data, the actual string, on the heap:

fn main() {
    let s1 = String::from("Hello");
}

fn main() {
    let mut s1 = String::from("Hello");
    s1.push(' ');
    s1.push_str("world");
    // DON'T DO THIS AT HOME! For educational purposes only.
    // String provides no guarantees about its layout, so this could lead to
    // undefined behavior.
    unsafe {
        let (ptr, capacity, len): (usize, usize, usize) = std::mem::transmute(s1);
        println!("ptr = {ptr:#x}, len = {len}, capacity = {capacity}");
    }
}

Gestione automatica della Memoria

Traditionally, languages have fallen into two broad categories:

• Full control via manual memory management: C, C++, Pascal, ...
  • Programmer decides when to allocate or free heap memory.
  • Programmer must determine whether a pointer still points to valid memory.
  • Studies show, programmers make mistakes.
• Full safety via automatic memory management at runtime: Java, Python, Go, Haskell, ...
  • A runtime system ensures that memory is not freed until it can no longer be referenced.
  • Typically implemented with reference counting, garbage collection, or RAII.

Rust offre un nuovo mix:

Full control and safety via compile time enforcement of correct memory management.

Lo fa con un concetto di ownership (proprietà) esplicito.

Proprietà

All variable bindings have a scope where they are valid and it is an error to use a variable outside its scope:

struct Point(i32, i32);

fn main() {
    {
        let p = Point(3, 4);
        println!("x: {}", p.0);
    }
    println!("y: {}", p.1);
}

We say that the variable owns the value. Every Rust value has precisely one owner at all times.

At the end of the scope, the variable is dropped and the data is freed. A destructor can run here to free up resources.

Semantica di move

An assignment will transfer ownership between variables:

fn main() {
    let s1: String = String::from("Hello!");
    let s2: String = s1;
    println!("s2: {s2}");
    // println!("s1: {s1}");
}

• The assignment of s1 to s2 transfers ownership.
• When s1 goes out of scope, nothing happens: it does not own anything.
• When s2 goes out of scope, the string data is freed.

Prima di muovere a s2:

Dopo il muovere a s2:

When you pass a value to a function, the value is assigned to the function parameter. This transfers ownership:

fn say_hello(name: String) {
    println!("Hello {name}")
}

fn main() {
    let name = String::from("Alice");
    say_hello(name);
    // say_hello(name);
}

std::string s1 = "Cpp";
std::string s2 = s1;  // Duplicate the data in s1.

Clone

Sometimes you want to make a copy of a value. The Clone trait accomplishes this.

#[derive(Default)]
struct Backends {
    hostnames: Vec<String>,
    weights: Vec<f64>,
}

impl Backends {
    fn set_hostnames(&mut self, hostnames: &Vec<String>) {
        self.hostnames = hostnames.clone();
        self.weights = hostnames.iter().map(|_| 1.0).collect();
    }
}

Tipi Composti

While move semantics are the default, certain types are copied by default:

fn main() {
    let x = 42;
    let y = x;
    println!("x: {x}"); // would not be accessible if not Copy
    println!("y: {y}");
}

Questi tipi implementano il trait Copy.

You can opt-in your own types to use copy semantics:

#[derive(Copy, Clone, Debug)]
struct Point(i32, i32);

fn main() {
    let p1 = Point(3, 4);
    let p2 = p1;
    println!("p1: {p1:?}");
    println!("p2: {p2:?}");
}

• After the assignment, both p1 and p2 own their own data.
• We can also use p1.clone() to explicitly copy the data.

Il trait Drop

Values which implement Drop can specify code to run when they go out of scope:

struct Droppable {
    name: &'static str,
}

impl Drop for Droppable {
    fn drop(&mut self) {
        println!("Dropping {}", self.name);
    }
}

fn main() {
    let a = Droppable { name: "a" };
    {
        let b = Droppable { name: "b" };
        {
            let c = Droppable { name: "c" };
            let d = Droppable { name: "d" };
            println!("Exiting block B");
        }
        println!("Exiting block A");
    }
    drop(a);
    println!("Exiting main");
}

Exercise: Builder Type

In this example, we will implement a complex data type that owns all of its data. We will use the "builder pattern" to support building a new value piece-by-piece, using convenience functions.

Fill in the missing pieces.

#[derive(Debug)]
enum Language {
    Rust,
    Java,
    Perl,
}

#[derive(Clone, Debug)]
struct Dependency {
    name: String,
    version_expression: String,
}

/// A representation of a software package.
#[derive(Debug)]
struct Package {
    name: String,
    version: String,
    authors: Vec<String>,
    dependencies: Vec<Dependency>,
    language: Option<Language>,
}

impl Package {
    /// Return a representation of this package as a dependency, for use in
    /// building other packages.
    fn as_dependency(&self) -> Dependency {
        todo!("1")
    }
}

/// A builder for a Package. Use `build()` to create the `Package` itself.
struct PackageBuilder(Package);

impl PackageBuilder {
    fn new(name: impl Into<String>) -> Self {
        todo!("2")
    }

    /// Set the package version.
    fn version(mut self, version: impl Into<String>) -> Self {
        self.0.version = version.into();
        self
    }

    /// Set the package authors.
    fn authors(mut self, authors: Vec<String>) -> Self {
        todo!("3")
    }

    /// Add an additional dependency.
    fn dependency(mut self, dependency: Dependency) -> Self {
        todo!("4")
    }

    /// Set the language. If not set, language defaults to None.
    fn language(mut self, language: Language) -> Self {
        todo!("5")
    }

    fn build(self) -> Package {
        self.0
    }
}

fn main() {
    let base64 = PackageBuilder::new("base64").version("0.13").build();
    println!("base64: {base64:?}");
    let log =
        PackageBuilder::new("log").version("0.4").language(Language::Rust).build();
    println!("log: {log:?}");
    let serde = PackageBuilder::new("serde")
        .authors(vec!["djmitche".into()])
        .version(String::from("4.0"))
        .dependency(base64.as_dependency())
        .dependency(log.as_dependency())
        .build();
    println!("serde: {serde:?}");
}

Soluzioni

#[derive(Debug)]
enum Language {
    Rust,
    Java,
    Perl,
}

#[derive(Clone, Debug)]
struct Dependency {
    name: String,
    version_expression: String,
}

/// A representation of a software package.
#[derive(Debug)]
struct Package {
    name: String,
    version: String,
    authors: Vec<String>,
    dependencies: Vec<Dependency>,
    language: Option<Language>,
}

impl Package {
    /// Return a representation of this package as a dependency, for use in
    /// building other packages.
    fn as_dependency(&self) -> Dependency {
        Dependency {
            name: self.name.clone(),
            version_expression: self.version.clone(),
        }
    }
}

/// A builder for a Package. Use `build()` to create the `Package` itself.
struct PackageBuilder(Package);

impl PackageBuilder {
    fn new(name: impl Into<String>) -> Self {
        Self(Package {
            name: name.into(),
            version: "0.1".into(),
            authors: vec![],
            dependencies: vec![],
            language: None,
        })
    }

    /// Set the package version.
    fn version(mut self, version: impl Into<String>) -> Self {
        self.0.version = version.into();
        self
    }

    /// Set the package authors.
    fn authors(mut self, authors: Vec<String>) -> Self {
        self.0.authors = authors;
        self
    }

    /// Add an additional dependency.
    fn dependency(mut self, dependency: Dependency) -> Self {
        self.0.dependencies.push(dependency);
        self
    }

    /// Set the language. If not set, language defaults to None.
    fn language(mut self, language: Language) -> Self {
        self.0.language = Some(language);
        self
    }

    fn build(self) -> Package {
        self.0
    }
}

fn main() {
    let base64 = PackageBuilder::new("base64").version("0.13").build();
    println!("base64: {base64:?}");
    let log =
        PackageBuilder::new("log").version("0.4").language(Language::Rust).build();
    println!("log: {log:?}");
    let serde = PackageBuilder::new("serde")
        .authors(vec!["djmitche".into()])
        .version(String::from("4.0"))
        .dependency(base64.as_dependency())
        .dependency(log.as_dependency())
        .build();
    println!("serde: {serde:?}");
}

Smart Pointers

This segment should take about 45 minutes. It contains:

Box<T>

Box is an owned pointer to data on the heap:

fn main() {
    let five = Box::new(5);
    println!("five: {}", *five);
}

Box<T> implements Deref<Target = T>, which means that you can call methods from T directly on a Box<T>.

Recursive data types or data types with dynamic sizes need to use a Box:

#[derive(Debug)]
enum List<T> {
    /// A non-empty list: first element and the rest of the list.
    Element(T, Box<List<T>>),
    /// An empty list.
    Nil,
}

fn main() {
    let list: List<i32> =
        List::Element(1, Box::new(List::Element(2, Box::new(List::Nil))));
    println!("{list:?}");
}

#[derive(Debug)]
enum List<T> {
    Element(T, Box<List<T>>),
    Nil,
}

fn main() {
    let list: List<i32> =
        List::Element(1, Box::new(List::Element(2, Box::new(List::Nil))));
    println!("{list:?}");
}

Rc

Rc is a reference-counted shared pointer. Use this when you need to refer to the same data from multiple places:

use std::rc::Rc;

fn main() {
    let a = Rc::new(10);
    let b = Rc::clone(&a);

    println!("a: {a}");
    println!("b: {b}");
}

• See Arc and Mutex if you are in a multi-threaded context.
• You can downgrade a shared pointer into a Weak pointer to create cycles that will get dropped.

Exercise: Binary Tree

A binary tree is a tree-type data structure where every node has two children (left and right). We will create a tree where each node stores a value. For a given node N, all nodes in a N's left subtree contain smaller values, and all nodes in N's right subtree will contain larger values.

Implement the following types, so that the given tests pass.

Extra Credit: implement an iterator over a binary tree that returns the values in order.

/// A node in the binary tree.
#[derive(Debug)]
struct Node<T: Ord> {
    value: T,
    left: Subtree<T>,
    right: Subtree<T>,
}

/// A possibly-empty subtree.
#[derive(Debug)]
struct Subtree<T: Ord>(Option<Box<Node<T>>>);

/// A container storing a set of values, using a binary tree.
///
/// If the same value is added multiple times, it is only stored once.
#[derive(Debug)]
pub struct BinaryTree<T: Ord> {
    root: Subtree<T>,
}

// Implement `new`, `insert`, `len`, and `has`.

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn len() {
        let mut tree = BinaryTree::new();
        assert_eq!(tree.len(), 0);
        tree.insert(2);
        assert_eq!(tree.len(), 1);
        tree.insert(1);
        assert_eq!(tree.len(), 2);
        tree.insert(2); // not a unique item
        assert_eq!(tree.len(), 2);
    }

    #[test]
    fn has() {
        let mut tree = BinaryTree::new();
        fn check_has(tree: &BinaryTree<i32>, exp: &[bool]) {
            let got: Vec<bool> =
                (0..exp.len()).map(|i| tree.has(&(i as i32))).collect();
            assert_eq!(&got, exp);
        }

        check_has(&tree, &[false, false, false, false, false]);
        tree.insert(0);
        check_has(&tree, &[true, false, false, false, false]);
        tree.insert(4);
        check_has(&tree, &[true, false, false, false, true]);
        tree.insert(4);
        check_has(&tree, &[true, false, false, false, true]);
        tree.insert(3);
        check_has(&tree, &[true, false, false, true, true]);
    }

    #[test]
    fn unbalanced() {
        let mut tree = BinaryTree::new();
        for i in 0..100 {
            tree.insert(i);
        }
        assert_eq!(tree.len(), 100);
        assert!(tree.has(&50));
    }
}

Soluzioni

use std::cmp::Ordering;

/// A node in the binary tree.
#[derive(Debug)]
struct Node<T: Ord> {
    value: T,
    left: Subtree<T>,
    right: Subtree<T>,
}

/// A possibly-empty subtree.
#[derive(Debug)]
struct Subtree<T: Ord>(Option<Box<Node<T>>>);

/// A container storing a set of values, using a binary tree.
///
/// If the same value is added multiple times, it is only stored once.
#[derive(Debug)]
pub struct BinaryTree<T: Ord> {
    root: Subtree<T>,
}

impl<T: Ord> BinaryTree<T> {
    fn new() -> Self {
        Self { root: Subtree::new() }
    }

    fn insert(&mut self, value: T) {
        self.root.insert(value);
    }

    fn has(&self, value: &T) -> bool {
        self.root.has(value)
    }

    fn len(&self) -> usize {
        self.root.len()
    }
}

impl<T: Ord> Subtree<T> {
    fn new() -> Self {
        Self(None)
    }

    fn insert(&mut self, value: T) {
        match &mut self.0 {
            None => self.0 = Some(Box::new(Node::new(value))),
            Some(n) => match value.cmp(&n.value) {
                Ordering::Less => n.left.insert(value),
                Ordering::Equal => {}
                Ordering::Greater => n.right.insert(value),
            },
        }
    }

    fn has(&self, value: &T) -> bool {
        match &self.0 {
            None => false,
            Some(n) => match value.cmp(&n.value) {
                Ordering::Less => n.left.has(value),
                Ordering::Equal => true,
                Ordering::Greater => n.right.has(value),
            },
        }
    }

    fn len(&self) -> usize {
        match &self.0 {
            None => 0,
            Some(n) => 1 + n.left.len() + n.right.len(),
        }
    }
}

impl<T: Ord> Node<T> {
    fn new(value: T) -> Self {
        Self { value, left: Subtree::new(), right: Subtree::new() }
    }
}

fn main() {
    let mut tree = BinaryTree::new();
    tree.insert("foo");
    assert_eq!(tree.len(), 1);
    tree.insert("bar");
    assert!(tree.has(&"foo"));
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn len() {
        let mut tree = BinaryTree::new();
        assert_eq!(tree.len(), 0);
        tree.insert(2);
        assert_eq!(tree.len(), 1);
        tree.insert(1);
        assert_eq!(tree.len(), 2);
        tree.insert(2); // not a unique item
        assert_eq!(tree.len(), 2);
    }

    #[test]
    fn has() {
        let mut tree = BinaryTree::new();
        fn check_has(tree: &BinaryTree<i32>, exp: &[bool]) {
            let got: Vec<bool> =
                (0..exp.len()).map(|i| tree.has(&(i as i32))).collect();
            assert_eq!(&got, exp);
        }

        check_has(&tree, &[false, false, false, false, false]);
        tree.insert(0);
        check_has(&tree, &[true, false, false, false, false]);
        tree.insert(4);
        check_has(&tree, &[true, false, false, false, true]);
        tree.insert(4);
        check_has(&tree, &[true, false, false, false, true]);
        tree.insert(3);
        check_has(&tree, &[true, false, false, true, true]);
    }

    #[test]
    fn unbalanced() {
        let mut tree = BinaryTree::new();
        for i in 0..100 {
            tree.insert(i);
        }
        assert_eq!(tree.len(), 100);
        assert!(tree.has(&50));
    }
}

Welcome Back

Including 10 minute breaks, this session should take about 2 hours and 20 minutes. It contains:

Prestito (Borrowing)

This segment should take about 1 hour. It contains:

Prestito (Borrowing)

As we saw before, instead of transferring ownership when calling a function, you can let a function borrow the value:

#[derive(Debug)]
struct Point(i32, i32);

fn add(p1: &Point, p2: &Point) -> Point {
    Point(p1.0 + p2.0, p1.1 + p2.1)
}

fn main() {
    let p1 = Point(3, 4);
    let p2 = Point(10, 20);
    let p3 = add(&p1, &p2);
    println!("{p1:?} + {p2:?} = {p3:?}");
}

• The add function borrows two points and returns a new point.
• Il chiamante (caller) mantiene la proprietà (ownership) degli input.

Prestito (Borrowing)

Rust's borrow checker puts constraints on the ways you can borrow values. For a given value, at any time:

• You can have one or more shared references to the value, or
• You can have exactly one exclusive reference to the value.

fn main() {
    let mut a: i32 = 10;
    let b: &i32 = &a;

    {
        let c: &mut i32 = &mut a;
        *c = 20;
    }

    println!("a: {a}");
    println!("b: {b}");
}

Interoperabilità

In some situations, it's necessary to modify data behind a shared (read-only) reference. For example, a shared data structure might have an internal cache, and wish to update that cache from read-only methods.

The "interior mutability" pattern allows exclusive (mutable) access behind a shared reference. The standard library provides several ways to do this, all while still ensuring safety, typically by performing a runtime check.

RefCell

use std::cell::RefCell;
use std::rc::Rc;

#[derive(Debug, Default)]
struct Node {
    value: i64,
    children: Vec<Rc<RefCell<Node>>>,
}

impl Node {
    fn new(value: i64) -> Rc<RefCell<Node>> {
        Rc::new(RefCell::new(Node { value, ..Node::default() }))
    }

    fn sum(&self) -> i64 {
        self.value + self.children.iter().map(|c| c.borrow().sum()).sum::<i64>()
    }
}

fn main() {
    let root = Node::new(1);
    root.borrow_mut().children.push(Node::new(5));
    let subtree = Node::new(10);
    subtree.borrow_mut().children.push(Node::new(11));
    subtree.borrow_mut().children.push(Node::new(12));
    root.borrow_mut().children.push(subtree);

    println!("graph: {root:#?}");
    println!("graph sum: {}", root.borrow().sum());
}

Cell

Cell wraps a value and allows getting or setting the value, even with a shared reference to the Cell. However, it does not allow any references to the value. Since there are no references, borrowing rules cannot be broken.

Salute (Health) Statistics

You're working on implementing a health-monitoring system. As part of that, you need to keep track of users' health statistics.

You'll start with a stubbed function in an impl block as well as a User struct definition. Your goal is to implement the stubbed out method on the User struct defined in the impl block.

Copy the code below to https://play.rust-lang.org/ and fill in the missing method:

// TODO: remove this when you're done with your implementation.
#![allow(unused_variables, dead_code)]


#![allow(dead_code)]
pub struct User {
    name: String,
    age: u32,
    height: f32,
    visit_count: usize,
    last_blood_pressure: Option<(u32, u32)>,
}

pub struct Measurements {
    height: f32,
    blood_pressure: (u32, u32),
}

pub struct HealthReport<'a> {
    patient_name: &'a str,
    visit_count: u32,
    height_change: f32,
    blood_pressure_change: Option<(i32, i32)>,
}

impl User {
    pub fn new(name: String, age: u32, height: f32) -> Self {
        Self {
            name,
            age,
            height,
            visit_count: 0,
            last_blood_pressure: None,
        }
    }

    pub fn visit_doctor(&mut self, measurements: Measurements) -> HealthReport {
        todo!("Update a user's statistics based on measurements from a visit to the doctor")
    }
}

fn main() {
    let bob = User::new(String::from("Bob"), 32, 155.2);
    println!("I'm {} and my age is {}", bob.name, bob.age);
}

#[test]
fn test_visit() {
    let mut bob = User::new(String::from("Bob"), 32, 155.2);
    assert_eq!(bob.visit_count, 0);
    let report = bob.visit_doctor(Measurements {
        height: 156.1,
        blood_pressure: (120, 80),
    });
    assert_eq!(report.patient_name, "Bob");
    assert_eq!(report.visit_count, 1);
    assert_eq!(report.blood_pressure_change, None);

    let report = bob.visit_doctor(Measurements {
        height: 156.1,
        blood_pressure: (115, 76),
    });

    assert_eq!(report.visit_count, 2);
    assert_eq!(report.blood_pressure_change, Some((-5, -4)));
}

Soluzioni

#![allow(dead_code)]
pub struct User {
    name: String,
    age: u32,
    height: f32,
    visit_count: usize,
    last_blood_pressure: Option<(u32, u32)>,
}

pub struct Measurements {
    height: f32,
    blood_pressure: (u32, u32),
}

pub struct HealthReport<'a> {
    patient_name: &'a str,
    visit_count: u32,
    height_change: f32,
    blood_pressure_change: Option<(i32, i32)>,
}

impl User {
    pub fn new(name: String, age: u32, height: f32) -> Self {
        Self {
            name,
            age,
            height,
            visit_count: 0,
            last_blood_pressure: None,
        }
    }

    pub fn visit_doctor(&mut self, measurements: Measurements) -> HealthReport {
        self.visit_count += 1;
        let bp = measurements.blood_pressure;
        let report = HealthReport {
            patient_name: &self.name,
            visit_count: self.visit_count as u32,
            height_change: measurements.height - self.height,
            blood_pressure_change: match self.last_blood_pressure {
                Some(lbp) => {
                    Some((bp.0 as i32 - lbp.0 as i32, bp.1 as i32 - lbp.1 as i32))
                }
                None => None,
            },
        };
        self.height = measurements.height;
        self.last_blood_pressure = Some(bp);
        report
    }
}

fn main() {
    let bob = User::new(String::from("Bob"), 32, 155.2);
    println!("I'm {} and my age is {}", bob.name, bob.age);
}

#[test]
fn test_visit() {
    let mut bob = User::new(String::from("Bob"), 32, 155.2);
    assert_eq!(bob.visit_count, 0);
    let report = bob.visit_doctor(Measurements {
        height: 156.1,
        blood_pressure: (120, 80),
    });
    assert_eq!(report.patient_name, "Bob");
    assert_eq!(report.visit_count, 1);
    assert_eq!(report.blood_pressure_change, None);

    let report = bob.visit_doctor(Measurements {
        height: 156.1,
        blood_pressure: (115, 76),
    });

    assert_eq!(report.visit_count, 2);
    assert_eq!(report.blood_pressure_change, Some((-5, -4)));
}

Lifetime

This segment should take about 1 hour and 10 minutes. It contains:

Slice

Una slice ti offre una vista in una raccolta più ampia:

fn main() {
    let mut a: [i32; 6] = [10, 20, 30, 40, 50, 60];
    println!("a: {a:?}");

    let s: &[i32] = &a[2..4];

    println!("s: {s:?}");
}

• Le sezioni prendono in prestito i dati dal tipo della raccolta.
• Domanda: Cosa succede se modifichi a[3] prima di stampare s?

Riferimenti pendenti

We can now understand the two string types in Rust: &str is almost like &[char], but with its data stored in a variable-length encoding (UTF-8).

fn main() {
    let s1: &str = "World";
    println!("s1: {s1}");

    let mut s2: String = String::from("Hello ");
    println!("s2: {s2}");
    s2.push_str(s1);
    println!("s2: {s2}");

    let s3: &str = &s2[6..];
    println!("s3: {s3}");
}

Terminologia di Rust:

• &str un riferimento immutabile a uno slice di stringa.
• String è un buffer di stringa mutabile.

Lifetime in Chiamate a Funzione

A reference has a lifetime, which must not "outlive" the value it refers to. This is verified by the borrow checker.

The lifetime can be implicit - this is what we have seen so far. Lifetimes can also be explicit: &'a Point, &'document str. Lifetimes start with ' and 'a is a typical default name. Read &'a Point as "a borrowed Point which is valid for at least the lifetime a".

Lifetimes are always inferred by the compiler: you cannot assign a lifetime yourself. Explicit lifetime annotations create constraints where there is ambiguity; the compiler verifies that there is a valid solution.

Lifetimes become more complicated when considering passing values to and returning values from functions.

#[derive(Debug)]
struct Point(i32, i32);

fn left_most(p1: &Point, p2: &Point) -> &Point {
    if p1.0 < p2.0 {
        p1
    } else {
        p2
    }
}

fn main() {
    let p1: Point = Point(10, 10);
    let p2: Point = Point(20, 20);
    let p3 = left_most(&p1, &p2); // What is the lifetime of p3?
    println!("p3: {p3:?}");
}

fn left_most<'a>(p1: &'a Point, p2: &'a Point) -> &'a Point {

Lifetime in Chiamate a Funzione

Lifetimes for function arguments and return values must be fully specified, but Rust allows lifetimes to be elided in most cases with a few simple rules. This is not inference -- it is just a syntactic shorthand.

• Each argument which does not have a lifetime annotation is given one.
• If there is only one argument lifetime, it is given to all un-annotated return values.
• If there are multiple argument lifetimes, but the first one is for self, that lifetime is given to all un-annotated return values.

#[derive(Debug)]
struct Point(i32, i32);

fn cab_distance(p1: &Point, p2: &Point) -> i32 {
    (p1.0 - p2.0).abs() + (p1.1 - p2.1).abs()
}

fn nearest<'a>(points: &'a [Point], query: &Point) -> Option<&'a Point> {
    let mut nearest = None;
    for p in points {
        if let Some((_, nearest_dist)) = nearest {
            let dist = cab_distance(p, query);
            if dist < nearest_dist {
                nearest = Some((p, dist));
            }
        } else {
            nearest = Some((p, cab_distance(p, query)));
        };
    }
    nearest.map(|(p, _)| p)
}

fn main() {
    println!(
        "{:?}",
        nearest(
            &[Point(1, 0), Point(1, 0), Point(-1, 0), Point(0, -1),],
            &Point(0, 2)
        )
    );
}

fn nearest<'a, 'q'>(points: &'a [Point], query: &'q Point) -> Option<&'q Point> {

Lifetime in Strutture Dati

If a data type stores borrowed data, it must be annotated with a lifetime:

#[derive(Debug)]
struct Highlight<'doc>(&'doc str);

fn erase(text: String) {
    println!("Bye {text}!");
}

fn main() {
    let text = String::from("The quick brown fox jumps over the lazy dog.");
    let fox = Highlight(&text[4..19]);
    let dog = Highlight(&text[35..43]);
    // erase(text);
    println!("{fox:?}");
    println!("{dog:?}");
}

Exercise: Protobuf Parsing

In this exercise, you will build a parser for the protobuf binary encoding. Don't worry, it's simpler than it seems! This illustrates a common parsing pattern, passing slices of data. The underlying data itself is never copied.

Fully parsing a protobuf message requires knowing the types of the fields, indexed by their field numbers. That is typically provided in a proto file. In this exercise, we'll encode that information into match statements in functions that get called for each field.

We'll use the following proto:

message PhoneNumber {
  optional string number = 1;
  optional string type = 2;
}

message Person {
  optional string name = 1;
  optional int32 id = 2;
  repeated PhoneNumber phones = 3;
}

A proto message is encoded as a series of fields, one after the next. Each is implemented as a "tag" followed by the value. The tag contains a field number (e.g., 2 for the id field of a Person message) and a wire type defining how the payload should be determined from the byte stream.

Integers, including the tag, are represented with a variable-length encoding called VARINT. Luckily, parse_varint is defined for you below. The given code also defines callbacks to handle Person and PhoneNumber fields, and to parse a message into a series of calls to those callbacks.

What remains for you is to implement the parse_field function and the ProtoMessage trait for Person and PhoneNumber.

use std::convert::TryFrom;
use thiserror::Error;

#[derive(Debug, Error)]
enum Error {
    #[error("Invalid varint")]
    InvalidVarint,
    #[error("Invalid wire-type")]
    InvalidWireType,
    #[error("Unexpected EOF")]
    UnexpectedEOF,
    #[error("Invalid length")]
    InvalidSize(#[from] std::num::TryFromIntError),
    #[error("Unexpected wire-type)")]
    UnexpectedWireType,
    #[error("Invalid string (not UTF-8)")]
    InvalidString,
}

/// A wire type as seen on the wire.
enum WireType {
    /// The Varint WireType indicates the value is a single VARINT.
    Varint,
    //I64,  -- not needed for this exercise
    /// The Len WireType indicates that the value is a length represented as a
    /// VARINT followed by exactly that number of bytes.
    Len,
    /// The I32 WireType indicates that the value is precisely 4 bytes in
    /// little-endian order containing a 32-bit signed integer.
    I32,
}

#[derive(Debug)]
/// A field's value, typed based on the wire type.
enum FieldValue<'a> {
    Varint(u64),
    //I64(i64),  -- not needed for this exercise
    Len(&'a [u8]),
    I32(i32),
}

#[derive(Debug)]
/// A field, containing the field number and its value.
struct Field<'a> {
    field_num: u64,
    value: FieldValue<'a>,
}

trait ProtoMessage<'a>: Default + 'a {
    fn add_field(&mut self, field: Field<'a>) -> Result<(), Error>;
}

impl TryFrom<u64> for WireType {
    type Error = Error;

    fn try_from(value: u64) -> Result<WireType, Error> {
        Ok(match value {
            0 => WireType::Varint,
            //1 => WireType::I64,  -- not needed for this exercise
            2 => WireType::Len,
            5 => WireType::I32,
            _ => return Err(Error::InvalidWireType),
        })
    }
}

impl<'a> FieldValue<'a> {
    fn as_string(&self) -> Result<&'a str, Error> {
        let FieldValue::Len(data) = self else {
            return Err(Error::UnexpectedWireType);
        };
        std::str::from_utf8(data).map_err(|_| Error::InvalidString)
    }

    fn as_bytes(&self) -> Result<&'a [u8], Error> {
        let FieldValue::Len(data) = self else {
            return Err(Error::UnexpectedWireType);
        };
        Ok(data)
    }

    fn as_u64(&self) -> Result<u64, Error> {
        let FieldValue::Varint(value) = self else {
            return Err(Error::UnexpectedWireType);
        };
        Ok(*value)
    }
}

/// Parse a VARINT, returning the parsed value and the remaining bytes.
fn parse_varint(data: &[u8]) -> Result<(u64, &[u8]), Error> {
    for i in 0..7 {
        let Some(b) = data.get(i) else {
            return Err(Error::InvalidVarint);
        };
        if b & 0x80 == 0 {
            // This is the last byte of the VARINT, so convert it to
            // a u64 and return it.
            let mut value = 0u64;
            for b in data[..=i].iter().rev() {
                value = (value << 7) | (b & 0x7f) as u64;
            }
            return Ok((value, &data[i + 1..]));
        }
    }

    // More than 7 bytes is invalid.
    Err(Error::InvalidVarint)
}

/// Convert a tag into a field number and a WireType.
fn unpack_tag(tag: u64) -> Result<(u64, WireType), Error> {
    let field_num = tag >> 3;
    let wire_type = WireType::try_from(tag & 0x7)?;
    Ok((field_num, wire_type))
}


/// Parse a field, returning the remaining bytes
fn parse_field(data: &[u8]) -> Result<(Field, &[u8]), Error> {
    let (tag, remainder) = parse_varint(data)?;
    let (field_num, wire_type) = unpack_tag(tag)?;
    let (fieldvalue, remainder) = match wire_type {
        _ => todo!("Based on the wire type, build a Field, consuming as many bytes as necessary.")
    };
    todo!("Return the field, and any un-consumed bytes.")
}

/// Parse a message in the given data, calling `T::add_field` for each field in
/// the message.
///
/// The entire input is consumed.
fn parse_message<'a, T: ProtoMessage<'a>>(mut data: &'a [u8]) -> Result<T, Error> {
    let mut result = T::default();
    while !data.is_empty() {
        let parsed = parse_field(data)?;
        result.add_field(parsed.0)?;
        data = parsed.1;
    }
    Ok(result)
}

#[derive(Debug, Default)]
struct PhoneNumber<'a> {
    number: &'a str,
    type_: &'a str,
}

#[derive(Debug, Default)]
struct Person<'a> {
    name: &'a str,
    id: u64,
    phone: Vec<PhoneNumber<'a>>,
}

// TODO: Implement ProtoMessage for Person and PhoneNumber.

fn main() {
    let person: Person = parse_message(&[
        0x0a, 0x07, 0x6d, 0x61, 0x78, 0x77, 0x65, 0x6c, 0x6c, 0x10, 0x2a, 0x1a,
        0x16, 0x0a, 0x0e, 0x2b, 0x31, 0x32, 0x30, 0x32, 0x2d, 0x35, 0x35, 0x35,
        0x2d, 0x31, 0x32, 0x31, 0x32, 0x12, 0x04, 0x68, 0x6f, 0x6d, 0x65, 0x1a,
        0x18, 0x0a, 0x0e, 0x2b, 0x31, 0x38, 0x30, 0x30, 0x2d, 0x38, 0x36, 0x37,
        0x2d, 0x35, 0x33, 0x30, 0x38, 0x12, 0x06, 0x6d, 0x6f, 0x62, 0x69, 0x6c,
        0x65,
    ])
    .unwrap();
    println!("{:#?}", person);
}

Soluzioni

use std::convert::TryFrom;
use thiserror::Error;

#[derive(Debug, Error)]
enum Error {
    #[error("Invalid varint")]
    InvalidVarint,
    #[error("Invalid wire-type")]
    InvalidWireType,
    #[error("Unexpected EOF")]
    UnexpectedEOF,
    #[error("Invalid length")]
    InvalidSize(#[from] std::num::TryFromIntError),
    #[error("Unexpected wire-type)")]
    UnexpectedWireType,
    #[error("Invalid string (not UTF-8)")]
    InvalidString,
}

/// A wire type as seen on the wire.
enum WireType {
    /// The Varint WireType indicates the value is a single VARINT.
    Varint,
    //I64,  -- not needed for this exercise
    /// The Len WireType indicates that the value is a length represented as a
    /// VARINT followed by exactly that number of bytes.
    Len,
    /// The I32 WireType indicates that the value is precisely 4 bytes in
    /// little-endian order containing a 32-bit signed integer.
    I32,
}

#[derive(Debug)]
/// A field's value, typed based on the wire type.
enum FieldValue<'a> {
    Varint(u64),
    //I64(i64),  -- not needed for this exercise
    Len(&'a [u8]),
    I32(i32),
}

#[derive(Debug)]
/// A field, containing the field number and its value.
struct Field<'a> {
    field_num: u64,
    value: FieldValue<'a>,
}

trait ProtoMessage<'a>: Default + 'a {
    fn add_field(&mut self, field: Field<'a>) -> Result<(), Error>;
}

impl TryFrom<u64> for WireType {
    type Error = Error;

    fn try_from(value: u64) -> Result<WireType, Error> {
        Ok(match value {
            0 => WireType::Varint,
            //1 => WireType::I64,  -- not needed for this exercise
            2 => WireType::Len,
            5 => WireType::I32,
            _ => return Err(Error::InvalidWireType),
        })
    }
}

impl<'a> FieldValue<'a> {
    fn as_string(&self) -> Result<&'a str, Error> {
        let FieldValue::Len(data) = self else {
            return Err(Error::UnexpectedWireType);
        };
        std::str::from_utf8(data).map_err(|_| Error::InvalidString)
    }

    fn as_bytes(&self) -> Result<&'a [u8], Error> {
        let FieldValue::Len(data) = self else {
            return Err(Error::UnexpectedWireType);
        };
        Ok(data)
    }

    fn as_u64(&self) -> Result<u64, Error> {
        let FieldValue::Varint(value) = self else {
            return Err(Error::UnexpectedWireType);
        };
        Ok(*value)
    }
}

/// Parse a VARINT, returning the parsed value and the remaining bytes.
fn parse_varint(data: &[u8]) -> Result<(u64, &[u8]), Error> {
    for i in 0..7 {
        let Some(b) = data.get(i) else {
            return Err(Error::InvalidVarint);
        };
        if b & 0x80 == 0 {
            // This is the last byte of the VARINT, so convert it to
            // a u64 and return it.
            let mut value = 0u64;
            for b in data[..=i].iter().rev() {
                value = (value << 7) | (b & 0x7f) as u64;
            }
            return Ok((value, &data[i + 1..]));
        }
    }

    // More than 7 bytes is invalid.
    Err(Error::InvalidVarint)
}

/// Convert a tag into a field number and a WireType.
fn unpack_tag(tag: u64) -> Result<(u64, WireType), Error> {
    let field_num = tag >> 3;
    let wire_type = WireType::try_from(tag & 0x7)?;
    Ok((field_num, wire_type))
}

/// Parse a field, returning the remaining bytes
fn parse_field(data: &[u8]) -> Result<(Field, &[u8]), Error> {
    let (tag, remainder) = parse_varint(data)?;
    let (field_num, wire_type) = unpack_tag(tag)?;
    let (fieldvalue, remainder) = match wire_type {
        WireType::Varint => {
            let (value, remainder) = parse_varint(remainder)?;
            (FieldValue::Varint(value), remainder)
        }
        WireType::Len => {
            let (len, remainder) = parse_varint(remainder)?;
            let len: usize = len.try_into()?;
            if remainder.len() < len {
                return Err(Error::UnexpectedEOF);
            }
            let (value, remainder) = remainder.split_at(len);
            (FieldValue::Len(value), remainder)
        }
        WireType::I32 => {
            if remainder.len() < 4 {
                return Err(Error::UnexpectedEOF);
            }
            let (value, remainder) = remainder.split_at(4);
            // Unwrap error because `value` is definitely 4 bytes long.
            let value = i32::from_le_bytes(value.try_into().unwrap());
            (FieldValue::I32(value), remainder)
        }
    };
    Ok((Field { field_num, value: fieldvalue }, remainder))
}

/// Parse a message in the given data, calling `T::add_field` for each field in
/// the message.
///
/// The entire input is consumed.
fn parse_message<'a, T: ProtoMessage<'a>>(mut data: &'a [u8]) -> Result<T, Error> {
    let mut result = T::default();
    while !data.is_empty() {
        let parsed = parse_field(data)?;
        result.add_field(parsed.0)?;
        data = parsed.1;
    }
    Ok(result)
}

#[derive(Debug, Default)]
struct PhoneNumber<'a> {
    number: &'a str,
    type_: &'a str,
}

#[derive(Debug, Default)]
struct Person<'a> {
    name: &'a str,
    id: u64,
    phone: Vec<PhoneNumber<'a>>,
}

impl<'a> ProtoMessage<'a> for Person<'a> {
    fn add_field(&mut self, field: Field<'a>) -> Result<(), Error> {
        match field.field_num {
            1 => self.name = field.value.as_string()?,
            2 => self.id = field.value.as_u64()?,
            3 => self.phone.push(parse_message(field.value.as_bytes()?)?),
            _ => {} // skip everything else
        }
        Ok(())
    }
}

impl<'a> ProtoMessage<'a> for PhoneNumber<'a> {
    fn add_field(&mut self, field: Field<'a>) -> Result<(), Error> {
        match field.field_num {
            1 => self.number = field.value.as_string()?,
            2 => self.type_ = field.value.as_string()?,
            _ => {} // skip everything else
        }
        Ok(())
    }
}

fn main() {
    let person: Person = parse_message(&[
        0x0a, 0x07, 0x6d, 0x61, 0x78, 0x77, 0x65, 0x6c, 0x6c, 0x10, 0x2a, 0x1a,
        0x16, 0x0a, 0x0e, 0x2b, 0x31, 0x32, 0x30, 0x32, 0x2d, 0x35, 0x35, 0x35,
        0x2d, 0x31, 0x32, 0x31, 0x32, 0x12, 0x04, 0x68, 0x6f, 0x6d, 0x65, 0x1a,
        0x18, 0x0a, 0x0e, 0x2b, 0x31, 0x38, 0x30, 0x30, 0x2d, 0x38, 0x36, 0x37,
        0x2d, 0x35, 0x33, 0x30, 0x38, 0x12, 0x06, 0x6d, 0x6f, 0x62, 0x69, 0x6c,
        0x65,
    ])
    .unwrap();
    println!("{:#?}", person);
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn as_string() {
        assert!(FieldValue::Varint(10).as_string().is_err());
        assert!(FieldValue::I32(10).as_string().is_err());
        assert_eq!(FieldValue::Len(b"hello").as_string().unwrap(), "hello");
    }

    #[test]
    fn as_bytes() {
        assert!(FieldValue::Varint(10).as_bytes().is_err());
        assert!(FieldValue::I32(10).as_bytes().is_err());
        assert_eq!(FieldValue::Len(b"hello").as_bytes().unwrap(), b"hello");
    }

    #[test]
    fn as_u64() {
        assert_eq!(FieldValue::Varint(10).as_u64().unwrap(), 10u64);
        assert!(FieldValue::I32(10).as_u64().is_err());
        assert!(FieldValue::Len(b"hello").as_u64().is_err());
    }
}

Welcome to Day 4

Today we will cover topics relating to building large-scale software in Rust:

• Iterators: a deep dive on the Iterator trait.
• Modules and visibility.
• Testing.
• Error handling: panics, Result, and the try operator ?.
• Unsafe Rust: the escape hatch when you can't express yourself in safe Rust.

Schedule

Including 10 minute breaks, this session should take about 3 hours and 5 minutes. It contains:

Iteratori

This segment should take about 45 minutes. It contains:

Iterator

The Iterator trait supports iterating over values in a collection. It requires a next method and provides lots of methods. Many standard library types implement Iterator, and you can implement it yourself, too:

struct Fibonacci {
    curr: u32,
    next: u32,
}

impl Iterator for Fibonacci {
    type Item = u32;

    fn next(&mut self) -> Option<Self::Item> {
        let new_next = self.curr + self.next;
        self.curr = self.next;
        self.next = new_next;
        Some(self.curr)
    }
}

fn main() {
    let fib = Fibonacci { curr: 0, next: 1 };
    for (i, n) in fib.enumerate().take(5) {
        println!("fib({i}): {n}");
    }
}

IntoIterator

The Iterator trait tells you how to iterate once you have created an iterator. The related trait IntoIterator defines how to create an iterator for a type. It is used automatically by the for loop.

struct Grid {
    x_coords: Vec<u32>,
    y_coords: Vec<u32>,
}

impl IntoIterator for Grid {
    type Item = (u32, u32);
    type IntoIter = GridIter;
    fn into_iter(self) -> GridIter {
        GridIter { grid: self, i: 0, j: 0 }
    }
}

struct GridIter {
    grid: Grid,
    i: usize,
    j: usize,
}

impl Iterator for GridIter {
    type Item = (u32, u32);

    fn next(&mut self) -> Option<(u32, u32)> {
        if self.i >= self.grid.x_coords.len() {
            self.i = 0;
            self.j += 1;
            if self.j >= self.grid.y_coords.len() {
                return None;
            }
        }
        let res = Some((self.grid.x_coords[self.i], self.grid.y_coords[self.j]));
        self.i += 1;
        res
    }
}

fn main() {
    let grid = Grid { x_coords: vec![3, 5, 7, 9], y_coords: vec![10, 20, 30, 40] };
    for (x, y) in grid {
        println!("point = {x}, {y}");
    }
}

FromIterator

FromIterator lets you build a collection from an Iterator.

fn main() {
    let primes = vec![2, 3, 5, 7];
    let prime_squares = primes.into_iter().map(|p| p * p).collect::<Vec<_>>();
    println!("prime_squares: {prime_squares:?}");
}

fn collect<B>(self) -> B
where
    B: FromIterator<Self::Item>,
    Self: Sized

Exercise: Iterator Method Chaining

In this exercise, you will need to find and use some of the provided methods in the Iterator trait to implement a complex calculation.

Copy the following code to https://play.rust-lang.org/ and make the tests pass. Use an iterator expression and collect the result to construct the return value.

#![allow(unused)]
fn main() {
/// Calculate the differences between elements of `values` offset by `offset`,
/// wrapping around from the end of `values` to the beginning.
///
/// Element `n` of the result is `values[(n+offset)%len] - values[n]`.
fn offset_differences<N>(offset: usize, values: Vec<N>) -> Vec<N>
where
    N: Copy + std::ops::Sub<Output = N>,
{
    unimplemented!()
}

#[test]
fn test_offset_one() {
    assert_eq!(offset_differences(1, vec![1, 3, 5, 7]), vec![2, 2, 2, -6]);
    assert_eq!(offset_differences(1, vec![1, 3, 5]), vec![2, 2, -4]);
    assert_eq!(offset_differences(1, vec![1, 3]), vec![2, -2]);
}

#[test]
fn test_larger_offsets() {
    assert_eq!(offset_differences(2, vec![1, 3, 5, 7]), vec![4, 4, -4, -4]);
    assert_eq!(offset_differences(3, vec![1, 3, 5, 7]), vec![6, -2, -2, -2]);
    assert_eq!(offset_differences(4, vec![1, 3, 5, 7]), vec![0, 0, 0, 0]);
    assert_eq!(offset_differences(5, vec![1, 3, 5, 7]), vec![2, 2, 2, -6]);
}

#[test]
fn test_custom_type() {
    assert_eq!(
        offset_differences(1, vec![1.0, 11.0, 5.0, 0.0]),
        vec![10.0, -6.0, -5.0, 1.0]
    );
}

#[test]
fn test_degenerate_cases() {
    assert_eq!(offset_differences(1, vec![0]), vec![0]);
    assert_eq!(offset_differences(1, vec![1]), vec![0]);
    let empty: Vec<i32> = vec![];
    assert_eq!(offset_differences(1, empty), vec![]);
}
}

Soluzioni

/// Calculate the differences between elements of `values` offset by `offset`,
/// wrapping around from the end of `values` to the beginning.
///
/// Element `n` of the result is `values[(n+offset)%len] - values[n]`.
fn offset_differences<N>(offset: usize, values: Vec<N>) -> Vec<N>
where
    N: Copy + std::ops::Sub<Output = N>,
{
    let a = (&values).into_iter();
    let b = (&values).into_iter().cycle().skip(offset);
    a.zip(b).map(|(a, b)| *b - *a).take(values.len()).collect()
}

#[test]
fn test_offset_one() {
    assert_eq!(offset_differences(1, vec![1, 3, 5, 7]), vec![2, 2, 2, -6]);
    assert_eq!(offset_differences(1, vec![1, 3, 5]), vec![2, 2, -4]);
    assert_eq!(offset_differences(1, vec![1, 3]), vec![2, -2]);
}

#[test]
fn test_larger_offsets() {
    assert_eq!(offset_differences(2, vec![1, 3, 5, 7]), vec![4, 4, -4, -4]);
    assert_eq!(offset_differences(3, vec![1, 3, 5, 7]), vec![6, -2, -2, -2]);
    assert_eq!(offset_differences(4, vec![1, 3, 5, 7]), vec![0, 0, 0, 0]);
    assert_eq!(offset_differences(5, vec![1, 3, 5, 7]), vec![2, 2, 2, -6]);
}

#[test]
fn test_custom_type() {
    assert_eq!(
        offset_differences(1, vec![1.0, 11.0, 5.0, 0.0]),
        vec![10.0, -6.0, -5.0, 1.0]
    );
}

#[test]
fn test_degenerate_cases() {
    assert_eq!(offset_differences(1, vec![0]), vec![0]);
    assert_eq!(offset_differences(1, vec![1]), vec![0]);
    let empty: Vec<i32> = vec![];
    assert_eq!(offset_differences(1, empty), vec![]);
}

fn main() {}

Moduli

This segment should take about 40 minutes. It contains:

Moduli

We have seen how impl blocks let us namespace functions to a type.

Similarly, mod lets us namespace types and functions:

mod foo {
    pub fn do_something() {
        println!("In the foo module");
    }
}

mod bar {
    pub fn do_something() {
        println!("In the bar module");
    }
}

fn main() {
    foo::do_something();
    bar::do_something();
}

Gerarchia del filesystem

Omitting the module content will tell Rust to look for it in another file:

mod garden;

This tells rust that the garden module content is found at src/garden.rs. Similarly, a garden::vegetables module can be found at src/garden/vegetables.rs.

The crate root is in:

• src/lib.rs (for a library crate)
• src/main.rs (for a binary crate)

Modules defined in files can be documented, too, using "inner doc comments". These document the item that contains them -- in this case, a module.

//! This module implements the garden, including a highly performant germination
//! implementation.

// Re-export types from this module.
pub use garden::Garden;
pub use seeds::SeedPacket;

/// Sow the given seed packets.
pub fn sow(seeds: Vec<SeedPacket>) {
    todo!()
}

/// Harvest the produce in the garden that is ready.
pub fn harvest(garden: &mut Garden) {
    todo!()
}

Visibilità

Modules are a privacy boundary:

• Module items are private by default (hides implementation details).
• Parent and sibling items are always visible.
• In other words, if an item is visible in module foo, it's visible in all the descendants of foo.

mod outer {
    fn private() {
        println!("outer::private");
    }

    pub fn public() {
        println!("outer::public");
    }

    mod inner {
        fn private() {
            println!("outer::inner::private");
        }

        pub fn public() {
            println!("outer::inner::public");
            super::private();
        }
    }
}

fn main() {
    outer::public();
}

use, super, self

A module can bring symbols from another module into scope with use. You will typically see something like this at the top of each module:

use std::collections::HashSet;
use std::process::abort;

Percorsi (Paths)

Paths are resolved as follows:

1. As a relative path:

   • foo or self::foo refers to foo in the current module,
   • super::foo refers to foo in the parent module.

2. As an absolute path:

   • crate::foo refers to foo in the root of the current crate,
   • bar::foo refers to foo in the bar crate.

Exercise: Modules for a GUI Library

In this exercise, you will reorganize a small GUI Library implementation. This library defines a Widget trait and a few implementations of that trait, as well as a main function.

It is typical to put each type or set of closely-related types into its own module, so each widget type should get its own module.

Cargo Setup

The Rust playground only supports one file, so you will need to make a Cargo project on your local filesystem:

cargo init gui-modules
cd gui-modules
cargo run

Edit the resulting src/main.rs to add mod statements, and add additional files in the src directory.

Source

Here's the single-module implementation of the GUI library:

pub trait Widget {
    /// Natural width of `self`.
    fn width(&self) -> usize;

    /// Draw the widget into a buffer.
    fn draw_into(&self, buffer: &mut dyn std::fmt::Write);

    /// Draw the widget on standard output.
    fn draw(&self) {
        let mut buffer = String::new();
        self.draw_into(&mut buffer);
        println!("{buffer}");
    }
}

pub struct Label {
    label: String,
}

impl Label {
    fn new(label: &str) -> Label {
        Label { label: label.to_owned() }
    }
}

pub struct Button {
    label: Label,
}

impl Button {
    fn new(label: &str) -> Button {
        Button { label: Label::new(label) }
    }
}

pub struct Window {
    title: String,
    widgets: Vec<Box<dyn Widget>>,
}

impl Window {
    fn new(title: &str) -> Window {
        Window { title: title.to_owned(), widgets: Vec::new() }
    }

    fn add_widget(&mut self, widget: Box<dyn Widget>) {
        self.widgets.push(widget);
    }

    fn inner_width(&self) -> usize {
        std::cmp::max(
            self.title.chars().count(),
            self.widgets.iter().map(|w| w.width()).max().unwrap_or(0),
        )
    }
}

impl Widget for Window {
    fn width(&self) -> usize {
        // Add 4 paddings for borders
        self.inner_width() + 4
    }

    fn draw_into(&self, buffer: &mut dyn std::fmt::Write) {
        let mut inner = String::new();
        for widget in &self.widgets {
            widget.draw_into(&mut inner);
        }

        let inner_width = self.inner_width();

        // TODO: Change draw_into to return Result<(), std::fmt::Error>. Then use the
        // ?-operator here instead of .unwrap().
        writeln!(buffer, "+-{:-<inner_width$}-+", "").unwrap();
        writeln!(buffer, "| {:^inner_width$} |", &self.title).unwrap();
        writeln!(buffer, "+={:=<inner_width$}=+", "").unwrap();
        for line in inner.lines() {
            writeln!(buffer, "| {:inner_width$} |", line).unwrap();
        }
        writeln!(buffer, "+-{:-<inner_width$}-+", "").unwrap();
    }
}

impl Widget for Button {
    fn width(&self) -> usize {
        self.label.width() + 8 // add a bit of padding
    }

    fn draw_into(&self, buffer: &mut dyn std::fmt::Write) {
        let width = self.width();
        let mut label = String::new();
        self.label.draw_into(&mut label);

        writeln!(buffer, "+{:-<width$}+", "").unwrap();
        for line in label.lines() {
            writeln!(buffer, "|{:^width$}|", &line).unwrap();
        }
        writeln!(buffer, "+{:-<width$}+", "").unwrap();
    }
}

impl Widget for Label {
    fn width(&self) -> usize {
        self.label.lines().map(|line| line.chars().count()).max().unwrap_or(0)
    }

    fn draw_into(&self, buffer: &mut dyn std::fmt::Write) {
        writeln!(buffer, "{}", &self.label).unwrap();
    }
}

fn main() {
    let mut window = Window::new("Rust GUI Demo 1.23");
    window.add_widget(Box::new(Label::new("This is a small text GUI demo.")));
    window.add_widget(Box::new(Button::new("Click me!")));
    window.draw();
}

Soluzioni

src
├── main.rs
├── widgets
│   ├── button.rs
│   ├── label.rs
│   └── window.rs
└── widgets.rs

// ---- src/widgets.rs ----
mod button;
mod label;
mod window;

pub trait Widget {
    /// Natural width of `self`.
    fn width(&self) -> usize;

    /// Draw the widget into a buffer.
    fn draw_into(&self, buffer: &mut dyn std::fmt::Write);

    /// Draw the widget on standard output.
    fn draw(&self) {
        let mut buffer = String::new();
        self.draw_into(&mut buffer);
        println!("{buffer}");
    }
}

pub use button::Button;
pub use label::Label;
pub use window::Window;

// ---- src/widgets/label.rs ----
use super::Widget;

pub struct Label {
    label: String,
}

impl Label {
    pub fn new(label: &str) -> Label {
        Label { label: label.to_owned() }
    }
}

impl Widget for Label {
    fn width(&self) -> usize {
        // ANCHOR_END: Label-width
        self.label.lines().map(|line| line.chars().count()).max().unwrap_or(0)
    }

    // ANCHOR: Label-draw_into
    fn draw_into(&self, buffer: &mut dyn std::fmt::Write) {
        // ANCHOR_END: Label-draw_into
        writeln!(buffer, "{}", &self.label).unwrap();
    }
}

// ---- src/widgets/button.rs ----
use super::{Label, Widget};

pub struct Button {
    label: Label,
}

impl Button {
    pub fn new(label: &str) -> Button {
        Button { label: Label::new(label) }
    }
}

impl Widget for Button {
    fn width(&self) -> usize {
        // ANCHOR_END: Button-width
        self.label.width() + 8 // add a bit of padding
    }

    // ANCHOR: Button-draw_into
    fn draw_into(&self, buffer: &mut dyn std::fmt::Write) {
        // ANCHOR_END: Button-draw_into
        let width = self.width();
        let mut label = String::new();
        self.label.draw_into(&mut label);

        writeln!(buffer, "+{:-<width$}+", "").unwrap();
        for line in label.lines() {
            writeln!(buffer, "|{:^width$}|", &line).unwrap();
        }
        writeln!(buffer, "+{:-<width$}+", "").unwrap();
    }
}

// ---- src/widgets/window.rs ----
use super::Widget;

pub struct Window {
    title: String,
    widgets: Vec<Box<dyn Widget>>,
}

impl Window {
    pub fn new(title: &str) -> Window {
        Window { title: title.to_owned(), widgets: Vec::new() }
    }

    pub fn add_widget(&mut self, widget: Box<dyn Widget>) {
        self.widgets.push(widget);
    }

    fn inner_width(&self) -> usize {
        std::cmp::max(
            self.title.chars().count(),
            self.widgets.iter().map(|w| w.width()).max().unwrap_or(0),
        )
    }
}

impl Widget for Window {
    fn width(&self) -> usize {
        // ANCHOR_END: Window-width
        // Add 4 paddings for borders
        self.inner_width() + 4
    }

    // ANCHOR: Window-draw_into
    fn draw_into(&self, buffer: &mut dyn std::fmt::Write) {
        // ANCHOR_END: Window-draw_into
        let mut inner = String::new();
        for widget in &self.widgets {
            widget.draw_into(&mut inner);
        }

        let inner_width = self.inner_width();

        // TODO: after learning about error handling, you can change
        // draw_into to return Result<(), std::fmt::Error>. Then use
        // the ?-operator here instead of .unwrap().
        writeln!(buffer, "+-{:-<inner_width$}-+", "").unwrap();
        writeln!(buffer, "| {:^inner_width$} |", &self.title).unwrap();
        writeln!(buffer, "+={:=<inner_width$}=+", "").unwrap();
        for line in inner.lines() {
            writeln!(buffer, "| {:inner_width$} |", line).unwrap();
        }
        writeln!(buffer, "+-{:-<inner_width$}-+", "").unwrap();
    }
}

// ---- src/main.rs ----
mod widgets;

use widgets::Widget;

fn main() {
    let mut window = widgets::Window::new("Rust GUI Demo 1.23");
    window
        .add_widget(Box::new(widgets::Label::new("This is a small text GUI demo.")));
    window.add_widget(Box::new(widgets::Button::new("Click me!")));
    window.draw();
}

Testare

This segment should take about 1 hour and 5 minutes. It contains:

Test per unità

Rust and Cargo come with a simple unit test framework:

• Unit tests are supported throughout your code.

• Integration tests are supported via the tests/ directory.

Tests are marked with #[test]. Unit tests are often put in a nested tests module, using #[cfg(test)] to conditionally compile them only when building tests.

fn first_word(text: &str) -> &str {
    match text.find(' ') {
        Some(idx) => &text[..idx],
        None => &text,
    }
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn test_empty() {
        assert_eq!(first_word(""), "");
    }

    #[test]
    fn test_single_word() {
        assert_eq!(first_word("Hello"), "Hello");
    }

    #[test]
    fn test_multiple_words() {
        assert_eq!(first_word("Hello World"), "Hello");
    }
}

• This lets you unit test private helpers.
• The #[cfg(test)] attribute is only active when you run cargo test.

Altri progetti

Test di integrazione

If you want to test your library as a client, use an integration test.

Create a .rs file under tests/:

// tests/my_library.rs
use my_library::init;

#[test]
fn test_init() {
    assert!(init().is_ok());
}

These tests only have access to the public API of your crate.

Test nella documentazione

Rust has built-in support for documentation tests:

#![allow(unused)]
fn main() {
/// Shortens a string to the given length.
///
/// ```
/// # use playground::shorten_string;
/// assert_eq!(shorten_string("Hello World", 5), "Hello");
/// assert_eq!(shorten_string("Hello World", 20), "Hello World");
/// ```
pub fn shorten_string(s: &str, length: usize) -> &str {
    &s[..std::cmp::min(length, s.len())]
}
}

• Code blocks in /// comments are automatically seen as Rust code.
• The code will be compiled and executed as part of cargo test.
• Adding # in the code will hide it from the docs, but will still compile/run it.
• Test the above code on the Rust Playground.

Crates Utili

Rust comes with only basic support for writing tests.

Here are some additional crates which we recommend for writing tests:

• googletest: Comprehensive test assertion library in the tradition of GoogleTest for C++.
• proptest: Property-based testing for Rust.
• rstest: Support for fixtures and parameterised tests.

GoogleTest

The GoogleTest crate allows for flexible test assertions using matchers:

use googletest::prelude::*;

#[googletest::test]
fn test_elements_are() {
    let value = vec!["foo", "bar", "baz"];
    expect_that!(value, elements_are!(eq("foo"), lt("xyz"), starts_with("b")));
}

If we change the last element to "!", the test fails with a structured error message pin-pointing the error:

---- test_elements_are stdout ----
Value of: value
Expected: has elements:
  0. is equal to "foo"
  1. is less than "xyz"
  2. starts with prefix "!"
Actual: ["foo", "bar", "baz"],
  where element #2 is "baz", which does not start with "!"
  at src/testing/googletest.rs:6:5
Error: See failure output above

#[test]
fn test_multiline_string_diff() {
    let haiku = "Memory safety found,\n\
                 Rust's strong typing guides the way,\n\
                 Secure code you'll write.";
    assert_that!(
        haiku,
        eq("Memory safety found,\n\
            Rust's silly humor guides the way,\n\
            Secure code you'll write.")
    );
}

    Value of: haiku
Expected: is equal to "Memory safety found,\nRust's silly humor guides the way,\nSecure code you'll write."
Actual: "Memory safety found,\nRust's strong typing guides the way,\nSecure code you'll write.",
  which isn't equal to "Memory safety found,\nRust's silly humor guides the way,\nSecure code you'll write."
Difference(-actual / +expected):
 Memory safety found,
-Rust's strong typing guides the way,
+Rust's silly humor guides the way,
 Secure code you'll write.
  at src/testing/googletest.rs:17:5

Mocking

For mocking, Mockall is a widely used library. You need to refactor your code to use traits, which you can then quickly mock:

use std::time::Duration;

#[mockall::automock]
pub trait Pet {
    fn is_hungry(&self, since_last_meal: Duration) -> bool;
}

#[test]
fn test_robot_dog() {
    let mut mock_dog = MockPet::new();
    mock_dog.expect_is_hungry().return_const(true);
    assert_eq!(mock_dog.is_hungry(Duration::from_secs(10)), true);
}

#[test]
fn test_robot_cat() {
    let mut mock_cat = MockPet::new();
    mock_cat
        .expect_is_hungry()
        .with(mockall::predicate::gt(Duration::from_secs(3 * 3600)))
        .return_const(true);
    mock_cat.expect_is_hungry().return_const(false);
    assert_eq!(mock_cat.is_hungry(Duration::from_secs(1 * 3600)), false);
    assert_eq!(mock_cat.is_hungry(Duration::from_secs(5 * 3600)), true);
}

Compiler Lints and Clippy

The Rust compiler produces fantastic error messages, as well as helpful built-in lints. Clippy provides even more lints, organized into groups that can be enabled per-project.

#[deny(clippy::cast_possible_truncation)]
fn main() {
    let x = 3;
    while (x < 70000) {
        x *= 2;
    }
    println!("X probably fits in a u16, right? {}", x as u16);
}

Algoritmo di Luhn

Algoritmo di Luhn

The Luhn algorithm is used to validate credit card numbers. The algorithm takes a string as input and does the following to validate the credit card number:

• Ignore all spaces. Reject number with less than two digits.

• Moving from right to left, double every second digit: for the number 1234, we double 3 and 1. For the number 98765, we double 6 and 8.

• After doubling a digit, sum the digits if the result is greater than 9. So doubling 7 becomes 14 which becomes 1 + 4 = 5.

• Somma tutte le cifre non raddoppiate e raddoppiate.

• The credit card number is valid if the sum ends with 0.

The provided code provides a buggy implementation of the luhn algorithm, along with two basic unit tests that confirm that most the algorithm is implemented correctly.

Copy the code below to https://play.rust-lang.org/ and write additional tests to uncover bugs in the provided implementation, fixing any bugs you find.

#![allow(unused)]
fn main() {
pub fn luhn(cc_number: &str) -> bool {
    let mut sum = 0;
    let mut double = false;

    for c in cc_number.chars().rev() {
        if let Some(digit) = c.to_digit(10) {
            if double {
                let double_digit = digit * 2;
                sum +=
                    if double_digit > 9 { double_digit - 9 } else { double_digit };
            } else {
                sum += digit;
            }
            double = !double;
        } else {
            continue;
        }
    }

    sum % 10 == 0
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn test_valid_cc_number() {
        assert!(luhn("4263 9826 4026 9299"));
        assert!(luhn("4539 3195 0343 6467"));
        assert!(luhn("7992 7398 713"));
    }

    #[test]
    fn test_invalid_cc_number() {
        assert!(!luhn("4223 9826 4026 9299"));
        assert!(!luhn("4539 3195 0343 6476"));
        assert!(!luhn("8273 1232 7352 0569"));
    }
}
}

Soluzioni

// This is the buggy version that appears in the problem.
#[cfg(never)]
pub fn luhn(cc_number: &str) -> bool {
    let mut sum = 0;
    let mut double = false;

    for c in cc_number.chars().rev() {
        if let Some(digit) = c.to_digit(10) {
            if double {
                let double_digit = digit * 2;
                sum +=
                    if double_digit > 9 { double_digit - 9 } else { double_digit };
            } else {
                sum += digit;
            }
            double = !double;
        } else {
            continue;
        }
    }

    sum % 10 == 0
}

// This is the solution and passes all of the tests below.
pub fn luhn(cc_number: &str) -> bool {
    let mut sum = 0;
    let mut double = false;
    let mut digits = 0;

    for c in cc_number.chars().rev() {
        if let Some(digit) = c.to_digit(10) {
            digits += 1;
            if double {
                let double_digit = digit * 2;
                sum +=
                    if double_digit > 9 { double_digit - 9 } else { double_digit };
            } else {
                sum += digit;
            }
            double = !double;
        } else if c.is_whitespace() {
            continue;
        } else {
            return false;
        }
    }

    digits >= 2 && sum % 10 == 0
}

fn main() {
    let cc_number = "1234 5678 1234 5670";
    println!(
        "Is {cc_number} a valid credit card number? {}",
        if luhn(cc_number) { "yes" } else { "no" }
    );
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn test_valid_cc_number() {
        assert!(luhn("4263 9826 4026 9299"));
        assert!(luhn("4539 3195 0343 6467"));
        assert!(luhn("7992 7398 713"));
    }

    #[test]
    fn test_invalid_cc_number() {
        assert!(!luhn("4223 9826 4026 9299"));
        assert!(!luhn("4539 3195 0343 6476"));
        assert!(!luhn("8273 1232 7352 0569"));
    }

    #[test]
    fn test_non_digit_cc_number() {
        assert!(!luhn("foo"));
        assert!(!luhn("foo 0 0"));
    }

    #[test]
    fn test_empty_cc_number() {
        assert!(!luhn(""));
        assert!(!luhn(" "));
        assert!(!luhn("  "));
        assert!(!luhn("    "));
    }

    #[test]
    fn test_single_digit_cc_number() {
        assert!(!luhn("0"));
    }

    #[test]
    fn test_two_digit_cc_number() {
        assert!(luhn(" 0 0 "));
    }
}

Welcome Back

Including 10 minute breaks, this session should take about 2 hours. It contains:

Gestione degli errori

This segment should take about 45 minutes. It contains:

Panico (Panics)

Rust handles fatal errors with a "panic".

Rust will trigger a panic if a fatal error happens at runtime:

fn main() {
    let v = vec![10, 20, 30];
    println!("v[100]: {}", v[100]);
}

• Panics are for unrecoverable and unexpected errors.
  • Panics are symptoms of bugs in the program.
  • Runtime failures like failed bounds checks can panic
  • Assertions (such as assert!) panic on failure
  • Purpose-specific panics can use the panic! macro.
• A panic will "unwind" the stack, dropping values just as if the functions had returned.
• Use non-panicking APIs (such as Vec::get) if crashing is not acceptable.

use std::panic;

fn main() {
    let result = panic::catch_unwind(|| "No problem here!");
    println!("{result:?}");

    let result = panic::catch_unwind(|| {
        panic!("oh no!");
    });
    println!("{result:?}");
}

Iteratore (Iterator)

Runtime errors like connection-refused or file-not-found are handled with the Result type, but matching this type on every call can be cumbersome. The try-operator ? is used to return errors to the caller. It lets you turn the common

match some_expression {
    Ok(value) => value,
    Err(err) => return Err(err),
}

into the much simpler

some_expression?

We can use this to simplify our error handling code:

use std::io::Read;
use std::{fs, io};

fn read_username(path: &str) -> Result<String, io::Error> {
    let username_file_result = fs::File::open(path);
    let mut username_file = match username_file_result {
        Ok(file) => file,
        Err(err) => return Err(err),
    };

    let mut username = String::new();
    match username_file.read_to_string(&mut username) {
        Ok(_) => Ok(username),
        Err(err) => Err(err),
    }
}

fn main() {
    //fs::write("config.dat", "alice").unwrap();
    let username = read_username("config.dat");
    println!("username or error: {username:?}");
}

Conversione implicita

The effective expansion of ? is a little more complicated than previously indicated:

expression?

funziona allo stesso modo di

match expression {
    Ok(value) => value,
    Err(err)  => return Err(From::from(err)),
}

The From::from call here means we attempt to convert the error type to the type returned by the function. This makes it easy to encapsulate errors into higher-level errors.

Esempio

use std::error::Error;
use std::fmt::{self, Display, Formatter};
use std::fs::File;
use std::io::{self, Read};

#[derive(Debug)]
enum ReadUsernameError {
    IoError(io::Error),
    EmptyUsername(String),
}

impl Error for ReadUsernameError {}

impl Display for ReadUsernameError {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self {
            Self::IoError(e) => write!(f, "IO error: {e}"),
            Self::EmptyUsername(path) => write!(f, "Found no username in {path}"),
        }
    }
}

impl From<io::Error> for ReadUsernameError {
    fn from(err: io::Error) -> Self {
        Self::IoError(err)
    }
}

fn read_username(path: &str) -> Result<String, ReadUsernameError> {
    let mut username = String::with_capacity(100);
    File::open(path)?.read_to_string(&mut username)?;
    if username.is_empty() {
        return Err(ReadUsernameError::EmptyUsername(String::from(path)));
    }
    Ok(username)
}

fn main() {
    //fs::write("config.dat", "").unwrap();
    let username = read_username("config.dat");
    println!("username or error: {username:?}");
}

Tipi di errori dinamici

Sometimes we want to allow any type of error to be returned without writing our own enum covering all the different possibilities. The std::error::Error trait makes it easy to create a trait object that can contain any error.

use std::error::Error;
use std::fs;
use std::io::Read;

fn read_count(path: &str) -> Result<i32, Box<dyn Error>> {
    let mut count_str = String::new();
    fs::File::open(path)?.read_to_string(&mut count_str)?;
    let count: i32 = count_str.parse()?;
    Ok(count)
}

fn main() {
    fs::write("count.dat", "1i3").unwrap();
    match read_count("count.dat") {
        Ok(count) => println!("Count: {count}"),
        Err(err) => println!("Error: {err}"),
    }
}

thiserror and anyhow

The thiserror and anyhow crates are widely used to simplify error handling.

• thiserror is often used in libraries to create custom error types that implement From<T>.
• anyhow is often used by applications to help with error handling in functions, including adding contextual information to your errors.

use anyhow::{bail, Context, Result};
use std::fs;
use std::io::Read;
use thiserror::Error;

#[derive(Clone, Debug, Eq, Error, PartialEq)]
#[error("Found no username in {0}")]
struct EmptyUsernameError(String);

fn read_username(path: &str) -> Result<String> {
    let mut username = String::with_capacity(100);
    fs::File::open(path)
        .with_context(|| format!("Failed to open {path}"))?
        .read_to_string(&mut username)
        .context("Failed to read")?;
    if username.is_empty() {
        bail!(EmptyUsernameError(path.to_string()));
    }
    Ok(username)
}

fn main() {
    //fs::write("config.dat", "").unwrap();
    match read_username("config.dat") {
        Ok(username) => println!("Username: {username}"),
        Err(err) => println!("Error: {err:?}"),
    }
}

Gestione strutturata degli errori con Result

The following implements a very simple parser for an expression language. However, it handles errors by panicking. Rewrite it to instead use idiomatic error handling and propagate errors to a return from main. Feel free to use thiserror and anyhow.

HINT: start by fixing error handling in the parse function. Once that is working correctly, update Tokenizer to implement Iterator<Item=Result<Token, TokenizerError>> and handle that in the parser.

use std::iter::Peekable;
use std::str::Chars;

/// An arithmetic operator.
#[derive(Debug, PartialEq, Clone, Copy)]
enum Op {
    Add,
    Sub,
}

/// A token in the expression language.
#[derive(Debug, PartialEq)]
enum Token {
    Number(String),
    Identifier(String),
    Operator(Op),
}

/// An expression in the expression language.
#[derive(Debug, PartialEq)]
enum Expression {
    /// A reference to a variable.
    Var(String),
    /// A literal number.
    Number(u32),
    /// A binary operation.
    Operation(Box<Expression>, Op, Box<Expression>),
}

fn tokenize(input: &str) -> Tokenizer {
    return Tokenizer(input.chars().peekable());
}

struct Tokenizer<'a>(Peekable<Chars<'a>>);

impl<'a> Iterator for Tokenizer<'a> {
    type Item = Token;

    fn next(&mut self) -> Option<Token> {
        let c = self.0.next()?;
        match c {
            '0'..='9' => {
                let mut num = String::from(c);
                while let Some(c @ '0'..='9') = self.0.peek() {
                    num.push(*c);
                    self.0.next();
                }
                Some(Token::Number(num))
            }
            'a'..='z' => {
                let mut ident = String::from(c);
                while let Some(c @ ('a'..='z' | '_' | '0'..='9')) = self.0.peek() {
                    ident.push(*c);
                    self.0.next();
                }
                Some(Token::Identifier(ident))
            }
            '+' => Some(Token::Operator(Op::Add)),
            '-' => Some(Token::Operator(Op::Sub)),
            _ => panic!("Unexpected character {c}"),
        }
    }
}

fn parse(input: &str) -> Expression {
    let mut tokens = tokenize(input);

    fn parse_expr<'a>(tokens: &mut Tokenizer<'a>) -> Expression {
        let Some(tok) = tokens.next() else {
            panic!("Unexpected end of input");
        };
        let expr = match tok {
            Token::Number(num) => {
                let v = num.parse().expect("Invalid 32-bit integer'");
                Expression::Number(v)
            }
            Token::Identifier(ident) => Expression::Var(ident),
            Token::Operator(_) => panic!("Unexpected token {tok:?}"),
        };
        // Look ahead to parse a binary operation if present.
        match tokens.next() {
            None => expr,
            Some(Token::Operator(op)) => Expression::Operation(
                Box::new(expr),
                op,
                Box::new(parse_expr(tokens)),
            ),
            Some(tok) => panic!("Unexpected token {tok:?}"),
        }
    }

    parse_expr(&mut tokens)
}

fn main() {
    let expr = parse("10+foo+20-30");
    println!("{expr:?}");
}

Soluzioni

use thiserror::Error;
use std::iter::Peekable;
use std::str::Chars;

/// An arithmetic operator.
#[derive(Debug, PartialEq, Clone, Copy)]
enum Op {
    Add,
    Sub,
}

/// A token in the expression language.
#[derive(Debug, PartialEq)]
enum Token {
    Number(String),
    Identifier(String),
    Operator(Op),
}

/// An expression in the expression language.
#[derive(Debug, PartialEq)]
enum Expression {
    /// A reference to a variable.
    Var(String),
    /// A literal number.
    Number(u32),
    /// A binary operation.
    Operation(Box<Expression>, Op, Box<Expression>),
}

fn tokenize(input: &str) -> Tokenizer {
    return Tokenizer(input.chars().peekable());
}

#[derive(Debug, Error)]
enum TokenizerError {
    #[error("Unexpected character '{0}' in input")]
    UnexpectedCharacter(char),
}

struct Tokenizer<'a>(Peekable<Chars<'a>>);

impl<'a> Iterator for Tokenizer<'a> {
    type Item = Result<Token, TokenizerError>;

    fn next(&mut self) -> Option<Result<Token, TokenizerError>> {
        let c = self.0.next()?;
        match c {
            '0'..='9' => {
                let mut num = String::from(c);
                while let Some(c @ '0'..='9') = self.0.peek() {
                    num.push(*c);
                    self.0.next();
                }
                Some(Ok(Token::Number(num)))
            }
            'a'..='z' => {
                let mut ident = String::from(c);
                while let Some(c @ ('a'..='z' | '_' | '0'..='9')) = self.0.peek() {
                    ident.push(*c);
                    self.0.next();
                }
                Some(Ok(Token::Identifier(ident)))
            }
            '+' => Some(Ok(Token::Operator(Op::Add))),
            '-' => Some(Ok(Token::Operator(Op::Sub))),
            _ => Some(Err(TokenizerError::UnexpectedCharacter(c))),
        }
    }
}

#[derive(Debug, Error)]
enum ParserError {
    #[error("Tokenizer error: {0}")]
    TokenizerError(#[from] TokenizerError),
    #[error("Unexpected end of input")]
    UnexpectedEOF,
    #[error("Unexpected token {0:?}")]
    UnexpectedToken(Token),
    #[error("Invalid number")]
    InvalidNumber(#[from] std::num::ParseIntError),
}

fn parse(input: &str) -> Result<Expression, ParserError> {
    let mut tokens = tokenize(input);

    fn parse_expr<'a>(
        tokens: &mut Tokenizer<'a>,
    ) -> Result<Expression, ParserError> {
        let tok = tokens.next().ok_or(ParserError::UnexpectedEOF)??;
        let expr = match tok {
            Token::Number(num) => {
                let v = num.parse()?;
                Expression::Number(v)
            }
            Token::Identifier(ident) => Expression::Var(ident),
            Token::Operator(_) => return Err(ParserError::UnexpectedToken(tok)),
        };
        // Look ahead to parse a binary operation if present.
        Ok(match tokens.next() {
            None => expr,
            Some(Ok(Token::Operator(op))) => Expression::Operation(
                Box::new(expr),
                op,
                Box::new(parse_expr(tokens)?),
            ),
            Some(Err(e)) => return Err(e.into()),
            Some(Ok(tok)) => return Err(ParserError::UnexpectedToken(tok)),
        })
    }

    parse_expr(&mut tokens)
}

fn main() -> anyhow::Result<()> {
    let expr = parse("10+foo+20-30")?;
    println!("{expr:?}");
    Ok(())
}

Unsafe Rust

This segment should take about 1 hour and 5 minutes. It contains:

Unsafe Rust

The Rust language has two parts:

• Safe Rust: memory safe, no undefined behavior possible.
• Unsafe Rust: can trigger undefined behavior if preconditions are violated.

We saw mostly safe Rust in this course, but it's important to know what Unsafe Rust is.

Unsafe code is usually small and isolated, and its correctness should be carefully documented. It is usually wrapped in a safe abstraction layer.

Unsafe Rust gives you access to five new capabilities:

• Dereference raw pointers.
• Access or modify mutable static variables.
• Access union fields.
• Call unsafe functions, including extern functions.
• Implement unsafe traits.

We will briefly cover unsafe capabilities next. For full details, please see Chapter 19.1 in the Rust Book and the Rustonomicon.

Dereferenziamento dei Puntatori (Pointers) Grezzi (Raw)

Creating pointers is safe, but dereferencing them requires unsafe:

fn main() {
    let mut s = String::from("careful!");

    let r1 = &mut s as *mut String;
    let r2 = r1 as *const String;

    // Safe because r1 and r2 were obtained from references and so are
    // guaranteed to be non-null and properly aligned, the objects underlying
    // the references from which they were obtained are live throughout the
    // whole unsafe block, and they are not accessed either through the
    // references or concurrently through any other pointers.
    unsafe {
        println!("r1 is: {}", *r1);
        *r1 = String::from("uhoh");
        println!("r2 is: {}", *r2);
    }

    // NOT SAFE. DO NOT DO THIS.
    /*
    let r3: &String = unsafe { &*r1 };
    drop(s);
    println!("r3 is: {}", *r3);
    */
}

Variabili Statiche Mutabili

It is safe to read an immutable static variable:

static HELLO_WORLD: &str = "Hello, world!";

fn main() {
    println!("HELLO_WORLD: {HELLO_WORLD}");
}

However, since data races can occur, it is unsafe to read and write mutable static variables:

static mut COUNTER: u32 = 0;

fn add_to_counter(inc: u32) {
    unsafe {
        COUNTER += inc;
    }
}

fn main() {
    add_to_counter(42);

    unsafe {
        println!("COUNTER: {COUNTER}");
    }
}

Unioni

Unions are like enums, but you need to track the active field yourself:

#[repr(C)]
union MyUnion {
    i: u8,
    b: bool,
}

fn main() {
    let u = MyUnion { i: 42 };
    println!("int: {}", unsafe { u.i });
    println!("bool: {}", unsafe { u.b }); // Undefined behavior!
}

Chiamare Funzioni Unsafe

Chiamare Funzioni Unsafe

A function or method can be marked unsafe if it has extra preconditions you must uphold to avoid undefined behaviour:

extern "C" {
    fn abs(input: i32) -> i32;
}

fn main() {
    let emojis = "🗻∈🌏";

    // Safe because the indices are in the correct order, within the bounds of
    // the string slice, and lie on UTF-8 sequence boundaries.
    unsafe {
        println!("emoji: {}", emojis.get_unchecked(0..4));
        println!("emoji: {}", emojis.get_unchecked(4..7));
        println!("emoji: {}", emojis.get_unchecked(7..11));
    }

    println!("char count: {}", count_chars(unsafe { emojis.get_unchecked(0..7) }));

    unsafe {
        // Undefined behavior if abs misbehaves.
        println!("Absolute value of -3 according to C: {}", abs(-3));
    }

    // Not upholding the UTF-8 encoding requirement breaks memory safety!
    // println!("emoji: {}", unsafe { emojis.get_unchecked(0..3) });
    // println!("char count: {}", count_chars(unsafe {
    // emojis.get_unchecked(0..3) }));
}

fn count_chars(s: &str) -> usize {
    s.chars().count()
}

Creare Funzioni Unsafe

You can mark your own functions as unsafe if they require particular conditions to avoid undefined behaviour.

/// Swaps the values pointed to by the given pointers.
///
/// # Safety
///
/// The pointers must be valid and properly aligned.
unsafe fn swap(a: *mut u8, b: *mut u8) {
    let temp = *a;
    *a = *b;
    *b = temp;
}

fn main() {
    let mut a = 42;
    let mut b = 66;

    // Safe because ...
    unsafe {
        swap(&mut a, &mut b);
    }

    println!("a = {}, b = {}", a, b);
}

Implementare Unsafe Traits

Like with functions, you can mark a trait as unsafe if the implementation must guarantee particular conditions to avoid undefined behaviour.

For example, the zerocopy crate has an unsafe trait that looks something like this:

use std::mem::size_of_val;
use std::slice;

/// ...
/// # Safety
/// The type must have a defined representation and no padding.
pub unsafe trait AsBytes {
    fn as_bytes(&self) -> &[u8] {
        unsafe {
            slice::from_raw_parts(
                self as *const Self as *const u8,
                size_of_val(self),
            )
        }
    }
}

// Safe because u32 has a defined representation and no padding.
unsafe impl AsBytes for u32 {}

Safe FFI Wrapper

Rust has great support for calling functions through a foreign function interface (FFI). We will use this to build a safe wrapper for the libc functions you would use from C to read the names of files in a directory.

You will want to consult the manual pages:

• opendir(3)
• readdir(3)
• closedir(3)

You will also want to browse the std::ffi module. There you find a number of string types which you need for the exercise:

You will convert between all these types:

• &str to CString: you need to allocate space for a trailing \0 character,
• CString to *const i8: you need a pointer to call C functions,
• *const i8 to &CStr: you need something which can find the trailing \0 character,
• &CStr to &[u8]: a slice of bytes is the universal interface for "some unknown data",
• &[u8] to &OsStr: &OsStr is a step towards OsString, use OsStrExt to create it,
• &OsStr to OsString: you need to clone the data in &OsStr to be able to return it and call readdir again.

The Nomicon also has a very useful chapter about FFI.

Copy the code below to https://play.rust-lang.org/ and fill in the missing functions and methods:

// TODO: remove this when you're done with your implementation.
#![allow(unused_imports, unused_variables, dead_code)]

mod ffi {
    use std::os::raw::{c_char, c_int};
    #[cfg(not(target_os = "macos"))]
    use std::os::raw::{c_long, c_uchar, c_ulong, c_ushort};

    // Opaque type. See https://doc.rust-lang.org/nomicon/ffi.html.
    #[repr(C)]
    pub struct DIR {
        _data: [u8; 0],
        _marker: core::marker::PhantomData<(*mut u8, core::marker::PhantomPinned)>,
    }

    // Layout according to the Linux man page for readdir(3), where ino_t and
    // off_t are resolved according to the definitions in
    // /usr/include/x86_64-linux-gnu/{sys/types.h, bits/typesizes.h}.
    #[cfg(not(target_os = "macos"))]
    #[repr(C)]
    pub struct dirent {
        pub d_ino: c_ulong,
        pub d_off: c_long,
        pub d_reclen: c_ushort,
        pub d_type: c_uchar,
        pub d_name: [c_char; 256],
    }

    // Layout according to the macOS man page for dir(5).
    #[cfg(all(target_os = "macos"))]
    #[repr(C)]
    pub struct dirent {
        pub d_fileno: u64,
        pub d_seekoff: u64,
        pub d_reclen: u16,
        pub d_namlen: u16,
        pub d_type: u8,
        pub d_name: [c_char; 1024],
    }

    extern "C" {
        pub fn opendir(s: *const c_char) -> *mut DIR;

        #[cfg(not(all(target_os = "macos", target_arch = "x86_64")))]
        pub fn readdir(s: *mut DIR) -> *const dirent;

        // See https://github.com/rust-lang/libc/issues/414 and the section on
        // _DARWIN_FEATURE_64_BIT_INODE in the macOS man page for stat(2).
        //
        // "Platforms that existed before these updates were available" refers
        // to macOS (as opposed to iOS / wearOS / etc.) on Intel and PowerPC.
        #[cfg(all(target_os = "macos", target_arch = "x86_64"))]
        #[link_name = "readdir$INODE64"]
        pub fn readdir(s: *mut DIR) -> *const dirent;

        pub fn closedir(s: *mut DIR) -> c_int;
    }
}

use std::ffi::{CStr, CString, OsStr, OsString};
use std::os::unix::ffi::OsStrExt;

#[derive(Debug)]
struct DirectoryIterator {
    path: CString,
    dir: *mut ffi::DIR,
}

impl DirectoryIterator {
    fn new(path: &str) -> Result<DirectoryIterator, String> {
        // Call opendir and return a Ok value if that worked,
        // otherwise return Err with a message.
        unimplemented!()
    }
}

impl Iterator for DirectoryIterator {
    type Item = OsString;
    fn next(&mut self) -> Option<OsString> {
        // Keep calling readdir until we get a NULL pointer back.
        unimplemented!()
    }
}

impl Drop for DirectoryIterator {
    fn drop(&mut self) {
        // Call closedir as needed.
        unimplemented!()
    }
}

fn main() -> Result<(), String> {
    let iter = DirectoryIterator::new(".")?;
    println!("files: {:#?}", iter.collect::<Vec<_>>());
    Ok(())
}

Soluzioni

mod ffi {
    use std::os::raw::{c_char, c_int};
    #[cfg(not(target_os = "macos"))]
    use std::os::raw::{c_long, c_uchar, c_ulong, c_ushort};

    // Opaque type. See https://doc.rust-lang.org/nomicon/ffi.html.
    #[repr(C)]
    pub struct DIR {
        _data: [u8; 0],
        _marker: core::marker::PhantomData<(*mut u8, core::marker::PhantomPinned)>,
    }

    // Layout according to the Linux man page for readdir(3), where ino_t and
    // off_t are resolved according to the definitions in
    // /usr/include/x86_64-linux-gnu/{sys/types.h, bits/typesizes.h}.
    #[cfg(not(target_os = "macos"))]
    #[repr(C)]
    pub struct dirent {
        pub d_ino: c_ulong,
        pub d_off: c_long,
        pub d_reclen: c_ushort,
        pub d_type: c_uchar,
        pub d_name: [c_char; 256],
    }

    // Layout according to the macOS man page for dir(5).
    #[cfg(all(target_os = "macos"))]
    #[repr(C)]
    pub struct dirent {
        pub d_fileno: u64,
        pub d_seekoff: u64,
        pub d_reclen: u16,
        pub d_namlen: u16,
        pub d_type: u8,
        pub d_name: [c_char; 1024],
    }

    extern "C" {
        pub fn opendir(s: *const c_char) -> *mut DIR;

        #[cfg(not(all(target_os = "macos", target_arch = "x86_64")))]
        pub fn readdir(s: *mut DIR) -> *const dirent;

        // See https://github.com/rust-lang/libc/issues/414 and the section on
        // _DARWIN_FEATURE_64_BIT_INODE in the macOS man page for stat(2).
        //
        // "Platforms that existed before these updates were available" refers
        // to macOS (as opposed to iOS / wearOS / etc.) on Intel and PowerPC.
        #[cfg(all(target_os = "macos", target_arch = "x86_64"))]
        #[link_name = "readdir$INODE64"]
        pub fn readdir(s: *mut DIR) -> *const dirent;

        pub fn closedir(s: *mut DIR) -> c_int;
    }
}

use std::ffi::{CStr, CString, OsStr, OsString};
use std::os::unix::ffi::OsStrExt;

#[derive(Debug)]
struct DirectoryIterator {
    path: CString,
    dir: *mut ffi::DIR,
}

impl DirectoryIterator {
    fn new(path: &str) -> Result<DirectoryIterator, String> {
        // Call opendir and return a Ok value if that worked,
        // otherwise return Err with a message.
        let path =
            CString::new(path).map_err(|err| format!("Invalid path: {err}"))?;
        // SAFETY: path.as_ptr() cannot be NULL.
        let dir = unsafe { ffi::opendir(path.as_ptr()) };
        if dir.is_null() {
            Err(format!("Could not open {:?}", path))
        } else {
            Ok(DirectoryIterator { path, dir })
        }
    }
}

impl Iterator for DirectoryIterator {
    type Item = OsString;
    fn next(&mut self) -> Option<OsString> {
        // Keep calling readdir until we get a NULL pointer back.
        // SAFETY: self.dir is never NULL.
        let dirent = unsafe { ffi::readdir(self.dir) };
        if dirent.is_null() {
            // We have reached the end of the directory.
            return None;
        }
        // SAFETY: dirent is not NULL and dirent.d_name is NUL
        // terminated.
        let d_name = unsafe { CStr::from_ptr((*dirent).d_name.as_ptr()) };
        let os_str = OsStr::from_bytes(d_name.to_bytes());
        Some(os_str.to_owned())
    }
}

impl Drop for DirectoryIterator {
    fn drop(&mut self) {
        // Call closedir as needed.
        if !self.dir.is_null() {
            // SAFETY: self.dir is not NULL.
            if unsafe { ffi::closedir(self.dir) } != 0 {
                panic!("Could not close {:?}", self.path);
            }
        }
    }
}

fn main() -> Result<(), String> {
    let iter = DirectoryIterator::new(".")?;
    println!("files: {:#?}", iter.collect::<Vec<_>>());
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::error::Error;

    #[test]
    fn test_nonexisting_directory() {
        let iter = DirectoryIterator::new("no-such-directory");
        assert!(iter.is_err());
    }

    #[test]
    fn test_empty_directory() -> Result<(), Box<dyn Error>> {
        let tmp = tempfile::TempDir::new()?;
        let iter = DirectoryIterator::new(
            tmp.path().to_str().ok_or("Non UTF-8 character in path")?,
        )?;
        let mut entries = iter.collect::<Vec<_>>();
        entries.sort();
        assert_eq!(entries, &[".", ".."]);
        Ok(())
    }

    #[test]
    fn test_nonempty_directory() -> Result<(), Box<dyn Error>> {
        let tmp = tempfile::TempDir::new()?;
        std::fs::write(tmp.path().join("foo.txt"), "The Foo Diaries\n")?;
        std::fs::write(tmp.path().join("bar.png"), "<PNG>\n")?;
        std::fs::write(tmp.path().join("crab.rs"), "//! Crab\n")?;
        let iter = DirectoryIterator::new(
            tmp.path().to_str().ok_or("Non UTF-8 character in path")?,
        )?;
        let mut entries = iter.collect::<Vec<_>>();
        entries.sort();
        assert_eq!(entries, &[".", "..", "bar.png", "crab.rs", "foo.txt"]);
        Ok(())
    }
}

Benvenuto a Rust in Android

Rust is supported for system software on Android. This means that you can write new services, libraries, drivers or even firmware in Rust (or improve existing code as needed).

We will attempt to call Rust from one of your own projects today. So try to find a little corner of your code base where we can move some lines of code to Rust. The fewer dependencies and "exotic" types the better. Something that parses some raw bytes would be ideal.

Setup

We will be using a Cuttlefish Android Virtual Device to test our code. Make sure you have access to one or create a new one with:

source build/envsetup.sh
lunch aosp_cf_x86_64_phone-trunk_staging-userdebug
acloud create

Please see the Android Developer Codelab for details.

Regole (Rules) di Build

The Android build system (Soong) supports Rust via a number of modules:

We will look at rust_binary and rust_library next.

Rust Binaries

Let us start with a simple application. At the root of an AOSP checkout, create the following files:

hello_rust/Android.bp:

rust_binary {
    name: "hello_rust",
    crate_name: "hello_rust",
    srcs: ["src/main.rs"],
}

hello_rust/src/main.rs:

//! Rust demo.

/// Prints a greeting to standard output.
fn main() {
    println!("Hello from Rust!");
}

You can now build, push, and run the binary:

m hello_rust
adb push "$ANDROID_PRODUCT_OUT/system/bin/hello_rust" /data/local/tmp
adb shell /data/local/tmp/hello_rust

Hello from Rust!

Rust Libraries

You use rust_library to create a new Rust library for Android.

Here we declare a dependency on two libraries:

• libgreeting, which we define below,
• libtextwrap, which is a crate already vendored in external/rust/crates/.

hello_rust/Android.bp:

rust_binary {
    name: "hello_rust_with_dep",
    crate_name: "hello_rust_with_dep",
    srcs: ["src/main.rs"],
    rustlibs: [
        "libgreetings",
        "libtextwrap",
    ],
    prefer_rlib: true, // Need this to avoid dynamic link error.
}

rust_library {
    name: "libgreetings",
    crate_name: "greetings",
    srcs: ["src/lib.rs"],
}

hello_rust/src/main.rs:

//! Rust demo.

use greetings::greeting;
use textwrap::fill;

/// Prints a greeting to standard output.
fn main() {
    println!("{}", fill(&greeting("Bob"), 24));
}

hello_rust/src/lib.rs:

//! Greeting library.

/// Greet `name`.
pub fn greeting(name: &str) -> String {
    format!("Hello {name}, it is very nice to meet you!")
}

You build, push, and run the binary like before:

m hello_rust_with_dep
adb push "$ANDROID_PRODUCT_OUT/system/bin/hello_rust_with_dep" /data/local/tmp
adb shell /data/local/tmp/hello_rust_with_dep

Hello Bob, it is very
nice to meet you!

AIDL

The Android Interface Definition Language (AIDL) is supported in Rust:

• Rust code can call existing AIDL servers,
• You can create new AIDL servers in Rust.

AIDL Interfaces

You declare the API of your service using an AIDL interface:

birthday_service/aidl/com/example/birthdayservice/IBirthdayService.aidl:

package com.example.birthdayservice;

/** Birthday service interface. */
interface IBirthdayService {
    /** Generate a Happy Birthday message. */
    String wishHappyBirthday(String name, int years);
}

birthday_service/aidl/Android.bp:

aidl_interface {
    name: "com.example.birthdayservice",
    srcs: ["com/example/birthdayservice/*.aidl"],
    unstable: true,
    backend: {
        rust: { // Rust is not enabled by default
            enabled: true,
        },
    },
}

Add vendor_available: true if your AIDL file is used by a binary in the vendor partition.

Service Implementation

We can now implement the AIDL service:

birthday_service/src/lib.rs:

//! Implementation of the `IBirthdayService` AIDL interface.
use com_example_birthdayservice::aidl::com::example::birthdayservice::IBirthdayService::IBirthdayService;
use com_example_birthdayservice::binder;

/// The `IBirthdayService` implementation.
pub struct BirthdayService;

impl binder::Interface for BirthdayService {}

impl IBirthdayService for BirthdayService {
    fn wishHappyBirthday(&self, name: &str, years: i32) -> binder::Result<String> {
        Ok(format!("Happy Birthday {name}, congratulations with the {years} years!"))
    }
}

birthday_service/Android.bp:

rust_library {
    name: "libbirthdayservice",
    srcs: ["src/lib.rs"],
    crate_name: "birthdayservice",
    rustlibs: [
        "com.example.birthdayservice-rust",
        "libbinder_rs",
    ],
}

AIDL Server

Finally, we can create a server which exposes the service:

birthday_service/src/server.rs:

//! Birthday service.
use birthdayservice::BirthdayService;
use com_example_birthdayservice::aidl::com::example::birthdayservice::IBirthdayService::BnBirthdayService;
use com_example_birthdayservice::binder;

const SERVICE_IDENTIFIER: &str = "birthdayservice";

/// Entry point for birthday service.
fn main() {
    let birthday_service = BirthdayService;
    let birthday_service_binder = BnBirthdayService::new_binder(
        birthday_service,
        binder::BinderFeatures::default(),
    );
    binder::add_service(SERVICE_IDENTIFIER, birthday_service_binder.as_binder())
        .expect("Failed to register service");
    binder::ProcessState::join_thread_pool()
}

birthday_service/Android.bp:

rust_binary {
    name: "birthday_server",
    crate_name: "birthday_server",
    srcs: ["src/server.rs"],
    rustlibs: [
        "com.example.birthdayservice-rust",
        "libbinder_rs",
        "libbirthdayservice",
    ],
    prefer_rlib: true, // To avoid dynamic link error.
}

Deploy

We can now build, push, and start the service:

m birthday_server
adb push "$ANDROID_PRODUCT_OUT/system/bin/birthday_server" /data/local/tmp
adb root
adb shell /data/local/tmp/birthday_server

In another terminal, check that the service runs:

adb shell service check birthdayservice

Service birthdayservice: found

You can also call the service with service call:

adb shell service call birthdayservice 1 s16 Bob i32 24

Result: Parcel(
  0x00000000: 00000000 00000036 00610048 00700070 '....6...H.a.p.p.'
  0x00000010: 00200079 00690042 00740072 00640068 'y. .B.i.r.t.h.d.'
  0x00000020: 00790061 00420020 0062006f 0020002c 'a.y. .B.o.b.,. .'
  0x00000030: 006f0063 0067006e 00610072 00750074 'c.o.n.g.r.a.t.u.'
  0x00000040: 0061006c 00690074 006e006f 00200073 'l.a.t.i.o.n.s. .'
  0x00000050: 00690077 00680074 00740020 00650068 'w.i.t.h. .t.h.e.'
  0x00000060: 00320020 00200034 00650079 00720061 ' .2.4. .y.e.a.r.'
  0x00000070: 00210073 00000000                   's.!.....        ')

AIDL Client

Finally, we can create a Rust client for our new service.

birthday_service/src/client.rs:

//! Birthday service.
use com_example_birthdayservice::aidl::com::example::birthdayservice::IBirthdayService::IBirthdayService;
use com_example_birthdayservice::binder;

const SERVICE_IDENTIFIER: &str = "birthdayservice";

/// Connect to the BirthdayService.
pub fn connect() -> Result<binder::Strong<dyn IBirthdayService>, binder::StatusCode>
{
    binder::get_interface(SERVICE_IDENTIFIER)
}

/// Call the birthday service.
fn main() -> Result<(), binder::Status> {
    let name = std::env::args().nth(1).unwrap_or_else(|| String::from("Bob"));
    let years = std::env::args()
        .nth(2)
        .and_then(|arg| arg.parse::<i32>().ok())
        .unwrap_or(42);

    binder::ProcessState::start_thread_pool();
    let service = connect().expect("Failed to connect to BirthdayService");
    let msg = service.wishHappyBirthday(&name, years)?;
    println!("{msg}");
    Ok(())
}

birthday_service/Android.bp:

rust_binary {
    name: "birthday_client",
    crate_name: "birthday_client",
    srcs: ["src/client.rs"],
    rustlibs: [
        "com.example.birthdayservice-rust",
        "libbinder_rs",
    ],
    prefer_rlib: true, // To avoid dynamic link error.
}

Notice that the client does not depend on libbirthdayservice.

Build, push, and run the client on your device:

m birthday_client
adb push "$ANDROID_PRODUCT_OUT/system/bin/birthday_client" /data/local/tmp
adb shell /data/local/tmp/birthday_client Charlie 60

Happy Birthday Charlie, congratulations with the 60 years!

Cambiare API

Let us extend the API with more functionality: we want to let clients specify a list of lines for the birthday card:

package com.example.birthdayservice;

/** Birthday service interface. */
interface IBirthdayService {
    /** Generate a Happy Birthday message. */
    String wishHappyBirthday(String name, int years, in String[] text);
}

Logging

You should use the log crate to automatically log to logcat (on-device) or stdout (on-host):

hello_rust_logs/Android.bp:

rust_binary {
    name: "hello_rust_logs",
    crate_name: "hello_rust_logs",
    srcs: ["src/main.rs"],
    rustlibs: [
        "liblog_rust",
        "liblogger",
    ],
    host_supported: true,
}

hello_rust_logs/src/main.rs:

//! Rust logging demo.

use log::{debug, error, info};

/// Logs a greeting.
fn main() {
    logger::init(
        logger::Config::default()
            .with_tag_on_device("rust")
            .with_min_level(log::Level::Trace),
    );
    debug!("Starting program.");
    info!("Things are going fine.");
    error!("Something went wrong!");
}

Build, push, and run the binary on your device:

m hello_rust_logs
adb push "$ANDROID_PRODUCT_OUT/system/bin/hello_rust_logs" /data/local/tmp
adb shell /data/local/tmp/hello_rust_logs

The logs show up in adb logcat:

adb logcat -s rust

09-08 08:38:32.454  2420  2420 D rust: hello_rust_logs: Starting program.
09-08 08:38:32.454  2420  2420 I rust: hello_rust_logs: Things are going fine.
09-08 08:38:32.454  2420  2420 E rust: hello_rust_logs: Something went wrong!

Interoperabilità

Rust has excellent support for interoperability with other languages. This means that you can:

• Call Rust functions from other languages.
• Call functions written in other languages from Rust.

When you call functions in a foreign language we say that you're using a foreign function interface, also known as FFI.

Interoperability with C

Rust has full support for linking object files with a C calling convention. Similarly, you can export Rust functions and call them from C.

You can do it by hand if you want:

extern "C" {
    fn abs(x: i32) -> i32;
}

fn main() {
    let x = -42;
    let abs_x = unsafe { abs(x) };
    println!("{x}, {abs_x}");
}

We already saw this in the Safe FFI Wrapper exercise.

This assumes full knowledge of the target platform. Not recommended for production.

We will look at better options next.

Using Bindgen

The bindgen tool can auto-generate bindings from a C header file.

First create a small C library:

interoperability/bindgen/libbirthday.h:

typedef struct card {
  const char* name;
  int years;
} card;

void print_card(const card* card);

interoperability/bindgen/libbirthday.c:

#include <stdio.h>
#include "libbirthday.h"

void print_card(const card* card) {
  printf("+--------------\n");
  printf("| Happy Birthday %s!\n", card->name);
  printf("| Congratulations with the %i years!\n", card->years);
  printf("+--------------\n");
}

Add this to your Android.bp file:

interoperability/bindgen/Android.bp:

cc_library {
    name: "libbirthday",
    srcs: ["libbirthday.c"],
}

Create a wrapper header file for the library (not strictly needed in this example):

interoperability/bindgen/libbirthday_wrapper.h:

#include "libbirthday.h"

You can now auto-generate the bindings:

interoperability/bindgen/Android.bp:

rust_bindgen {
    name: "libbirthday_bindgen",
    crate_name: "birthday_bindgen",
    wrapper_src: "libbirthday_wrapper.h",
    source_stem: "bindings",
    static_libs: ["libbirthday"],
}

Finally, we can use the bindings in our Rust program:

interoperability/bindgen/Android.bp:

rust_binary {
    name: "print_birthday_card",
    srcs: ["main.rs"],
    rustlibs: ["libbirthday_bindgen"],
}

interoperability/bindgen/main.rs:

//! Bindgen demo.

use birthday_bindgen::{card, print_card};

fn main() {
    let name = std::ffi::CString::new("Peter").unwrap();
    let card = card { name: name.as_ptr(), years: 42 };
    // SAFETY: `print_card` is safe to call with a valid `card` pointer.
    unsafe {
        print_card(&card as *const card);
    }
}

Build, push, and run the binary on your device:

m print_birthday_card
adb push "$ANDROID_PRODUCT_OUT/system/bin/print_birthday_card" /data/local/tmp
adb shell /data/local/tmp/print_birthday_card

Finally, we can run auto-generated tests to ensure the bindings work:

interoperability/bindgen/Android.bp:

rust_test {
    name: "libbirthday_bindgen_test",
    srcs: [":libbirthday_bindgen"],
    crate_name: "libbirthday_bindgen_test",
    test_suites: ["general-tests"],
    auto_gen_config: true,
    clippy_lints: "none", // Generated file, skip linting
    lints: "none",
}

atest libbirthday_bindgen_test

Calling Rust

Exporting Rust functions and types to C is easy:

interoperability/rust/libanalyze/analyze.rs

//! Rust FFI demo.
#![deny(improper_ctypes_definitions)]

use std::os::raw::c_int;

/// Analyze the numbers.
#[no_mangle]
pub extern "C" fn analyze_numbers(x: c_int, y: c_int) {
    if x < y {
        println!("x ({x}) is smallest!");
    } else {
        println!("y ({y}) is probably larger than x ({x})");
    }
}

interoperability/rust/libanalyze/analyze.h

#ifndef ANALYSE_H
#define ANALYSE_H

extern "C" {
void analyze_numbers(int x, int y);
}

#endif

interoperability/rust/libanalyze/Android.bp

rust_ffi {
    name: "libanalyze_ffi",
    crate_name: "analyze_ffi",
    srcs: ["analyze.rs"],
    include_dirs: ["."],
}

We can now call this from a C binary:

interoperability/rust/analyze/main.c

#include "analyze.h"

int main() {
  analyze_numbers(10, 20);
  analyze_numbers(123, 123);
  return 0;
}

interoperability/rust/analyze/Android.bp

cc_binary {
    name: "analyze_numbers",
    srcs: ["main.c"],
    static_libs: ["libanalyze_ffi"],
}

Build, push, and run the binary on your device:

m analyze_numbers
adb push "$ANDROID_PRODUCT_OUT/system/bin/analyze_numbers" /data/local/tmp
adb shell /data/local/tmp/analyze_numbers

Con C++

The CXX crate makes it possible to do safe interoperability between Rust and C++.

The overall approach looks like this:

Moduli (Module) di Test

CXX relies on a description of the function signatures that will be exposed from each language to the other. You provide this description using extern blocks in a Rust module annotated with the #[cxx::bridge] attribute macro.

#[allow(unsafe_op_in_unsafe_fn)]
#[cxx::bridge(namespace = "org::blobstore")]
mod ffi {
    // Shared structs with fields visible to both languages.
    struct BlobMetadata {
        size: usize,
        tags: Vec<String>,
    }

    // Rust types and signatures exposed to C++.
    extern "Rust" {
        type MultiBuf;

        fn next_chunk(buf: &mut MultiBuf) -> &[u8];
    }

    // C++ types and signatures exposed to Rust.
    unsafe extern "C++" {
        include!("include/blobstore.h");

        type BlobstoreClient;

        fn new_blobstore_client() -> UniquePtr<BlobstoreClient>;
        fn put(self: Pin<&mut BlobstoreClient>, parts: &mut MultiBuf) -> u64;
        fn tag(self: Pin<&mut BlobstoreClient>, blobid: u64, tag: &str);
        fn metadata(&self, blobid: u64) -> BlobMetadata;
    }
}

Rust Bridge Declarations

#[cxx::bridge]
mod ffi {
    extern "Rust" {
        type MyType; // Opaque type
        fn foo(&self); // Method on `MyType`
        fn bar() -> Box<MyType>; // Free function
    }
}

struct MyType(i32);

impl MyType {
    fn foo(&self) {
        println!("{}", self.0);
    }
}

fn bar() -> Box<MyType> {
    Box::new(MyType(123))
}

Generated C++

#[cxx::bridge]
mod ffi {
    // Rust types and signatures exposed to C++.
    extern "Rust" {
        type MultiBuf;

        fn next_chunk(buf: &mut MultiBuf) -> &[u8];
    }
}

Results in (roughly) the following C++:

struct MultiBuf final : public ::rust::Opaque {
  ~MultiBuf() = delete;

private:
  friend ::rust::layout;
  struct layout {
    static ::std::size_t size() noexcept;
    static ::std::size_t align() noexcept;
  };
};

::rust::Slice<::std::uint8_t const> next_chunk(::org::blobstore::MultiBuf &buf) noexcept;

C++ Bridge Declarations

#[cxx::bridge]
mod ffi {
    // C++ types and signatures exposed to Rust.
    unsafe extern "C++" {
        include!("include/blobstore.h");

        type BlobstoreClient;

        fn new_blobstore_client() -> UniquePtr<BlobstoreClient>;
        fn put(self: Pin<&mut BlobstoreClient>, parts: &mut MultiBuf) -> u64;
        fn tag(self: Pin<&mut BlobstoreClient>, blobid: u64, tag: &str);
        fn metadata(&self, blobid: u64) -> BlobMetadata;
    }
}

Results in (roughly) the following Rust:

#[repr(C)]
pub struct BlobstoreClient {
    _private: ::cxx::private::Opaque,
}

pub fn new_blobstore_client() -> ::cxx::UniquePtr<BlobstoreClient> {
    extern "C" {
        #[link_name = "org$blobstore$cxxbridge1$new_blobstore_client"]
        fn __new_blobstore_client() -> *mut BlobstoreClient;
    }
    unsafe { ::cxx::UniquePtr::from_raw(__new_blobstore_client()) }
}

impl BlobstoreClient {
    pub fn put(&self, parts: &mut MultiBuf) -> u64 {
        extern "C" {
            #[link_name = "org$blobstore$cxxbridge1$BlobstoreClient$put"]
            fn __put(
                _: &BlobstoreClient,
                parts: *mut ::cxx::core::ffi::c_void,
            ) -> u64;
        }
        unsafe {
            __put(self, parts as *mut MultiBuf as *mut ::cxx::core::ffi::c_void)
        }
    }
}

// ...

Tipi Scalari

#[cxx::bridge]
mod ffi {
    #[derive(Clone, Debug, Hash)]
    struct PlayingCard {
        suit: Suit,
        value: u8,  // A=1, J=11, Q=12, K=13
    }

    enum Suit {
        Clubs,
        Diamonds,
        Hearts,
        Spades,
    }
}

Shared Enums

#[cxx::bridge]
mod ffi {
    enum Suit {
        Clubs,
        Diamonds,
        Hearts,
        Spades,
    }
}

Generated Rust:

#![allow(unused)]
fn main() {
#[derive(Copy, Clone, PartialEq, Eq)]
#[repr(transparent)]
pub struct Suit {
    pub repr: u8,
}

#[allow(non_upper_case_globals)]
impl Suit {
    pub const Clubs: Self = Suit { repr: 0 };
    pub const Diamonds: Self = Suit { repr: 1 };
    pub const Hearts: Self = Suit { repr: 2 };
    pub const Spades: Self = Suit { repr: 3 };
}
}

Generated C++:

enum class Suit : uint8_t {
  Clubs = 0,
  Diamonds = 1,
  Hearts = 2,
  Spades = 3,
};

Gestione degli errori

#[cxx::bridge]
mod ffi {
    extern "Rust" {
        fn fallible(depth: usize) -> Result<String>;
    }
}

fn fallible(depth: usize) -> anyhow::Result<String> {
    if depth == 0 {
        return Err(anyhow::Error::msg("fallible1 requires depth > 0"));
    }

    Ok("Success!".into())
}

Gestione degli errori

#[cxx::bridge]
mod ffi {
    unsafe extern "C++" {
        include!("example/include/example.h");
        fn fallible(depth: usize) -> Result<String>;
    }
}

fn main() {
    if let Err(err) = ffi::fallible(99) {
        eprintln!("Error: {}", err);
        process::exit(1);
    }
}

Additional Types

Building in Android

Create a cc_library_static to build the C++ library, including the CXX generated header and source file.

cc_library_static {
    name: "libcxx_test_cpp",
    srcs: ["cxx_test.cpp"],
    generated_headers: [
        "cxx-bridge-header",
        "libcxx_test_bridge_header"
    ],
    generated_sources: ["libcxx_test_bridge_code"],
}

Building in Android

Create two genrules: One to generate the CXX header, and one to generate the CXX source file. These are then used as inputs to the cc_library_static.

// Generate a C++ header containing the C++ bindings
// to the Rust exported functions in lib.rs.
genrule {
    name: "libcxx_test_bridge_header",
    tools: ["cxxbridge"],
    cmd: "$(location cxxbridge) $(in) --header > $(out)",
    srcs: ["lib.rs"],
    out: ["lib.rs.h"],
}

// Generate the C++ code that Rust calls into.
genrule {
    name: "libcxx_test_bridge_code",
    tools: ["cxxbridge"],
    cmd: "$(location cxxbridge) $(in) > $(out)",
    srcs: ["lib.rs"],
    out: ["lib.rs.cc"],
}

Building in Android

Create a rust_binary that depends on libcxx and your cc_library_static.

rust_binary {
    name: "cxx_test",
    srcs: ["lib.rs"],
    rustlibs: ["libcxx"],
    static_libs: ["libcxx_test_cpp"],
}

Interoperability with Java

Java can load shared objects via Java Native Interface (JNI). The jni crate allows you to create a compatible library.

First, we create a Rust function to export to Java:

interoperability/java/src/lib.rs:

#![allow(unused)]
fn main() {
//! Rust <-> Java FFI demo.

use jni::objects::{JClass, JString};
use jni::sys::jstring;
use jni::JNIEnv;

/// HelloWorld::hello method implementation.
#[no_mangle]
pub extern "system" fn Java_HelloWorld_hello(
    env: JNIEnv,
    _class: JClass,
    name: JString,
) -> jstring {
    let input: String = env.get_string(name).unwrap().into();
    let greeting = format!("Hello, {input}!");
    let output = env.new_string(greeting).unwrap();
    output.into_inner()
}
}

interoperability/java/Android.bp:

rust_ffi_shared {
    name: "libhello_jni",
    crate_name: "hello_jni",
    srcs: ["src/lib.rs"],
    rustlibs: ["libjni"],
}

We then call this function from Java:

interoperability/java/HelloWorld.java:

class HelloWorld {
    private static native String hello(String name);

    static {
        System.loadLibrary("hello_jni");
    }

    public static void main(String[] args) {
        String output = HelloWorld.hello("Alice");
        System.out.println(output);
    }
}

interoperability/java/Android.bp:

java_binary {
    name: "helloworld_jni",
    srcs: ["HelloWorld.java"],
    main_class: "HelloWorld",
    required: ["libhello_jni"],
}

Finally, you can build, sync, and run the binary:

m helloworld_jni
adb sync  # requires adb root && adb remount
adb shell /system/bin/helloworld_jni

Esercizi

This is a group exercise: We will look at one of the projects you work with and try to integrate some Rust into it. Some suggestions:

• Call your AIDL service with a client written in Rust.

• Move a function from your project to Rust and call it.

Welcome to Rust in Chromium

Rust is supported for third-party libraries in Chromium, with first-party glue code to connect between Rust and existing Chromium C++ code.

Today, we'll call into Rust to do something silly with strings. If you've got a corner of the code where you're displaying a UTF8 string to the user, feel free to follow this recipe in your part of the codebase instead of the exact part we talk about.

Setup

Make sure you can build and run Chromium. Any platform and set of build flags is OK, so long as your code is relatively recent (commit position 1223636 onwards, corresponding to November 2023):

gn gen out/Debug
autoninja -C out/Debug chrome
out/Debug/chrome # or on Mac, out/Debug/Chromium.app/Contents/MacOS/Chromium

(A component, debug build is recommended for quickest iteration time. This is the default!)

See How to build Chromium if you aren't already at that point. Be warned: setting up to build Chromium takes time.

It's also recommended that you have Visual Studio code installed.

About the exercises

This part of the course has a series of exercises which build on each other. We'll be doing them spread throughout the course instead of just at the end. If you don't have time to complete a certain part, don't worry: you can catch up in the next slot.

Comparing Chromium and Cargo Ecosystems

Rust community typically uses cargo and libraries from crates.io. Chromium is built using gn and ninja and a curated set of dependencies.

When writing code in Rust, your choices are:

• Use gn and ninja with the help of the templates from //build/rust/*.gni (e.g. rust_static_library that we'll meet later). This uses Chromium's audited toolchain and crates.
• Use cargo, but restrict yourself to Chromium's audited toolchain and crates
• Use cargo, trusting a toolchain and/or crates downloaded from the internet

From here on we'll be focusing on gn and ninja, because this is how Rust code can be built into the Chromium browser. At the same time, Cargo is an important part of the Rust ecosystem and you should keep it in your toolbox.

Mini exercise

Split into small groups and:

• Brainstorm scenarios where cargo may offer an advantage and assess the risk profile of these scenarios.
• Discuss which tools, libraries, and groups of people need to be trusted when using gn and ninja, offline cargo, etc.

Chromium Rust policy

Chromium does not yet allow first-party Rust except in rare cases as approved by Chromium's Area Tech Leads.

Chromium's policy on third party libraries is outlined here - Rust is allowed for third party libraries under various circumstances, including if they're the best option for performance or for security.

Very few Rust libraries directly expose a C/C++ API, so that means that nearly all such libraries will require a small amount of first-party glue code.

First-party Rust glue code for a particular third-party crate should normally be kept in third_party/rust/<crate>/<version>/wrapper.

Because of this, today's course will be heavily focused on:

• Bringing in third-party Rust libraries ("crates")
• Writing glue code to be able to use those crates from Chromium C++.

If this policy changes over time, the course will evolve to keep up.

Build rules

Rust code is usually built using cargo. Chromium builds with gn and ninja for efficiency --- its static rules allow maximum parallelism. Rust is no exception.

Adding Rust code to Chromium

In some existing Chromium BUILD.gn file, declare a rust_static_library:

import("//build/rust/rust_static_library.gni")

rust_static_library("my_rust_lib") {
  crate_root = "lib.rs"
  sources = [ "lib.rs" ]
}

You can also add deps on other Rust targets. Later we'll use this to depend upon third party code.

Including unsafe Rust Code

Unsafe Rust code is forbidden in rust_static_library by default --- it won't compile. If you need unsafe Rust code, add allow_unsafe = true to the gn target. (Later in the course we'll see circumstances where this is necessary.)

import("//build/rust/rust_static_library.gni")

rust_static_library("my_rust_lib") {
  crate_root = "lib.rs"
  sources = [
    "lib.rs",
    "hippopotamus.rs"
  ]
  allow_unsafe = true
}

Depending on Rust Code from Chromium C++

Simply add the above target to the deps of some Chromium C++ target.

import("//build/rust/rust_static_library.gni")

rust_static_library("my_rust_lib") {
  crate_root = "lib.rs"
  sources = [ "lib.rs" ]
}

# or source_set, static_library etc.
component("preexisting_cpp") {
  deps = [ ":my_rust_lib" ]
}

Visual Studio Code

Types are elided in Rust code, which makes a good IDE even more useful than for C++. Visual Studio code works well for Rust in Chromium. To use it,

• Ensure your VSCode has the rust-analyzer extension, not earlier forms of Rust support
• gn gen out/Debug --export-rust-project (or equivalent for your output directory)
• ln -s out/Debug/rust-project.json rust-project.json

Build rules exercise

In your Chromium build, add a new Rust target to //ui/base/BUILD.gn containing:

#![allow(unused)]
fn main() {
#[no_mangle]
pub extern "C" fn hello_from_rust() {
    println!("Hello from Rust!")
}
}

Important: note that no_mangle here is considered a type of unsafety by the Rust compiler, so you'll need to to allow unsafe code in your gn target.

Add this new Rust target as a dependency of //ui/base:base. Declare this function at the top of ui/base/resource/resource_bundle.cc (later, we'll see how this can be automated by bindings generation tools):

extern "C" void hello_from_rust();

Call this function from somewhere in ui/base/resource/resource_bundle.cc - we suggest the top of ResourceBundle::MaybeMangleLocalizedString. Build and run Chromium, and ensure that "Hello from Rust!" is printed lots of times.

If you use VSCode, now set up Rust to work well in VSCode. It will be useful in subsequent exercises. If you've succeeded, you will be able to use right-click "Go to definition" on println!.

Where to find help

• The options available to the rust_static_library gn template
• Information about #[no_mangle]
• Information about extern "C"
• Information about gn's --export-rust-project switch
• How to install rust-analyzer in VSCode

Testare

Rust community typically authors unit tests in a module placed in the same source file as the code being tested. This was covered earlier in the course and looks like this:

#![allow(unused)]
fn main() {
#[cfg(test)]
mod tests {
    #[test]
    fn my_test() {
        todo!()
    }
}
}

In Chromium we place unit tests in a separate source file and we continue to follow this practice for Rust --- this makes tests consistently discoverable and helps to avoid rebuilding .rs files a second time (in the test configuration).

This results in the following options for testing Rust code in Chromium:

• Native Rust tests (i.e. #[test]). Discouraged outside of //third_party/rust.
• gtest tests authored in C++ and exercising Rust via FFI calls. Sufficient when Rust code is just a thin FFI layer and the existing unit tests provide sufficient coverage for the feature.
• gtest tests authored in Rust and using the crate under test through its public API (using pub mod for_testing { ... } if needed). This is the subject of the next few slides.

rust_gtest_interop Library

The rust_gtest_interop library provides a way to:

• Use a Rust function as a gtest testcase (using the #[gtest(...)] attribute)
• Use expect_eq! and similar macros (similar to assert_eq! but not panicking and not terminating the test when the assertion fails).

Example:

use rust_gtest_interop::prelude::*;

#[gtest(MyRustTestSuite, MyAdditionTest)]
fn test_addition() {
    expect_eq!(2 + 2, 4);
}

GN Rules for Rust Tests

The simplest way to build Rust gtest tests is to add them to an existing test binary that already contains tests authored in C++. For example:

test("ui_base_unittests") {
  ...
  sources += [ "my_rust_lib_unittest.rs" ]
  deps += [ ":my_rust_lib" ]
}

Authoring Rust tests in a separate static_library also works, but requires manually declaring the dependency on the support libraries:

rust_static_library("my_rust_lib_unittests") {
  testonly = true
  is_gtest_unittests = true
  crate_root = "my_rust_lib_unittest.rs"
  sources = [ "my_rust_lib_unittest.rs" ]
  deps = [
    ":my_rust_lib",
    "//testing/rust_gtest_interop",
  ]
}

test("ui_base_unittests") {
  ...
  deps += [ ":my_rust_lib_unittests" ]
}

chromium::import! Macro

After adding :my_rust_lib to GN deps, we still need to learn how to import and use my_rust_lib from my_rust_lib_unittest.rs. We haven't provided an explicit crate_name for my_rust_lib so its crate name is computed based on the full target path and name. Fortunately we can avoid working with such an unwieldy name by using the chromium::import! macro from the automatically-imported chromium crate:

chromium::import! {
    "//ui/base:my_rust_lib";
}

use my_rust_lib::my_function_under_test;

Under the covers the macro expands to something similar to:

extern crate ui_sbase_cmy_urust_ulib as my_rust_lib;

use my_rust_lib::my_function_under_test;

More information can be found in the doc comment of the chromium::import macro.

Testing exercise

Time for another exercise!

In your Chromium build:

• Add a testable function next to hello_from_rust. Some suggestions: adding two integers received as arguments, computing the nth Fibonacci number, summing integers in a slice, etc.
• Add a separate ..._unittest.rs file with a test for the new function.
• Add the new tests to BUILD.gn.
• Build the tests, run them, and verify that the new test works.

Interoperabilità con C

The Rust community offers multiple options for C++/Rust interop, with new tools being developed all the time. At the moment, Chromium uses a tool called CXX.

You describe your whole language boundary in an interface definition language (which looks a lot like Rust) and then CXX tools generate declarations for functions and types in both Rust and C++.

See the CXX tutorial for a full example of using this.

Esempi

CXX requires that the whole C++/Rust boundary is declared in cxx::bridge modules inside .rs source code.

#[cxx::bridge]
mod ffi {
    extern "Rust" {
        type MultiBuf;

        fn next_chunk(buf: &mut MultiBuf) -> &[u8];
    }

    unsafe extern "C++" {
        include!("example/include/blobstore.h");

        type BlobstoreClient;

        fn new_blobstore_client() -> UniquePtr<BlobstoreClient>;
        fn put(self: &BlobstoreClient, buf: &mut MultiBuf) -> Result<u64>;
    }
}

// Definitions of Rust types and functions go here

Limitations of CXX

By far the most useful page when using CXX is the type reference.

CXX fundamentally suits cases where:

• Your Rust-C++ interface is sufficiently simple that you can declare all of it.
• You're using only the types natively supported by CXX already, for example std::unique_ptr, std::string, &[u8] etc.

It has many limitations --- for example lack of support for Rust's Option type.

These limitations constrain us to using Rust in Chromium only for well isolated "leaf nodes" rather than for arbitrary Rust-C++ interop. When considering a use-case for Rust in Chromium, a good starting point is to draft the CXX bindings for the language boundary to see if it appears simple enough.

Gestione degli errori

CXX's support for Result<T,E> relies on C++ exceptions, so we can't use that in Chromium. Alternatives:

• The T part of Result<T, E> can be:

  • Returned via out parameters (e.g. via &mut T). This requires that T can be passed across the FFI boundary - for example T has to be:
    • A primitive type (like u32 or usize)
    • A type natively supported by cxx (like UniquePtr<T>) that has a suitable default value to use in a failure case (unlike Box<T>).
  • Retained on the Rust side, and exposed via reference. This may be needed when T is a Rust type, which cannot be passed across the FFI boundary, and cannot be stored in UniquePtr<T>.

• The E part of Result<T, E> can be:

  • Returned as a boolean (e.g. true representing success, and false representing failure)
  • Preserving error details is in theory possible, but so far hasn't been needed in practice.

CXX Error Handling: QR Example

The QR code generator is an example where a boolean is used to communicate success vs failure, and where the successful result can be passed across the FFI boundary:

#[cxx::bridge(namespace = "qr_code_generator")]
mod ffi {
    extern "Rust" {
        fn generate_qr_code_using_rust(
            data: &[u8],
            min_version: i16,
            out_pixels: Pin<&mut CxxVector<u8>>,
            out_qr_size: &mut usize,
        ) -> bool;
    }
}

CXX Error Handling: PNG Example

A prototype of a PNG decoder illustrates what can be done when the successful result cannot be passed across the FFI boundary:

#[cxx::bridge(namespace = "gfx::rust_bindings")]
mod ffi {
    extern "Rust" {
        /// This returns an FFI-friendly equivalent of `Result<PngReader<'a>,
        /// ()>`.
        fn new_png_reader<'a>(input: &'a [u8]) -> Box<ResultOfPngReader<'a>>;

        /// C++ bindings for the `crate::png::ResultOfPngReader` type.
        type ResultOfPngReader<'a>;
        fn is_err(self: &ResultOfPngReader) -> bool;
        fn unwrap_as_mut<'a, 'b>(
            self: &'b mut ResultOfPngReader<'a>,
        ) -> &'b mut PngReader<'a>;

        /// C++ bindings for the `crate::png::PngReader` type.
        type PngReader<'a>;
        fn height(self: &PngReader) -> u32;
        fn width(self: &PngReader) -> u32;
        fn read_rgba8(self: &mut PngReader, output: &mut [u8]) -> bool;
    }
}

Using cxx in Chromium

In Chromium, we define an independent #[cxx::bridge] mod for each leaf-node where we want to use Rust. You'd typically have one for each rust_static_library. Just add

cxx_bindings = [ "my_rust_file.rs" ]
   # list of files containing #[cxx::bridge], not all source files
allow_unsafe = true

to your existing rust_static_library target alongside crate_root and sources.

C++ headers will be generated at a sensible location, so you can just

#include "ui/base/my_rust_file.rs.h"

You will find some utility functions in //base to convert to/from Chromium C++ types to CXX Rust types --- for example SpanToRustSlice.

Exercise: Interoperability with C++

Part one

• In the Rust file you previously created, add a #[cxx::bridge] which specifies a single function, to be called from C++, called hello_from_rust, taking no parameters and returning no value.
• Modify your previous hello_from_rust function to remove extern "C" and #[no_mangle]. This is now just a standard Rust function.
• Modify your gn target to build these bindings.
• In your C++ code, remove the forward-declaration of hello_from_rust. Instead, include the generated header file.
• Build and run!

Part two

It's a good idea to play with CXX a little. It helps you think about how flexible Rust in Chromium actually is.

Some things to try:

• Call back into C++ from Rust. You will need:
  • An additional header file which you can include! from your cxx::bridge. You'll need to declare your C++ function in that new header file.
  • An unsafe block to call such a function, or alternatively specify the unsafe keyword in your #[cxx::bridge] as described here.
  • You may also need to #include "third_party/rust/cxx/v1/crate/include/cxx.h"
• Pass a C++ string from C++ into Rust.
• Pass a reference to a C++ object into Rust.
• Intentionally get the Rust function signatures mismatched from the #[cxx::bridge], and get used to the errors you see.
• Intentionally get the C++ function signatures mismatched from the #[cxx::bridge], and get used to the errors you see.
• Pass a std::unique_ptr of some type from C++ into Rust, so that Rust can own some C++ object.
• Create a Rust object and pass it into C++, so that C++ owns it. (Hint: you need a Box).
• Declare some methods on a C++ type. Call them from Rust.
• Declare some methods on a Rust type. Call them from C++.

Part three

Now you understand the strengths and limitations of CXX interop, think of a couple of use-cases for Rust in Chromium where the interface would be sufficiently simple. Sketch how you might define that interface.

Where to find help

• The cxx binding reference
• The rust_static_library gn template

Adding Third Party Crates

Rust libraries are called "crates" and are found at crates.io. It's very easy for Rust crates to depend upon one another. So they do!

For a Chromium engineer, this has pros and cons:

• All crates use a common build system so we can automate their inclusion into Chromium...
• ... but, crates typically have transitive dependencies, so you will likely have to bring in multiple libraries.

We'll discuss:

• How to put a crate in the Chromium source code tree
• How to make gn build rules for it
• How to audit its source code for sufficient safety.

Configuring the Cargo.toml file to add crates

Chromium has a single set of centrally-managed direct crate dependencies. These are managed through a single Cargo.toml:

[dependencies]
bitflags = "1"
cfg-if = "1"
cxx = "1"
# lots more...

As with any other Cargo.toml, you can specify more details about the dependencies --- most commonly, you'll want to specify the features that you wish to enable in the crate.

When adding a crate to Chromium, you'll often need to provide some extra information in an additional file, gnrt_config.toml, which we'll meet next.

Configuring gnrt_config.toml

Alongside Cargo.toml is gnrt_config.toml. This contains Chromium-specific extensions to crate handling.

If you add a new crate, you should specify at least the group. This is one of:

#   'safe': The library satisfies the rule-of-2 and can be used in any process.
#   'sandbox': The library does not satisfy the rule-of-2 and must be used in
#              a sandboxed process such as the renderer or a utility process.
#   'test': The library is only used in tests.

For instance,

[crate.my-new-crate]
group = 'test' # only used in test code

Depending on the crate source code layout, you may also need to use this file to specify where its LICENSE file(s) can be found.

Later, we'll see some other things you will need to configure in this file to resolve problems.

Downloading Crates

A tool called gnrt knows how to download crates and how to generate BUILD.gn rules.

To start, download the crate you want like this:

cd chromium/src
vpython3 tools/crates/run_gnrt.py -- vendor

Although the gnrt tool is part of the Chromium source code, by running this command you will be downloading and running its dependencies from crates.io. See the earlier section discussing this security decision.

This vendor command may download:

• Your crate
• Direct and transitive dependencies
• New versions of other crates, as required by cargo to resolve the complete set of crates required by Chromium.

Chromium maintains patches for some crates, kept in //third_party/rust/chromium_crates_io/patches. These will be reapplied automatically, but if patching fails you may need to take manual action.

Generating gn Build Rules

Once you've downloaded the crate, generate the BUILD.gn files like this:

vpython3 tools/crates/run_gnrt.py -- gen

Now run git status. You should find:

• At least one new crate source code in third_party/rust/chromium_crates_io/vendor
• At least one new BUILD.gn in third_party/rust/<crate name>/v<major semver version>
• An appropriate README.chromium

The "major semver version" is a Rust "semver" version number.

Take a close look, especially at the things generated in third_party/rust.

Talk a little about semver --- and specifically the way that in Chromium it's to allow multiple incompatible versions of a crate, which is discouraged but sometimes necessary in the Cargo ecosystem.

Resolving Problems

If your build fails, it may be because of a build.rs: programs which do arbitrary things at build time. This is fundamentally at odds with the design of gn and ninja which aim for static, deterministic, build rules to maximize parallelism and repeatability of builds.

Some build.rs actions are automatically supported; others require action:

build script effect	Supported by our gn templates	Work required by you
Checking rustc version to configure features on and off	Si	None
Checking platform or CPU to configure features on and off	Si	None
Generating code	Si	Yes - specify in gnrt_config.toml
Building C/C++	No	Patch around it
Arbitrary other actions	No	Patch around it

Fortunately, most crates don't contain a build script, and fortunately, most build scripts only do the top two actions.

Build Scripts Which Generate Code

If ninja complains about missing files, check the build.rs to see if it writes source code files.

If so, modify gnrt_config.toml to add build-script-outputs to the crate. If this is a transitive dependency, that is, one on which Chromium code should not directly depend, also add allow-first-party-usage=false. There are several examples already in that file:

[crate.unicode-linebreak]
allow-first-party-usage = false
build-script-outputs = ["tables.rs"]

Now rerun gnrt.py -- gen to regenerate BUILD.gn files to inform ninja that this particular output file is input to subsequent build steps.

Build Scripts Which Build C++ or Take Arbitrary Actions

Some crates use the cc crate to build and link C/C++ libraries. Other crates parse C/C++ using bindgen within their build scripts. These actions can't be supported in a Chromium context --- our gn, ninja and LLVM build system is very specific in expressing relationships between build actions.

So, your options are:

• Avoid these crates
• Apply a patch to the crate.

Patches should be kept in third_party/rust/chromium_crates_io/patches/<crate> - see for example the patches against the cxx crate - and will be applied automatically by gnrt each time it upgrades the crate.

Depending on a Crate

Once you've added a third-party crate and generated build rules, depending on a crate is simple. Find your rust_static_library target, and add a dep on the :lib target within your crate.

Specifically,

For instance,

rust_static_library("my_rust_lib") {
  crate_root = "lib.rs"
  sources = [ "lib.rs" ]
  deps = [ "//third_party/rust/example_rust_crate/v1:lib" ]
}

Auditing Third Party Crates

Adding new libraries is subject to Chromium's standard policies, but of course also subject to security review. As you may be bringing in not just a single crate but also transitive dependencies, there may be a lot of code to review. On the other hand, safe Rust code can have limited negative side effects. How should you review it?

Over time Chromium aims to move to a process based around cargo vet.

Meanwhile, for each new crate addition, we are checking for the following:

• Understand why each crate is used. What's the relationship between crates? If the build system for each crate contains a build.rs or procedural macros, work out what they're for. Are they compatible with the way Chromium is normally built?
• Check each crate seems to be reasonably well maintained
• Use cd third-party/rust/chromium_crates_io; cargo audit to check for known vulnerabilities (first you'll need to cargo install cargo-audit, which ironically involves downloading lots of dependencies from the internet2)
• Ensure any unsafe code is good enough for the Rule of Two
• Check for any use of fs or net APIs
• Read all the code at a sufficient level to look for anything out of place that might have been maliciously inserted. (You can't realistically aim for 100% perfection here: there's often just too much code.)

These are just guidelines --- work with reviewers from security@chromium.org to work out the right way to become confident of the crate.

Checking Crates into Chromium Source Code

git status should reveal:

• Crate code in //third_party/rust/chromium_crates_io
• Metadata (BUILD.gn and README.chromium) in //third_party/rust/<crate>/<version>

Please also add an OWNERS file in the latter location.

You should land all this, along with your Cargo.toml and gnrt_config.toml changes, into the Chromium repo.

Important: you need to use git add -f because otherwise .gitignore files may result in some files being skipped.

As you do so, you might find presubmit checks fail because of non-inclusive language. This is because Rust crate data tends to include names of git branches, and many projects still use non-inclusive terminology there. So you may need to run:

infra/update_inclusive_language_presubmit_exempt_dirs.sh > infra/inclusive_language_presubmit_exempt_dirs.txt
git add -p infra/inclusive_language_presubmit_exempt_dirs.txt # add whatever changes are yours

Keeping Crates Up to Date

As the OWNER of any third party Chromium dependency, you are expected to keep it up to date with any security fixes. It is hoped that we will soon automate this for Rust crates, but for now, it's still your responsibility just as it is for any other third party dependency.

Esercizi

Add uwuify to Chromium, turning off the crate's default features. Assume that the crate will be used in shipping Chromium, but won't be used to handle untrustworthy input.

(In the next exercise we'll use uwuify from Chromium, but feel free to skip ahead and do that now if you like. Or, you could create a new rust_executable target which uses uwuify).

Students will need to download lots of transitive dependencies.

The total crates needed are:

• instant,
• lock_api,
• parking_lot,
• parking_lot_core,
• redox_syscall,
• scopeguard,
• smallvec, and
• uwuify.

If students are downloading even more than that, they probably forgot to turn off the default features.

Thanks to Daniel Liu for this crate!

Bringing It Together --- Exercise

In this exercise, you're going to add a whole new Chromium feature, bringing together everything you already learned.

The Brief from Product Management

A community of pixies has been discovered living in a remote rainforest. It's important that we get Chromium for Pixies delivered to them as soon as possible.

The requirement is to translate all Chromium's UI strings into Pixie language.

There's not time to wait for proper translations, but fortunately pixie language is very close to English, and it turns out there's a Rust crate which does the translation.

In fact, you already imported that crate in the previous exercise.

(Obviously, real translations of Chrome require incredible care and diligence. Don't ship this!)

Steps

Modify ResourceBundle::MaybeMangleLocalizedString so that it uwuifies all strings before display. In this special build of Chromium, it should always do this irrespective of the setting of mangle_localized_strings_.

If you've done everything right across all these exercises, congratulations, you should have created Chrome for pixies!

Students will likely need some hints here. Hints include:

• UTF16 vs UTF8. Students should be aware that Rust strings are always UTF8, and will probably decide that it's better to do the conversion on the C++ side using base::UTF16ToUTF8 and back again.
• If students decide to do the conversion on the Rust side, they'll need to consider String::from_utf16, consider error handling, and consider which CXX supported types can transfer a lot of u16s.
• Students may design the C++/Rust boundary in several different ways, e.g. taking and returning strings by value, or taking a mutable reference to a string. If a mutable reference is used, CXX will likely tell the student that they need to use Pin. You may need to explain what Pin does, and then explain why CXX needs it for mutable references to C++ data: the answer is that C++ data can't be moved around like Rust data, because it may contain self-referential pointers.
• The C++ target containing ResourceBundle::MaybeMangleLocalizedString will need to depend on a rust_static_library target. The student probably already did this.
• The rust_static_library target will need to depend on //third_party/rust/uwuify/v0_2:lib.

Soluzioni

Solutions to the Chromium exercises can be found in this series of CLs.

Welcome to Bare Metal Rust

This is a standalone one-day course about bare-metal Rust, aimed at people who are familiar with the basics of Rust (perhaps from completing the Comprehensive Rust course), and ideally also have some experience with bare-metal programming in some other language such as C.

Today we will talk about 'bare-metal' Rust: running Rust code without an OS underneath us. This will be divided into several parts:

• What is no_std Rust?
• Writing firmware for microcontrollers.
• Writing bootloader / kernel code for application processors.
• Some useful crates for bare-metal Rust development.

For the microcontroller part of the course we will use the BBC micro:bit v2 as an example. It's a development board based on the Nordic nRF51822 microcontroller with some LEDs and buttons, an I2C-connected accelerometer and compass, and an on-board SWD debugger.

To get started, install some tools we'll need later. On gLinux or Debian:

sudo apt install gcc-aarch64-linux-gnu gdb-multiarch libudev-dev picocom pkg-config qemu-system-arm
rustup update
rustup target add aarch64-unknown-none thumbv7em-none-eabihf
rustup component add llvm-tools-preview
cargo install cargo-binutils cargo-embed

And give users in the plugdev group access to the micro:bit programmer:

echo 'SUBSYSTEM=="usb", ATTR{idVendor}=="0d28", MODE="0664", GROUP="plugdev"' |\
  sudo tee /etc/udev/rules.d/50-microbit.rules
sudo udevadm control --reload-rules

On MacOS:

xcode-select --install
brew install gdb picocom qemu
brew install --cask gcc-aarch64-embedded
rustup update
rustup target add aarch64-unknown-none thumbv7em-none-eabihf
rustup component add llvm-tools-preview
cargo install cargo-binutils cargo-embed

no_std

core	alloc	std
Slices, &str, CStr NonZeroU8... Option, Result Display, Debug, write!... Iterator panic!, assert_eq!... NonNull and all the usual pointer-related functions Future and async/await fence, AtomicBool, AtomicPtr, AtomicU32... Duration	Box, Cow, Arc, Rc Vec, BinaryHeap, BtreeMap, LinkedList, VecDeque String, CString, format!	Error HashMap Mutex, Condvar, Barrier, Once, RwLock, mpsc File and the rest of fs println!, Read, Write, Stdin, Stdout and the rest of io Path, OsString net Command, Child, ExitCode spawn, sleep and the rest of thread SystemTime, Instant

• HashMap depends on RNG.
• std re-exports the contents of both core and alloc.

A minimal no_std program

#![no_main]
#![no_std]

use core::panic::PanicInfo;

#[panic_handler]
fn panic(_panic: &PanicInfo) -> ! {
    loop {}
}

• This will compile to an empty binary.
• std provides a panic handler; without it we must provide our own.
• It can also be provided by another crate, such as panic-halt.
• Depending on the target, you may need to compile with panic = "abort" to avoid an error about eh_personality.
• Note that there is no main or any other entry point; it's up to you to define your own entry point. This will typically involve a linker script and some assembly code to set things up ready for Rust code to run.

alloc

To use alloc you must implement a global (heap) allocator.

#![no_main]
#![no_std]

extern crate alloc;
extern crate panic_halt as _;

use alloc::string::ToString;
use alloc::vec::Vec;
use buddy_system_allocator::LockedHeap;

#[global_allocator]
static HEAP_ALLOCATOR: LockedHeap<32> = LockedHeap::<32>::new();

static mut HEAP: [u8; 65536] = [0; 65536];

pub fn entry() {
    // Safe because `HEAP` is only used here and `entry` is only called once.
    unsafe {
        // Give the allocator some memory to allocate.
        HEAP_ALLOCATOR.lock().init(HEAP.as_mut_ptr() as usize, HEAP.len());
    }

    // Now we can do things that require heap allocation.
    let mut v = Vec::new();
    v.push("A string".to_string());
}

• buddy_system_allocator is a third-party crate implementing a basic buddy system allocator. Other crates are available, or you can write your own or hook into your existing allocator.
• The const parameter of LockedHeap is the max order of the allocator; i.e. in this case it can allocate regions of up to 2**32 bytes.
• If any crate in your dependency tree depends on alloc then you must have exactly one global allocator defined in your binary. Usually this is done in the top-level binary crate.
• extern crate panic_halt as _ is necessary to ensure that the panic_halt crate is linked in so we get its panic handler.
• This example will build but not run, as it doesn't have an entry point.

Microcontroller

The cortex_m_rt crate provides (among other things) a reset handler for Cortex M microcontrollers.

#![no_main]
#![no_std]

extern crate panic_halt as _;

mod interrupts;

use cortex_m_rt::entry;

#[entry]
fn main() -> ! {
    loop {}
}

Next we'll look at how to access peripherals, with increasing levels of abstraction.

• The cortex_m_rt::entry macro requires that the function have type fn() -> !, because returning to the reset handler doesn't make sense.
• Run the example with cargo embed --bin minimal

Raw MMIO

Most microcontrollers access peripherals via memory-mapped IO. Let's try turning on an LED on our micro:bit:

#![no_main]
#![no_std]

extern crate panic_halt as _;

mod interrupts;

use core::mem::size_of;
use cortex_m_rt::entry;

/// GPIO port 0 peripheral address
const GPIO_P0: usize = 0x5000_0000;

// GPIO peripheral offsets
const PIN_CNF: usize = 0x700;
const OUTSET: usize = 0x508;
const OUTCLR: usize = 0x50c;

// PIN_CNF fields
const DIR_OUTPUT: u32 = 0x1;
const INPUT_DISCONNECT: u32 = 0x1 << 1;
const PULL_DISABLED: u32 = 0x0 << 2;
const DRIVE_S0S1: u32 = 0x0 << 8;
const SENSE_DISABLED: u32 = 0x0 << 16;

#[entry]
fn main() -> ! {
    // Configure GPIO 0 pins 21 and 28 as push-pull outputs.
    let pin_cnf_21 = (GPIO_P0 + PIN_CNF + 21 * size_of::<u32>()) as *mut u32;
    let pin_cnf_28 = (GPIO_P0 + PIN_CNF + 28 * size_of::<u32>()) as *mut u32;
    // Safe because the pointers are to valid peripheral control registers, and
    // no aliases exist.
    unsafe {
        pin_cnf_21.write_volatile(
            DIR_OUTPUT
                | INPUT_DISCONNECT
                | PULL_DISABLED
                | DRIVE_S0S1
                | SENSE_DISABLED,
        );
        pin_cnf_28.write_volatile(
            DIR_OUTPUT
                | INPUT_DISCONNECT
                | PULL_DISABLED
                | DRIVE_S0S1
                | SENSE_DISABLED,
        );
    }

    // Set pin 28 low and pin 21 high to turn the LED on.
    let gpio0_outset = (GPIO_P0 + OUTSET) as *mut u32;
    let gpio0_outclr = (GPIO_P0 + OUTCLR) as *mut u32;
    // Safe because the pointers are to valid peripheral control registers, and
    // no aliases exist.
    unsafe {
        gpio0_outclr.write_volatile(1 << 28);
        gpio0_outset.write_volatile(1 << 21);
    }

    loop {}
}

• GPIO 0 pin 21 is connected to the first column of the LED matrix, and pin 28 to the first row.

Run the example with:

cargo embed --bin mmio

Peripheral Access Crates

svd2rust generates mostly-safe Rust wrappers for memory-mapped peripherals from CMSIS-SVD files.

#![no_main]
#![no_std]

extern crate panic_halt as _;

use cortex_m_rt::entry;
use nrf52833_pac::Peripherals;

#[entry]
fn main() -> ! {
    let p = Peripherals::take().unwrap();
    let gpio0 = p.P0;

    // Configure GPIO 0 pins 21 and 28 as push-pull outputs.
    gpio0.pin_cnf[21].write(|w| {
        w.dir().output();
        w.input().disconnect();
        w.pull().disabled();
        w.drive().s0s1();
        w.sense().disabled();
        w
    });
    gpio0.pin_cnf[28].write(|w| {
        w.dir().output();
        w.input().disconnect();
        w.pull().disabled();
        w.drive().s0s1();
        w.sense().disabled();
        w
    });

    // Set pin 28 low and pin 21 high to turn the LED on.
    gpio0.outclr.write(|w| w.pin28().clear());
    gpio0.outset.write(|w| w.pin21().set());

    loop {}
}

• SVD (System View Description) files are XML files typically provided by silicon vendors which describe the memory map of the device.
  • They are organised by peripheral, register, field and value, with names, descriptions, addresses and so on.
  • SVD files are often buggy and incomplete, so there are various projects which patch the mistakes, add missing details, and publish the generated crates.
• cortex-m-rt provides the vector table, among other things.
• If you cargo install cargo-binutils then you can run cargo objdump --bin pac -- -d --no-show-raw-insn to see the resulting binary.

Run the example with:

cargo embed --bin pac

HAL crates

HAL crates for many microcontrollers provide wrappers around various peripherals. These generally implement traits from embedded-hal.

#![no_main]
#![no_std]

extern crate panic_halt as _;

use cortex_m_rt::entry;
use nrf52833_hal::gpio::{p0, Level};
use nrf52833_hal::pac::Peripherals;
use nrf52833_hal::prelude::*;

#[entry]
fn main() -> ! {
    let p = Peripherals::take().unwrap();

    // Create HAL wrapper for GPIO port 0.
    let gpio0 = p0::Parts::new(p.P0);

    // Configure GPIO 0 pins 21 and 28 as push-pull outputs.
    let mut col1 = gpio0.p0_28.into_push_pull_output(Level::High);
    let mut row1 = gpio0.p0_21.into_push_pull_output(Level::Low);

    // Set pin 28 low and pin 21 high to turn the LED on.
    col1.set_low().unwrap();
    row1.set_high().unwrap();

    loop {}
}

• set_low and set_high are methods on the embedded_hal OutputPin trait.
• HAL crates exist for many Cortex-M and RISC-V devices, including various STM32, GD32, nRF, NXP, MSP430, AVR and PIC microcontrollers.

Run the example with:

cargo embed --bin hal

Board support crates

Board support crates provide a further level of wrapping for a specific board for convenience.

#![no_main]
#![no_std]

extern crate panic_halt as _;

use cortex_m_rt::entry;
use microbit::hal::prelude::*;
use microbit::Board;

#[entry]
fn main() -> ! {
    let mut board = Board::take().unwrap();

    board.display_pins.col1.set_low().unwrap();
    board.display_pins.row1.set_high().unwrap();

    loop {}
}

• In this case the board support crate is just providing more useful names, and a bit of initialisation.
• The crate may also include drivers for some on-board devices outside of the microcontroller itself.
  • microbit-v2 includes a simple driver for the LED matrix.

Run the example with:

cargo embed --bin board_support

The type state pattern

#[entry]
fn main() -> ! {
    let p = Peripherals::take().unwrap();
    let gpio0 = p0::Parts::new(p.P0);

    let pin: P0_01<Disconnected> = gpio0.p0_01;

    // let gpio0_01_again = gpio0.p0_01; // Error, moved.
    let pin_input: P0_01<Input<Floating>> = pin.into_floating_input();
    if pin_input.is_high().unwrap() {
        // ...
    }
    let mut pin_output: P0_01<Output<OpenDrain>> = pin_input
        .into_open_drain_output(OpenDrainConfig::Disconnect0Standard1, Level::Low);
    pin_output.set_high().unwrap();
    // pin_input.is_high(); // Error, moved.

    let _pin2: P0_02<Output<OpenDrain>> = gpio0
        .p0_02
        .into_open_drain_output(OpenDrainConfig::Disconnect0Standard1, Level::Low);
    let _pin3: P0_03<Output<PushPull>> =
        gpio0.p0_03.into_push_pull_output(Level::Low);

    loop {}
}

• Pins don't implement Copy or Clone, so only one instance of each can exist. Once a pin is moved out of the port struct nobody else can take it.
• Changing the configuration of a pin consumes the old pin instance, so you can’t keep use the old instance afterwards.
• The type of a value indicates the state that it is in: e.g. in this case, the configuration state of a GPIO pin. This encodes the state machine into the type system, and ensures that you don't try to use a pin in a certain way without properly configuring it first. Illegal state transitions are caught at compile time.
• You can call is_high on an input pin and set_high on an output pin, but not vice-versa.
• Many HAL crates follow this pattern.

embedded-hal

The embedded-hal crate provides a number of traits covering common microcontroller peripherals.

• GPIO
• ADC
• I2C, SPI, UART, CAN
• RNG
• Timers
• Watchdogs

Other crates then implement drivers in terms of these traits, e.g. an accelerometer driver might need an I2C or SPI bus implementation.

• There are implementations for many microcontrollers, as well as other platforms such as Linux on Raspberry Pi.
• There is work in progress on an async version of embedded-hal, but it isn't stable yet.

probe-rs and cargo-embed

probe-rs is a handy toolset for embedded debugging, like OpenOCD but better integrated.

• SWD (Serial Wire Debug) and JTAG via CMSIS-DAP, ST-Link and J-Link probes
• GDB stub and Microsoft DAP (Debug Adapter Protocol) server
• Cargo integration

cargo-embed is a cargo subcommand to build and flash binaries, log RTT (Real Time Transfers) output and connect GDB. It's configured by an Embed.toml file in your project directory.

• CMSIS-DAP is an Arm standard protocol over USB for an in-circuit debugger to access the CoreSight Debug Access Port of various Arm Cortex processors. It's what the on-board debugger on the BBC micro:bit uses.
• ST-Link is a range of in-circuit debuggers from ST Microelectronics, J-Link is a range from SEGGER.
• The Debug Access Port is usually either a 5-pin JTAG interface or 2-pin Serial Wire Debug.
• probe-rs is a library which you can integrate into your own tools if you want to.
• The Microsoft Debug Adapter Protocol lets VSCode and other IDEs debug code running on any supported microcontroller.
• cargo-embed is a binary built using the probe-rs library.
• RTT (Real Time Transfers) is a mechanism to transfer data between the debug host and the target through a number of ringbuffers.

Debugging

Embed.toml:

[default.general]
chip = "nrf52833_xxAA"

[debug.gdb]
enabled = true

In one terminal under src/bare-metal/microcontrollers/examples/:

cargo embed --bin board_support debug

In another terminal in the same directory:

On gLinux or Debian:

gdb-multiarch target/thumbv7em-none-eabihf/debug/board_support --eval-command="target remote :1337"

On MacOS:

arm-none-eabi-gdb target/thumbv7em-none-eabihf/debug/board_support --eval-command="target remote :1337"

In GDB, try running:

b src/bin/board_support.rs:29
b src/bin/board_support.rs:30
b src/bin/board_support.rs:32
c
c
c

Other projects

• RTIC
  • "Real-Time Interrupt-driven Concurrency"
  • Shared resource management, message passing, task scheduling, timer queue
• Embassy
  • async executors with priorities, timers, networking, USB
• TockOS
  • Security-focused RTOS with preemptive scheduling and Memory Protection Unit support
• Hubris
  • Microkernel RTOS from Oxide Computer Company with memory protection, unprivileged drivers, IPC
• Bindings for FreeRTOS
• Some platforms have std implementations, e.g. esp-idf.

• RTIC can be considered either an RTOS or a concurrency framework.
  • It doesn't include any HALs.
  • It uses the Cortex-M NVIC (Nested Virtual Interrupt Controller) for scheduling rather than a proper kernel.
  • Cortex-M only.
• Google uses TockOS on the Haven microcontroller for Titan security keys.
• FreeRTOS is mostly written in C, but there are Rust bindings for writing applications.

Esercizi

We will read the direction from an I2C compass, and log the readings to a serial port.

After looking at the exercises, you can look at the solutions provided.

Bussola

We will read the direction from an I2C compass, and log the readings to a serial port. If you have time, try displaying it on the LEDs somehow too, or use the buttons somehow.

Hints:

• Check the documentation for the lsm303agr and microbit-v2 crates, as well as the micro:bit hardware.
• The LSM303AGR Inertial Measurement Unit is connected to the internal I2C bus.
• TWI is another name for I2C, so the I2C master peripheral is called TWIM.
• The LSM303AGR driver needs something implementing the embedded_hal::blocking::i2c::WriteRead trait. The microbit::hal::Twim struct implements this.
• You have a microbit::Board struct with fields for the various pins and peripherals.
• You can also look at the nRF52833 datasheet if you want, but it shouldn't be necessary for this exercise.

Download the exercise template and look in the compass directory for the following files.

src/main.rs:

#![no_main]
#![no_std]

extern crate panic_halt as _;

use core::fmt::Write;
use cortex_m_rt::entry;
use microbit::{hal::uarte::{Baudrate, Parity, Uarte}, Board};

#[entry]
fn main() -> ! {
    let board = Board::take().unwrap();

    // Configure serial port.
    let mut serial = Uarte::new(
        board.UARTE0,
        board.uart.into(),
        Parity::EXCLUDED,
        Baudrate::BAUD115200,
    );

    // Use the system timer as a delay provider.
    let mut delay = Delay::new(board.SYST);

    // Set up the I2C controller and Inertial Measurement Unit.
    // TODO

    writeln!(serial, "Ready.").unwrap();

    loop {
        // Read compass data and log it to the serial port.
        // TODO
    }
}

Cargo.toml (you shouldn't need to change this):

[workspace]

[package]
name = "compass"
version = "0.1.0"
edition = "2021"
publish = false

[dependencies]
cortex-m-rt = "0.7.3"
embedded-hal = "1.0.0"
lsm303agr = "0.3.0"
microbit-v2 = "0.13.0"
panic-halt = "0.2.0"

Embed.toml (you shouldn't need to change this):

[default.general]
chip = "nrf52833_xxAA"

[debug.gdb]
enabled = true

[debug.reset]
halt_afterwards = true

.cargo/config.toml (you shouldn't need to change this):

[build]
target = "thumbv7em-none-eabihf" # Cortex-M4F

[target.'cfg(all(target_arch = "arm", target_os = "none"))']
rustflags = ["-C", "link-arg=-Tlink.x"]

See the serial output on Linux with:

picocom --baud 115200 --imap lfcrlf /dev/ttyACM0

Or on Mac OS something like (the device name may be slightly different):

picocom --baud 115200 --imap lfcrlf /dev/tty.usbmodem14502

Use Ctrl+A Ctrl+Q to quit picocom.

Bare Metal Rust Morning Exercise

Bussola

(back to exercise)

#![no_main]
#![no_std]

extern crate panic_halt as _;

use core::fmt::Write;
use cortex_m_rt::entry;
use core::cmp::{max, min};
use lsm303agr::{
    AccelMode, AccelOutputDataRate, Lsm303agr, MagMode, MagOutputDataRate,
};
use microbit::display::blocking::Display;
use microbit::hal::prelude::*;
use microbit::hal::twim::Twim;
use microbit::hal::uarte::{Baudrate, Parity, Uarte};
use microbit::hal::{Delay, Timer};
use microbit::pac::twim0::frequency::FREQUENCY_A;
use microbit::Board;

const COMPASS_SCALE: i32 = 30000;
const ACCELEROMETER_SCALE: i32 = 700;

#[entry]
fn main() -> ! {
    let board = Board::take().unwrap();

    // Configure serial port.
    let mut serial = Uarte::new(
        board.UARTE0,
        board.uart.into(),
        Parity::EXCLUDED,
        Baudrate::BAUD115200,
    );

    // Use the system timer as a delay provider.
    let mut delay = Delay::new(board.SYST);

    // Set up the I2C controller and Inertial Measurement Unit.
    writeln!(serial, "Setting up IMU...").unwrap();
    let i2c = Twim::new(board.TWIM0, board.i2c_internal.into(), FREQUENCY_A::K100);
    let mut imu = Lsm303agr::new_with_i2c(i2c);
    imu.init().unwrap();
    imu.set_mag_mode_and_odr(
        &mut delay,
        MagMode::HighResolution,
        MagOutputDataRate::Hz50,
    )
    .unwrap();
    imu.set_accel_mode_and_odr(
        &mut delay,
        AccelMode::Normal,
        AccelOutputDataRate::Hz50,
    )
    .unwrap();
    let mut imu = imu.into_mag_continuous().ok().unwrap();

    // Set up display and timer.
    let mut timer = Timer::new(board.TIMER0);
    let mut display = Display::new(board.display_pins);

    let mut mode = Mode::Compass;
    let mut button_pressed = false;

    writeln!(serial, "Ready.").unwrap();

    loop {
        // Read compass data and log it to the serial port.
        while !(imu.mag_status().unwrap().xyz_new_data()
            && imu.accel_status().unwrap().xyz_new_data())
        {}
        let compass_reading = imu.magnetic_field().unwrap();
        let accelerometer_reading = imu.acceleration().unwrap();
        writeln!(
            serial,
            "{},{},{}\t{},{},{}",
            compass_reading.x_nt(),
            compass_reading.y_nt(),
            compass_reading.z_nt(),
            accelerometer_reading.x_mg(),
            accelerometer_reading.y_mg(),
            accelerometer_reading.z_mg(),
        )
        .unwrap();

        let mut image = [[0; 5]; 5];
        let (x, y) = match mode {
            Mode::Compass => (
                scale(-compass_reading.x_nt(), -COMPASS_SCALE, COMPASS_SCALE, 0, 4)
                    as usize,
                scale(compass_reading.y_nt(), -COMPASS_SCALE, COMPASS_SCALE, 0, 4)
                    as usize,
            ),
            Mode::Accelerometer => (
                scale(
                    accelerometer_reading.x_mg(),
                    -ACCELEROMETER_SCALE,
                    ACCELEROMETER_SCALE,
                    0,
                    4,
                ) as usize,
                scale(
                    -accelerometer_reading.y_mg(),
                    -ACCELEROMETER_SCALE,
                    ACCELEROMETER_SCALE,
                    0,
                    4,
                ) as usize,
            ),
        };
        image[y][x] = 255;
        display.show(&mut timer, image, 100);

        // If button A is pressed, switch to the next mode and briefly blink all LEDs
        // on.
        if board.buttons.button_a.is_low().unwrap() {
            if !button_pressed {
                mode = mode.next();
                display.show(&mut timer, [[255; 5]; 5], 200);
            }
            button_pressed = true;
        } else {
            button_pressed = false;
        }
    }
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
enum Mode {
    Compass,
    Accelerometer,
}

impl Mode {
    fn next(self) -> Self {
        match self {
            Self::Compass => Self::Accelerometer,
            Self::Accelerometer => Self::Compass,
        }
    }
}

fn scale(value: i32, min_in: i32, max_in: i32, min_out: i32, max_out: i32) -> i32 {
    let range_in = max_in - min_in;
    let range_out = max_out - min_out;
    cap(min_out + range_out * (value - min_in) / range_in, min_out, max_out)
}

fn cap(value: i32, min_value: i32, max_value: i32) -> i32 {
    max(min_value, min(value, max_value))
}

Application processors

So far we've talked about microcontrollers, such as the Arm Cortex-M series. Now let's try writing something for Cortex-A. For simplicity we'll just work with QEMU's aarch64 'virt' board.

• Broadly speaking, microcontrollers don't have an MMU or multiple levels of privilege (exception levels on Arm CPUs, rings on x86), while application processors do.
• QEMU supports emulating various different machines or board models for each architecture. The 'virt' board doesn't correspond to any particular real hardware, but is designed purely for virtual machines.

Preparativi per lavorare con Rust

Before we can start running Rust code, we need to do some initialisation.

.section .init.entry, "ax"
.global entry
entry:
    /*
     * Load and apply the memory management configuration, ready to enable MMU and
     * caches.
     */
    adrp x30, idmap
    msr ttbr0_el1, x30

    mov_i x30, .Lmairval
    msr mair_el1, x30

    mov_i x30, .Ltcrval
    /* Copy the supported PA range into TCR_EL1.IPS. */
    mrs x29, id_aa64mmfr0_el1
    bfi x30, x29, #32, #4

    msr tcr_el1, x30

    mov_i x30, .Lsctlrval

    /*
     * Ensure everything before this point has completed, then invalidate any
     * potentially stale local TLB entries before they start being used.
     */
    isb
    tlbi vmalle1
    ic iallu
    dsb nsh
    isb

    /*
     * Configure sctlr_el1 to enable MMU and cache and don't proceed until this
     * has completed.
     */
    msr sctlr_el1, x30
    isb

    /* Disable trapping floating point access in EL1. */
    mrs x30, cpacr_el1
    orr x30, x30, #(0x3 << 20)
    msr cpacr_el1, x30
    isb

    /* Zero out the bss section. */
    adr_l x29, bss_begin
    adr_l x30, bss_end
0:  cmp x29, x30
    b.hs 1f
    stp xzr, xzr, [x29], #16
    b 0b

1:  /* Prepare the stack. */
    adr_l x30, boot_stack_end
    mov sp, x30

    /* Set up exception vector. */
    adr x30, vector_table_el1
    msr vbar_el1, x30

    /* Call into Rust code. */
    bl main

    /* Loop forever waiting for interrupts. */
2:  wfi
    b 2b

• This is the same as it would be for C: initialising the processor state, zeroing the BSS, and setting up the stack pointer.
  • The BSS (block starting symbol, for historical reasons) is the part of the object file which containing statically allocated variables which are initialised to zero. They are omitted from the image, to avoid wasting space on zeroes. The compiler assumes that the loader will take care of zeroing them.
• The BSS may already be zeroed, depending on how memory is initialised and the image is loaded, but we zero it to be sure.
• We need to enable the MMU and cache before reading or writing any memory. If we don't:
  • Unaligned accesses will fault. We build the Rust code for the aarch64-unknown-none target which sets +strict-align to prevent the compiler generating unaligned accesses, so it should be fine in this case, but this is not necessarily the case in general.
  • If it were running in a VM, this can lead to cache coherency issues. The problem is that the VM is accessing memory directly with the cache disabled, while the host has cacheable aliases to the same memory. Even if the host doesn't explicitly access the memory, speculative accesses can lead to cache fills, and then changes from one or the other will get lost when the cache is cleaned or the VM enables the cache. (Cache is keyed by physical address, not VA or IPA.)
• For simplicity, we just use a hardcoded pagetable (see idmap.S) which identity maps the first 1 GiB of address space for devices, the next 1 GiB for DRAM, and another 1 GiB higher up for more devices. This matches the memory layout that QEMU uses.
• We also set up the exception vector (vbar_el1), which we'll see more about later.
• All examples this afternoon assume we will be running at exception level 1 (EL1). If you need to run at a different exception level you'll need to modify entry.S accordingly.

Inline assembly

Sometimes we need to use assembly to do things that aren't possible with Rust code. For example, to make an HVC (hypervisor call) to tell the firmware to power off the system:

#![no_main]
#![no_std]

use core::arch::asm;
use core::panic::PanicInfo;

mod exceptions;

const PSCI_SYSTEM_OFF: u32 = 0x84000008;

#[no_mangle]
extern "C" fn main(_x0: u64, _x1: u64, _x2: u64, _x3: u64) {
    // Safe because this only uses the declared registers and doesn't do
    // anything with memory.
    unsafe {
        asm!("hvc #0",
            inout("w0") PSCI_SYSTEM_OFF => _,
            inout("w1") 0 => _,
            inout("w2") 0 => _,
            inout("w3") 0 => _,
            inout("w4") 0 => _,
            inout("w5") 0 => _,
            inout("w6") 0 => _,
            inout("w7") 0 => _,
            options(nomem, nostack)
        );
    }

    loop {}
}

(If you actually want to do this, use the smccc crate which has wrappers for all these functions.)

• PSCI is the Arm Power State Coordination Interface, a standard set of functions to manage system and CPU power states, among other things. It is implemented by EL3 firmware and hypervisors on many systems.
• The 0 => _ syntax means initialise the register to 0 before running the inline assembly code, and ignore its contents afterwards. We need to use inout rather than in because the call could potentially clobber the contents of the registers.
• This main function needs to be #[no_mangle] and extern "C" because it is called from our entry point in entry.S.
• _x0–_x3 are the values of registers x0–x3, which are conventionally used by the bootloader to pass things like a pointer to the device tree. According to the standard aarch64 calling convention (which is what extern "C" specifies to use), registers x0–x7 are used for the first 8 arguments passed to a function, so entry.S doesn't need to do anything special except make sure it doesn't change these registers.
• Run the example in QEMU with make qemu_psci under src/bare-metal/aps/examples.

Volatile memory access for MMIO

• Use pointer::read_volatile and pointer::write_volatile.
• Never hold a reference.
• addr_of! lets you get fields of structs without creating an intermediate reference.

• Volatile access: read or write operations may have side-effects, so prevent the compiler or hardware from reordering, duplicating or eliding them.
  • Usually if you write and then read, e.g. via a mutable reference, the compiler may assume that the value read is the same as the value just written, and not bother actually reading memory.
• Some existing crates for volatile access to hardware do hold references, but this is unsound. Whenever a reference exist, the compiler may choose to dereference it.
• Use the addr_of! macro to get struct field pointers from a pointer to the struct.

Let's write a UART driver

The QEMU 'virt' machine has a PL011 UART, so let's write a driver for that.

const FLAG_REGISTER_OFFSET: usize = 0x18;
const FR_BUSY: u8 = 1 << 3;
const FR_TXFF: u8 = 1 << 5;

/// Minimal driver for a PL011 UART.
#[derive(Debug)]
pub struct Uart {
    base_address: *mut u8,
}

impl Uart {
    /// Constructs a new instance of the UART driver for a PL011 device at the
    /// given base address.
    ///
    /// # Safety
    ///
    /// The given base address must point to the 8 MMIO control registers of a
    /// PL011 device, which must be mapped into the address space of the process
    /// as device memory and not have any other aliases.
    pub unsafe fn new(base_address: *mut u8) -> Self {
        Self { base_address }
    }

    /// Writes a single byte to the UART.
    pub fn write_byte(&self, byte: u8) {
        // Wait until there is room in the TX buffer.
        while self.read_flag_register() & FR_TXFF != 0 {}

        // Safe because we know that the base address points to the control
        // registers of a PL011 device which is appropriately mapped.
        unsafe {
            // Write to the TX buffer.
            self.base_address.write_volatile(byte);
        }

        // Wait until the UART is no longer busy.
        while self.read_flag_register() & FR_BUSY != 0 {}
    }

    fn read_flag_register(&self) -> u8 {
        // Safe because we know that the base address points to the control
        // registers of a PL011 device which is appropriately mapped.
        unsafe { self.base_address.add(FLAG_REGISTER_OFFSET).read_volatile() }
    }
}

• Note that Uart::new is unsafe while the other methods are safe. This is because as long as the caller of Uart::new guarantees that its safety requirements are met (i.e. that there is only ever one instance of the driver for a given UART, and nothing else aliasing its address space), then it is always safe to call write_byte later because we can assume the necessary preconditions.
• We could have done it the other way around (making new safe but write_byte unsafe), but that would be much less convenient to use as every place that calls write_byte would need to reason about the safety
• This is a common pattern for writing safe wrappers of unsafe code: moving the burden of proof for soundness from a large number of places to a smaller number of places.

More traits

We derived the Debug trait. It would be useful to implement a few more traits too.

use core::fmt::{self, Write};

impl Write for Uart {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        for c in s.as_bytes() {
            self.write_byte(*c);
        }
        Ok(())
    }
}

// Safe because it just contains a pointer to device memory, which can be
// accessed from any context.
unsafe impl Send for Uart {}

• Implementing Write lets us use the write! and writeln! macros with our Uart type.
• Run the example in QEMU with make qemu_minimal under src/bare-metal/aps/examples.

A better UART driver

The PL011 actually has a bunch more registers, and adding offsets to construct pointers to access them is error-prone and hard to read. Plus, some of them are bit fields which would be nice to access in a structured way.

Offset	Register name	Width
0x00	DR	12
0x04	RSR	4
0x18	FR	9
0x20	ILPR	8
0x24	IBRD	16
0x28	FBRD	6
0x2c	LCR_H	8
0x30	CR	16
0x34	IFLS	6
0x38	IMSC	11
0x3c	RIS	11
0x40	MIS	11
0x44	ICR	11
0x48	DMACR	3

• There are also some ID registers which have been omitted for brevity.

Bitflag

The bitflags crate is useful for working with bitflags.

use bitflags::bitflags;

bitflags! {
    /// Flags from the UART flag register.
    #[repr(transparent)]
    #[derive(Copy, Clone, Debug, Eq, PartialEq)]
    struct Flags: u16 {
        /// Clear to send.
        const CTS = 1 << 0;
        /// Data set ready.
        const DSR = 1 << 1;
        /// Data carrier detect.
        const DCD = 1 << 2;
        /// UART busy transmitting data.
        const BUSY = 1 << 3;
        /// Receive FIFO is empty.
        const RXFE = 1 << 4;
        /// Transmit FIFO is full.
        const TXFF = 1 << 5;
        /// Receive FIFO is full.
        const RXFF = 1 << 6;
        /// Transmit FIFO is empty.
        const TXFE = 1 << 7;
        /// Ring indicator.
        const RI = 1 << 8;
    }
}

• The bitflags! macro creates a newtype something like Flags(u16), along with a bunch of method implementations to get and set flags.

Multiple registers

We can use a struct to represent the memory layout of the UART's registers.

#[repr(C, align(4))]
struct Registers {
    dr: u16,
    _reserved0: [u8; 2],
    rsr: ReceiveStatus,
    _reserved1: [u8; 19],
    fr: Flags,
    _reserved2: [u8; 6],
    ilpr: u8,
    _reserved3: [u8; 3],
    ibrd: u16,
    _reserved4: [u8; 2],
    fbrd: u8,
    _reserved5: [u8; 3],
    lcr_h: u8,
    _reserved6: [u8; 3],
    cr: u16,
    _reserved7: [u8; 3],
    ifls: u8,
    _reserved8: [u8; 3],
    imsc: u16,
    _reserved9: [u8; 2],
    ris: u16,
    _reserved10: [u8; 2],
    mis: u16,
    _reserved11: [u8; 2],
    icr: u16,
    _reserved12: [u8; 2],
    dmacr: u8,
    _reserved13: [u8; 3],
}

• #[repr(C)] tells the compiler to lay the struct fields out in order, following the same rules as C. This is necessary for our struct to have a predictable layout, as default Rust representation allows the compiler to (among other things) reorder fields however it sees fit.

Driver

Now let's use the new Registers struct in our driver.

/// Driver for a PL011 UART.
#[derive(Debug)]
pub struct Uart {
    registers: *mut Registers,
}

impl Uart {
    /// Constructs a new instance of the UART driver for a PL011 device at the
    /// given base address.
    ///
    /// # Safety
    ///
    /// The given base address must point to the 8 MMIO control registers of a
    /// PL011 device, which must be mapped into the address space of the process
    /// as device memory and not have any other aliases.
    pub unsafe fn new(base_address: *mut u32) -> Self {
        Self { registers: base_address as *mut Registers }
    }

    /// Writes a single byte to the UART.
    pub fn write_byte(&self, byte: u8) {
        // Wait until there is room in the TX buffer.
        while self.read_flag_register().contains(Flags::TXFF) {}

        // Safe because we know that self.registers points to the control
        // registers of a PL011 device which is appropriately mapped.
        unsafe {
            // Write to the TX buffer.
            addr_of_mut!((*self.registers).dr).write_volatile(byte.into());
        }

        // Wait until the UART is no longer busy.
        while self.read_flag_register().contains(Flags::BUSY) {}
    }

    /// Reads and returns a pending byte, or `None` if nothing has been
    /// received.
    pub fn read_byte(&self) -> Option<u8> {
        if self.read_flag_register().contains(Flags::RXFE) {
            None
        } else {
            let data = unsafe { addr_of!((*self.registers).dr).read_volatile() };
            // TODO: Check for error conditions in bits 8-11.
            Some(data as u8)
        }
    }

    fn read_flag_register(&self) -> Flags {
        // Safe because we know that self.registers points to the control
        // registers of a PL011 device which is appropriately mapped.
        unsafe { addr_of!((*self.registers).fr).read_volatile() }
    }
}

• Note the use of addr_of! / addr_of_mut! to get pointers to individual fields without creating an intermediate reference, which would be unsound.

Using it

Let's write a small program using our driver to write to the serial console, and echo incoming bytes.

#![no_main]
#![no_std]

mod exceptions;
mod pl011;

use crate::pl011::Uart;
use core::fmt::Write;
use core::panic::PanicInfo;
use log::error;
use smccc::psci::system_off;
use smccc::Hvc;

/// Base address of the primary PL011 UART.
const PL011_BASE_ADDRESS: *mut u32 = 0x900_0000 as _;

#[no_mangle]
extern "C" fn main(x0: u64, x1: u64, x2: u64, x3: u64) {
    // Safe because `PL011_BASE_ADDRESS` is the base address of a PL011 device,
    // and nothing else accesses that address range.
    let mut uart = unsafe { Uart::new(PL011_BASE_ADDRESS) };

    writeln!(uart, "main({x0:#x}, {x1:#x}, {x2:#x}, {x3:#x})").unwrap();

    loop {
        if let Some(byte) = uart.read_byte() {
            uart.write_byte(byte);
            match byte {
                b'\r' => {
                    uart.write_byte(b'\n');
                }
                b'q' => break,
                _ => {}
            }
        }
    }

    writeln!(uart, "Bye!").unwrap();
    system_off::<Hvc>().unwrap();
}

• As in the inline assembly example, this main function is called from our entry point code in entry.S. See the speaker notes there for details.
• Run the example in QEMU with make qemu under src/bare-metal/aps/examples.

Logging

It would be nice to be able to use the logging macros from the log crate. We can do this by implementing the Log trait.

use crate::pl011::Uart;
use core::fmt::Write;
use log::{LevelFilter, Log, Metadata, Record, SetLoggerError};
use spin::mutex::SpinMutex;

static LOGGER: Logger = Logger { uart: SpinMutex::new(None) };

struct Logger {
    uart: SpinMutex<Option<Uart>>,
}

impl Log for Logger {
    fn enabled(&self, _metadata: &Metadata) -> bool {
        true
    }

    fn log(&self, record: &Record) {
        writeln!(
            self.uart.lock().as_mut().unwrap(),
            "[{}] {}",
            record.level(),
            record.args()
        )
        .unwrap();
    }

    fn flush(&self) {}
}

/// Initialises UART logger.
pub fn init(uart: Uart, max_level: LevelFilter) -> Result<(), SetLoggerError> {
    LOGGER.uart.lock().replace(uart);

    log::set_logger(&LOGGER)?;
    log::set_max_level(max_level);
    Ok(())
}

• The unwrap in log is safe because we initialise LOGGER before calling set_logger.

Using it

We need to initialise the logger before we use it.

#![no_main]
#![no_std]

mod exceptions;
mod logger;
mod pl011;

use crate::pl011::Uart;
use core::panic::PanicInfo;
use log::{error, info, LevelFilter};
use smccc::psci::system_off;
use smccc::Hvc;

/// Base address of the primary PL011 UART.
const PL011_BASE_ADDRESS: *mut u32 = 0x900_0000 as _;

#[no_mangle]
extern "C" fn main(x0: u64, x1: u64, x2: u64, x3: u64) {
    // Safe because `PL011_BASE_ADDRESS` is the base address of a PL011 device,
    // and nothing else accesses that address range.
    let uart = unsafe { Uart::new(PL011_BASE_ADDRESS) };
    logger::init(uart, LevelFilter::Trace).unwrap();

    info!("main({x0:#x}, {x1:#x}, {x2:#x}, {x3:#x})");

    assert_eq!(x1, 42);

    system_off::<Hvc>().unwrap();
}

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    error!("{info}");
    system_off::<Hvc>().unwrap();
    loop {}
}

• Note that our panic handler can now log details of panics.
• Run the example in QEMU with make qemu_logger under src/bare-metal/aps/examples.

Eccezioni

AArch64 defines an exception vector table with 16 entries, for 4 types of exceptions (synchronous, IRQ, FIQ, SError) from 4 states (current EL with SP0, current EL with SPx, lower EL using AArch64, lower EL using AArch32). We implement this in assembly to save volatile registers to the stack before calling into Rust code:

use log::error;
use smccc::psci::system_off;
use smccc::Hvc;

#[no_mangle]
extern "C" fn sync_exception_current(_elr: u64, _spsr: u64) {
    error!("sync_exception_current");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn irq_current(_elr: u64, _spsr: u64) {
    error!("irq_current");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn fiq_current(_elr: u64, _spsr: u64) {
    error!("fiq_current");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn serr_current(_elr: u64, _spsr: u64) {
    error!("serr_current");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn sync_lower(_elr: u64, _spsr: u64) {
    error!("sync_lower");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn irq_lower(_elr: u64, _spsr: u64) {
    error!("irq_lower");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn fiq_lower(_elr: u64, _spsr: u64) {
    error!("fiq_lower");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn serr_lower(_elr: u64, _spsr: u64) {
    error!("serr_lower");
    system_off::<Hvc>().unwrap();
}

• EL is exception level; all our examples this afternoon run in EL1.
• For simplicity we aren't distinguishing between SP0 and SPx for the current EL exceptions, or between AArch32 and AArch64 for the lower EL exceptions.
• For this example we just log the exception and power down, as we don't expect any of them to actually happen.
• We can think of exception handlers and our main execution context more or less like different threads. Send and Sync will control what we can share between them, just like with threads. For example, if we want to share some value between exception handlers and the rest of the program, and it's Send but not Sync, then we'll need to wrap it in something like a Mutex and put it in a static.

Other projects

• oreboot
  • "coreboot without the C"
  • Supports x86, aarch64 and RISC-V.
  • Relies on LinuxBoot rather than having many drivers itself.
• Rust RaspberryPi OS tutorial
  • Initialisation, UART driver, simple bootloader, JTAG, exception levels, exception handling, page tables
  • Some dodginess around cache maintenance and initialisation in Rust, not necessarily a good example to copy for production code.
• cargo-call-stack
  • Static analysis to determine maximum stack usage.

• The RaspberryPi OS tutorial runs Rust code before the MMU and caches are enabled. This will read and write memory (e.g. the stack). However:
  • Without the MMU and cache, unaligned accesses will fault. It builds with aarch64-unknown-none which sets +strict-align to prevent the compiler generating unaligned accesses so it should be alright, but this is not necessarily the case in general.
  • If it were running in a VM, this can lead to cache coherency issues. The problem is that the VM is accessing memory directly with the cache disabled, while the host has cacheable aliases to the same memory. Even if the host doesn't explicitly access the memory, speculative accesses can lead to cache fills, and then changes from one or the other will get lost. Again this is alright in this particular case (running directly on the hardware with no hypervisor), but isn't a good pattern in general.

Crates utili

We'll go over a few crates which solve some common problems in bare-metal programming.

zerocopy

The zerocopy crate (from Fuchsia) provides traits and macros for safely converting between byte sequences and other types.

use zerocopy::AsBytes;

#[repr(u32)]
#[derive(AsBytes, Debug, Default)]
enum RequestType {
    #[default]
    In = 0,
    Out = 1,
    Flush = 4,
}

#[repr(C)]
#[derive(AsBytes, Debug, Default)]
struct VirtioBlockRequest {
    request_type: RequestType,
    reserved: u32,
    sector: u64,
}

fn main() {
    let request = VirtioBlockRequest {
        request_type: RequestType::Flush,
        sector: 42,
        ..Default::default()
    };

    assert_eq!(
        request.as_bytes(),
        &[4, 0, 0, 0, 0, 0, 0, 0, 42, 0, 0, 0, 0, 0, 0, 0]
    );
}

This is not suitable for MMIO (as it doesn't use volatile reads and writes), but can be useful for working with structures shared with hardware e.g. by DMA, or sent over some external interface.

• FromBytes can be implemented for types for which any byte pattern is valid, and so can safely be converted from an untrusted sequence of bytes.
• Attempting to derive FromBytes for these types would fail, because RequestType doesn't use all possible u32 values as discriminants, so not all byte patterns are valid.
• zerocopy::byteorder has types for byte-order aware numeric primitives.
• Run the example with cargo run under src/bare-metal/useful-crates/zerocopy-example/. (It won't run in the Playground because of the crate dependency.)

aarch64-paging

The aarch64-paging crate lets you create page tables according to the AArch64 Virtual Memory System Architecture.

use aarch64_paging::{
    idmap::IdMap,
    paging::{Attributes, MemoryRegion},
};

const ASID: usize = 1;
const ROOT_LEVEL: usize = 1;

// Create a new page table with identity mapping.
let mut idmap = IdMap::new(ASID, ROOT_LEVEL);
// Map a 2 MiB region of memory as read-only.
idmap.map_range(
    &MemoryRegion::new(0x80200000, 0x80400000),
    Attributes::NORMAL | Attributes::NON_GLOBAL | Attributes::READ_ONLY,
).unwrap();
// Set `TTBR0_EL1` to activate the page table.
idmap.activate();

• For now it only supports EL1, but support for other exception levels should be straightforward to add.
• This is used in Android for the Protected VM Firmware.
• There's no easy way to run this example, as it needs to run on real hardware or under QEMU.

buddy_system_allocator

buddy_system_allocator is a third-party crate implementing a basic buddy system allocator. It can be used both for LockedHeap implementing GlobalAlloc so you can use the standard alloc crate (as we saw before), or for allocating other address space. For example, we might want to allocate MMIO space for PCI BARs:

use buddy_system_allocator::FrameAllocator;
use core::alloc::Layout;

fn main() {
    let mut allocator = FrameAllocator::<32>::new();
    allocator.add_frame(0x200_0000, 0x400_0000);

    let layout = Layout::from_size_align(0x100, 0x100).unwrap();
    let bar = allocator
        .alloc_aligned(layout)
        .expect("Failed to allocate 0x100 byte MMIO region");
    println!("Allocated 0x100 byte MMIO region at {:#x}", bar);
}

• PCI BARs always have alignment equal to their size.
• Run the example with cargo run under src/bare-metal/useful-crates/allocator-example/. (It won't run in the Playground because of the crate dependency.)

tinyvec

Sometimes you want something which can be resized like a Vec, but without heap allocation. tinyvec provides this: a vector backed by an array or slice, which could be statically allocated or on the stack, which keeps track of how many elements are used and panics if you try to use more than are allocated.

use tinyvec::{array_vec, ArrayVec};

fn main() {
    let mut numbers: ArrayVec<[u32; 5]> = array_vec!(42, 66);
    println!("{numbers:?}");
    numbers.push(7);
    println!("{numbers:?}");
    numbers.remove(1);
    println!("{numbers:?}");
}

• tinyvec requires that the element type implement Default for initialisation.
• The Rust Playground includes tinyvec, so this example will run fine inline.

spin

std::sync::Mutex and the other synchronisation primitives from std::sync are not available in core or alloc. How can we manage synchronisation or interior mutability, such as for sharing state between different CPUs?

The spin crate provides spinlock-based equivalents of many of these primitives.

use spin::mutex::SpinMutex;

static counter: SpinMutex<u32> = SpinMutex::new(0);

fn main() {
    println!("count: {}", counter.lock());
    *counter.lock() += 2;
    println!("count: {}", counter.lock());
}

• Be careful to avoid deadlock if you take locks in interrupt handlers.
• spin also has a ticket lock mutex implementation; equivalents of RwLock, Barrier and Once from std::sync; and Lazy for lazy initialisation.
• The once_cell crate also has some useful types for late initialisation with a slightly different approach to spin::once::Once.
• The Rust Playground includes spin, so this example will run fine inline.

Android

To build a bare-metal Rust binary in AOSP, you need to use a rust_ffi_static Soong rule to build your Rust code, then a cc_binary with a linker script to produce the binary itself, and then a raw_binary to convert the ELF to a raw binary ready to be run.

rust_ffi_static {
    name: "libvmbase_example",
    defaults: ["vmbase_ffi_defaults"],
    crate_name: "vmbase_example",
    srcs: ["src/main.rs"],
    rustlibs: [
        "libvmbase",
    ],
}

cc_binary {
    name: "vmbase_example",
    defaults: ["vmbase_elf_defaults"],
    srcs: [
        "idmap.S",
    ],
    static_libs: [
        "libvmbase_example",
    ],
    linker_scripts: [
        "image.ld",
        ":vmbase_sections",
    ],
}

raw_binary {
    name: "vmbase_example_bin",
    stem: "vmbase_example.bin",
    src: ":vmbase_example",
    enabled: false,
    target: {
        android_arm64: {
            enabled: true,
        },
    },
}

vmbase

For VMs running under crosvm on aarch64, the vmbase library provides a linker script and useful defaults for the build rules, along with an entry point, UART console logging and more.

#![no_main]
#![no_std]

use vmbase::{main, println};

main!(main);

pub fn main(arg0: u64, arg1: u64, arg2: u64, arg3: u64) {
    println!("Hello world");
}

• The main! macro marks your main function, to be called from the vmbase entry point.
• The vmbase entry point handles console initialisation, and issues a PSCI_SYSTEM_OFF to shutdown the VM if your main function returns.

Esercizi

We will write a driver for the PL031 real-time clock device.

After looking at the exercises, you can look at the solutions provided.

RTC driver

The QEMU aarch64 virt machine has a PL031 real-time clock at 0x9010000. For this exercise, you should write a driver for it.

1. Use it to print the current time to the serial console. You can use the chrono crate for date/time formatting.
2. Use the match register and raw interrupt status to busy-wait until a given time, e.g. 3 seconds in the future. (Call core::hint::spin_loop inside the loop.)
3. Extension if you have time: Enable and handle the interrupt generated by the RTC match. You can use the driver provided in the arm-gic crate to configure the Arm Generic Interrupt Controller.
   • Use the RTC interrupt, which is wired to the GIC as IntId::spi(2).
   • Once the interrupt is enabled, you can put the core to sleep via arm_gic::wfi(), which will cause the core to sleep until it receives an interrupt.

Download the exercise template and look in the rtc directory for the following files.

src/main.rs:

#![no_main]
#![no_std]

mod exceptions;
mod logger;
mod pl011;

use crate::pl011::Uart;
use arm_gic::gicv3::GicV3;
use core::panic::PanicInfo;
use log::{error, info, trace, LevelFilter};
use smccc::psci::system_off;
use smccc::Hvc;

/// Base addresses of the GICv3.
const GICD_BASE_ADDRESS: *mut u64 = 0x800_0000 as _;
const GICR_BASE_ADDRESS: *mut u64 = 0x80A_0000 as _;

/// Base address of the primary PL011 UART.
const PL011_BASE_ADDRESS: *mut u32 = 0x900_0000 as _;

#[no_mangle]
extern "C" fn main(x0: u64, x1: u64, x2: u64, x3: u64) {
    // Safe because `PL011_BASE_ADDRESS` is the base address of a PL011 device,
    // and nothing else accesses that address range.
    let uart = unsafe { Uart::new(PL011_BASE_ADDRESS) };
    logger::init(uart, LevelFilter::Trace).unwrap();

    info!("main({:#x}, {:#x}, {:#x}, {:#x})", x0, x1, x2, x3);

    // Safe because `GICD_BASE_ADDRESS` and `GICR_BASE_ADDRESS` are the base
    // addresses of a GICv3 distributor and redistributor respectively, and
    // nothing else accesses those address ranges.
    let mut gic = unsafe { GicV3::new(GICD_BASE_ADDRESS, GICR_BASE_ADDRESS) };
    gic.setup();

    // TODO: Create instance of RTC driver and print current time.

    // TODO: Wait for 3 seconds.

    system_off::<Hvc>().unwrap();
}

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    error!("{info}");
    system_off::<Hvc>().unwrap();
    loop {}
}

src/exceptions.rs (you should only need to change this for the 3rd part of the exercise):

#![allow(unused)]
fn main() {
// Copyright 2023 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use arm_gic::gicv3::GicV3;
use log::{error, info, trace};
use smccc::psci::system_off;
use smccc::Hvc;

#[no_mangle]
extern "C" fn sync_exception_current(_elr: u64, _spsr: u64) {
    error!("sync_exception_current");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn irq_current(_elr: u64, _spsr: u64) {
    trace!("irq_current");
    let intid =
        GicV3::get_and_acknowledge_interrupt().expect("No pending interrupt");
    info!("IRQ {intid:?}");
}

#[no_mangle]
extern "C" fn fiq_current(_elr: u64, _spsr: u64) {
    error!("fiq_current");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn serr_current(_elr: u64, _spsr: u64) {
    error!("serr_current");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn sync_lower(_elr: u64, _spsr: u64) {
    error!("sync_lower");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn irq_lower(_elr: u64, _spsr: u64) {
    error!("irq_lower");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn fiq_lower(_elr: u64, _spsr: u64) {
    error!("fiq_lower");
    system_off::<Hvc>().unwrap();
}

#[no_mangle]
extern "C" fn serr_lower(_elr: u64, _spsr: u64) {
    error!("serr_lower");
    system_off::<Hvc>().unwrap();
}
}

src/logger.rs (you shouldn't need to change this):

#![allow(unused)]
fn main() {
// Copyright 2023 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// ANCHOR: main
use crate::pl011::Uart;
use core::fmt::Write;
use log::{LevelFilter, Log, Metadata, Record, SetLoggerError};
use spin::mutex::SpinMutex;

static LOGGER: Logger = Logger { uart: SpinMutex::new(None) };

struct Logger {
    uart: SpinMutex<Option<Uart>>,
}

impl Log for Logger {
    fn enabled(&self, _metadata: &Metadata) -> bool {
        true
    }

    fn log(&self, record: &Record) {
        writeln!(
            self.uart.lock().as_mut().unwrap(),
            "[{}] {}",
            record.level(),
            record.args()
        )
        .unwrap();
    }

    fn flush(&self) {}
}

/// Initialises UART logger.
pub fn init(uart: Uart, max_level: LevelFilter) -> Result<(), SetLoggerError> {
    LOGGER.uart.lock().replace(uart);

    log::set_logger(&LOGGER)?;
    log::set_max_level(max_level);
    Ok(())
}
}

src/pl011.rs (you shouldn't need to change this):

#![allow(unused)]
fn main() {
// Copyright 2023 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#![allow(unused)]

use core::fmt::{self, Write};
use core::ptr::{addr_of, addr_of_mut};

// ANCHOR: Flags
use bitflags::bitflags;

bitflags! {
    /// Flags from the UART flag register.
    #[repr(transparent)]
    #[derive(Copy, Clone, Debug, Eq, PartialEq)]
    struct Flags: u16 {
        /// Clear to send.
        const CTS = 1 << 0;
        /// Data set ready.
        const DSR = 1 << 1;
        /// Data carrier detect.
        const DCD = 1 << 2;
        /// UART busy transmitting data.
        const BUSY = 1 << 3;
        /// Receive FIFO is empty.
        const RXFE = 1 << 4;
        /// Transmit FIFO is full.
        const TXFF = 1 << 5;
        /// Receive FIFO is full.
        const RXFF = 1 << 6;
        /// Transmit FIFO is empty.
        const TXFE = 1 << 7;
        /// Ring indicator.
        const RI = 1 << 8;
    }
}
// ANCHOR_END: Flags

bitflags! {
    /// Flags from the UART Receive Status Register / Error Clear Register.
    #[repr(transparent)]
    #[derive(Copy, Clone, Debug, Eq, PartialEq)]
    struct ReceiveStatus: u16 {
        /// Framing error.
        const FE = 1 << 0;
        /// Parity error.
        const PE = 1 << 1;
        /// Break error.
        const BE = 1 << 2;
        /// Overrun error.
        const OE = 1 << 3;
    }
}

// ANCHOR: Registers
#[repr(C, align(4))]
struct Registers {
    dr: u16,
    _reserved0: [u8; 2],
    rsr: ReceiveStatus,
    _reserved1: [u8; 19],
    fr: Flags,
    _reserved2: [u8; 6],
    ilpr: u8,
    _reserved3: [u8; 3],
    ibrd: u16,
    _reserved4: [u8; 2],
    fbrd: u8,
    _reserved5: [u8; 3],
    lcr_h: u8,
    _reserved6: [u8; 3],
    cr: u16,
    _reserved7: [u8; 3],
    ifls: u8,
    _reserved8: [u8; 3],
    imsc: u16,
    _reserved9: [u8; 2],
    ris: u16,
    _reserved10: [u8; 2],
    mis: u16,
    _reserved11: [u8; 2],
    icr: u16,
    _reserved12: [u8; 2],
    dmacr: u8,
    _reserved13: [u8; 3],
}
// ANCHOR_END: Registers

// ANCHOR: Uart
/// Driver for a PL011 UART.
#[derive(Debug)]
pub struct Uart {
    registers: *mut Registers,
}

impl Uart {
    /// Constructs a new instance of the UART driver for a PL011 device at the
    /// given base address.
    ///
    /// # Safety
    ///
    /// The given base address must point to the MMIO control registers of a
    /// PL011 device, which must be mapped into the address space of the process
    /// as device memory and not have any other aliases.
    pub unsafe fn new(base_address: *mut u32) -> Self {
        Self { registers: base_address as *mut Registers }
    }

    /// Writes a single byte to the UART.
    pub fn write_byte(&self, byte: u8) {
        // Wait until there is room in the TX buffer.
        while self.read_flag_register().contains(Flags::TXFF) {}

        // Safe because we know that self.registers points to the control
        // registers of a PL011 device which is appropriately mapped.
        unsafe {
            // Write to the TX buffer.
            addr_of_mut!((*self.registers).dr).write_volatile(byte.into());
        }

        // Wait until the UART is no longer busy.
        while self.read_flag_register().contains(Flags::BUSY) {}
    }

    /// Reads and returns a pending byte, or `None` if nothing has been
    /// received.
    pub fn read_byte(&self) -> Option<u8> {
        if self.read_flag_register().contains(Flags::RXFE) {
            None
        } else {
            let data = unsafe { addr_of!((*self.registers).dr).read_volatile() };
            // TODO: Check for error conditions in bits 8-11.
            Some(data as u8)
        }
    }

    fn read_flag_register(&self) -> Flags {
        // Safe because we know that self.registers points to the control
        // registers of a PL011 device which is appropriately mapped.
        unsafe { addr_of!((*self.registers).fr).read_volatile() }
    }
}
// ANCHOR_END: Uart

impl Write for Uart {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        for c in s.as_bytes() {
            self.write_byte(*c);
        }
        Ok(())
    }
}

// Safe because it just contains a pointer to device memory, which can be
// accessed from any context.
unsafe impl Send for Uart {}
}

Cargo.toml (you shouldn't need to change this):

[workspace]

[package]
name = "rtc"
version = "0.1.0"
edition = "2021"
publish = false

[dependencies]
arm-gic = "0.1.0"
bitflags = "2.4.2"
chrono = { version = "0.4.24", default-features = false }
log = "0.4.17"
smccc = "0.1.1"
spin = "0.9.8"

[build-dependencies]
cc = "1.0.73"

build.rs (you shouldn't need to change this):

// Copyright 2023 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use cc::Build;
use std::env;

fn main() {
    #[cfg(target_os = "linux")]
    env::set_var("CROSS_COMPILE", "aarch64-linux-gnu");
    #[cfg(not(target_os = "linux"))]
    env::set_var("CROSS_COMPILE", "aarch64-none-elf");

    Build::new()
        .file("entry.S")
        .file("exceptions.S")
        .file("idmap.S")
        .compile("empty")
}

entry.S (you shouldn't need to change this):

/*
 * Copyright 2023 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

.macro adr_l, reg:req, sym:req
	adrp \reg, \sym
	add \reg, \reg, :lo12:\sym
.endm

.macro mov_i, reg:req, imm:req
	movz \reg, :abs_g3:\imm
	movk \reg, :abs_g2_nc:\imm
	movk \reg, :abs_g1_nc:\imm
	movk \reg, :abs_g0_nc:\imm
.endm

.set .L_MAIR_DEV_nGnRE,	0x04
.set .L_MAIR_MEM_WBWA,	0xff
.set .Lmairval, .L_MAIR_DEV_nGnRE | (.L_MAIR_MEM_WBWA << 8)

/* 4 KiB granule size for TTBR0_EL1. */
.set .L_TCR_TG0_4KB, 0x0 << 14
/* 4 KiB granule size for TTBR1_EL1. */
.set .L_TCR_TG1_4KB, 0x2 << 30
/* Disable translation table walk for TTBR1_EL1, generating a translation fault instead. */
.set .L_TCR_EPD1, 0x1 << 23
/* Translation table walks for TTBR0_EL1 are inner sharable. */
.set .L_TCR_SH_INNER, 0x3 << 12
/*
 * Translation table walks for TTBR0_EL1 are outer write-back read-allocate write-allocate
 * cacheable.
 */
.set .L_TCR_RGN_OWB, 0x1 << 10
/*
 * Translation table walks for TTBR0_EL1 are inner write-back read-allocate write-allocate
 * cacheable.
 */
.set .L_TCR_RGN_IWB, 0x1 << 8
/* Size offset for TTBR0_EL1 is 2**39 bytes (512 GiB). */
.set .L_TCR_T0SZ_512, 64 - 39
.set .Ltcrval, .L_TCR_TG0_4KB | .L_TCR_TG1_4KB | .L_TCR_EPD1 | .L_TCR_RGN_OWB
.set .Ltcrval, .Ltcrval | .L_TCR_RGN_IWB | .L_TCR_SH_INNER | .L_TCR_T0SZ_512

/* Stage 1 instruction access cacheability is unaffected. */
.set .L_SCTLR_ELx_I, 0x1 << 12
/* SP alignment fault if SP is not aligned to a 16 byte boundary. */
.set .L_SCTLR_ELx_SA, 0x1 << 3
/* Stage 1 data access cacheability is unaffected. */
.set .L_SCTLR_ELx_C, 0x1 << 2
/* EL0 and EL1 stage 1 MMU enabled. */
.set .L_SCTLR_ELx_M, 0x1 << 0
/* Privileged Access Never is unchanged on taking an exception to EL1. */
.set .L_SCTLR_EL1_SPAN, 0x1 << 23
/* SETEND instruction disabled at EL0 in aarch32 mode. */
.set .L_SCTLR_EL1_SED, 0x1 << 8
/* Various IT instructions are disabled at EL0 in aarch32 mode. */
.set .L_SCTLR_EL1_ITD, 0x1 << 7
.set .L_SCTLR_EL1_RES1, (0x1 << 11) | (0x1 << 20) | (0x1 << 22) | (0x1 << 28) | (0x1 << 29)
.set .Lsctlrval, .L_SCTLR_ELx_M | .L_SCTLR_ELx_C | .L_SCTLR_ELx_SA | .L_SCTLR_EL1_ITD | .L_SCTLR_EL1_SED
.set .Lsctlrval, .Lsctlrval | .L_SCTLR_ELx_I | .L_SCTLR_EL1_SPAN | .L_SCTLR_EL1_RES1

/**
 * This is a generic entry point for an image. It carries out the operations required to prepare the
 * loaded image to be run. Specifically, it zeroes the bss section using registers x25 and above,
 * prepares the stack, enables floating point, and sets up the exception vector. It preserves x0-x3
 * for the Rust entry point, as these may contain boot parameters.
 */
.section .init.entry, "ax"
.global entry
entry:
	/* Load and apply the memory management configuration, ready to enable MMU and caches. */
	adrp x30, idmap
	msr ttbr0_el1, x30

	mov_i x30, .Lmairval
	msr mair_el1, x30

	mov_i x30, .Ltcrval
	/* Copy the supported PA range into TCR_EL1.IPS. */
	mrs x29, id_aa64mmfr0_el1
	bfi x30, x29, #32, #4

	msr tcr_el1, x30

	mov_i x30, .Lsctlrval

	/*
	 * Ensure everything before this point has completed, then invalidate any potentially stale
	 * local TLB entries before they start being used.
	 */
	isb
	tlbi vmalle1
	ic iallu
	dsb nsh
	isb

	/*
	 * Configure sctlr_el1 to enable MMU and cache and don't proceed until this has completed.
	 */
	msr sctlr_el1, x30
	isb

	/* Disable trapping floating point access in EL1. */
	mrs x30, cpacr_el1
	orr x30, x30, #(0x3 << 20)
	msr cpacr_el1, x30
	isb

	/* Zero out the bss section. */
	adr_l x29, bss_begin
	adr_l x30, bss_end
0:	cmp x29, x30
	b.hs 1f
	stp xzr, xzr, [x29], #16
	b 0b

1:	/* Prepare the stack. */
	adr_l x30, boot_stack_end
	mov sp, x30

	/* Set up exception vector. */
	adr x30, vector_table_el1
	msr vbar_el1, x30

	/* Call into Rust code. */
	bl main

	/* Loop forever waiting for interrupts. */
2:	wfi
	b 2b

exceptions.S (you shouldn't need to change this):

/*
 * Copyright 2023 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/**
 * Saves the volatile registers onto the stack. This currently takes 14
 * instructions, so it can be used in exception handlers with 18 instructions
 * left.
 *
 * On return, x0 and x1 are initialised to elr_el2 and spsr_el2 respectively,
 * which can be used as the first and second arguments of a subsequent call.
 */
.macro save_volatile_to_stack
	/* Reserve stack space and save registers x0-x18, x29 & x30. */
	stp x0, x1, [sp, #-(8 * 24)]!
	stp x2, x3, [sp, #8 * 2]
	stp x4, x5, [sp, #8 * 4]
	stp x6, x7, [sp, #8 * 6]
	stp x8, x9, [sp, #8 * 8]
	stp x10, x11, [sp, #8 * 10]
	stp x12, x13, [sp, #8 * 12]
	stp x14, x15, [sp, #8 * 14]
	stp x16, x17, [sp, #8 * 16]
	str x18, [sp, #8 * 18]
	stp x29, x30, [sp, #8 * 20]

	/*
	 * Save elr_el1 & spsr_el1. This such that we can take nested exception
	 * and still be able to unwind.
	 */
	mrs x0, elr_el1
	mrs x1, spsr_el1
	stp x0, x1, [sp, #8 * 22]
.endm

/**
 * Restores the volatile registers from the stack. This currently takes 14
 * instructions, so it can be used in exception handlers while still leaving 18
 * instructions left; if paired with save_volatile_to_stack, there are 4
 * instructions to spare.
 */
.macro restore_volatile_from_stack
	/* Restore registers x2-x18, x29 & x30. */
	ldp x2, x3, [sp, #8 * 2]
	ldp x4, x5, [sp, #8 * 4]
	ldp x6, x7, [sp, #8 * 6]
	ldp x8, x9, [sp, #8 * 8]
	ldp x10, x11, [sp, #8 * 10]
	ldp x12, x13, [sp, #8 * 12]
	ldp x14, x15, [sp, #8 * 14]
	ldp x16, x17, [sp, #8 * 16]
	ldr x18, [sp, #8 * 18]
	ldp x29, x30, [sp, #8 * 20]

	/* Restore registers elr_el1 & spsr_el1, using x0 & x1 as scratch. */
	ldp x0, x1, [sp, #8 * 22]
	msr elr_el1, x0
	msr spsr_el1, x1

	/* Restore x0 & x1, and release stack space. */
	ldp x0, x1, [sp], #8 * 24
.endm

/**
 * This is a generic handler for exceptions taken at the current EL while using
 * SP0. It behaves similarly to the SPx case by first switching to SPx, doing
 * the work, then switching back to SP0 before returning.
 *
 * Switching to SPx and calling the Rust handler takes 16 instructions. To
 * restore and return we need an additional 16 instructions, so we can implement
 * the whole handler within the allotted 32 instructions.
 */
.macro current_exception_sp0 handler:req
	msr spsel, #1
	save_volatile_to_stack
	bl \handler
	restore_volatile_from_stack
	msr spsel, #0
	eret
.endm

/**
 * This is a generic handler for exceptions taken at the current EL while using
 * SPx. It saves volatile registers, calls the Rust handler, restores volatile
 * registers, then returns.
 *
 * This also works for exceptions taken from EL0, if we don't care about
 * non-volatile registers.
 *
 * Saving state and jumping to the Rust handler takes 15 instructions, and
 * restoring and returning also takes 15 instructions, so we can fit the whole
 * handler in 30 instructions, under the limit of 32.
 */
.macro current_exception_spx handler:req
	save_volatile_to_stack
	bl \handler
	restore_volatile_from_stack
	eret
.endm

.section .text.vector_table_el1, "ax"
.global vector_table_el1
.balign 0x800
vector_table_el1:
sync_cur_sp0:
	current_exception_sp0 sync_exception_current

.balign 0x80
irq_cur_sp0:
	current_exception_sp0 irq_current

.balign 0x80
fiq_cur_sp0:
	current_exception_sp0 fiq_current

.balign 0x80
serr_cur_sp0:
	current_exception_sp0 serr_current

.balign 0x80
sync_cur_spx:
	current_exception_spx sync_exception_current

.balign 0x80
irq_cur_spx:
	current_exception_spx irq_current

.balign 0x80
fiq_cur_spx:
	current_exception_spx fiq_current

.balign 0x80
serr_cur_spx:
	current_exception_spx serr_current

.balign 0x80
sync_lower_64:
	current_exception_spx sync_lower

.balign 0x80
irq_lower_64:
	current_exception_spx irq_lower

.balign 0x80
fiq_lower_64:
	current_exception_spx fiq_lower

.balign 0x80
serr_lower_64:
	current_exception_spx serr_lower

.balign 0x80
sync_lower_32:
	current_exception_spx sync_lower

.balign 0x80
irq_lower_32:
	current_exception_spx irq_lower

.balign 0x80
fiq_lower_32:
	current_exception_spx fiq_lower

.balign 0x80
serr_lower_32:
	current_exception_spx serr_lower

idmap.S (you shouldn't need to change this):

/*
 * Copyright 2023 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

.set .L_TT_TYPE_BLOCK, 0x1
.set .L_TT_TYPE_PAGE,  0x3
.set .L_TT_TYPE_TABLE, 0x3

/* Access flag. */
.set .L_TT_AF, 0x1 << 10
/* Not global. */
.set .L_TT_NG, 0x1 << 11
.set .L_TT_XN, 0x3 << 53

.set .L_TT_MT_DEV, 0x0 << 2			// MAIR #0 (DEV_nGnRE)
.set .L_TT_MT_MEM, (0x1 << 2) | (0x3 << 8)	// MAIR #1 (MEM_WBWA), inner shareable

.set .L_BLOCK_DEV, .L_TT_TYPE_BLOCK | .L_TT_MT_DEV | .L_TT_AF | .L_TT_XN
.set .L_BLOCK_MEM, .L_TT_TYPE_BLOCK | .L_TT_MT_MEM | .L_TT_AF | .L_TT_NG

.section ".rodata.idmap", "a", %progbits
.global idmap
.align 12
idmap:
	/* level 1 */
	.quad		.L_BLOCK_DEV | 0x0		    // 1 GiB of device mappings
	.quad		.L_BLOCK_MEM | 0x40000000	// 1 GiB of DRAM
	.fill		254, 8, 0x0			// 254 GiB of unmapped VA space
	.quad		.L_BLOCK_DEV | 0x4000000000 // 1 GiB of device mappings
	.fill		255, 8, 0x0			// 255 GiB of remaining VA space

image.ld (you shouldn't need to change this):

/*
 * Copyright 2023 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/*
 * Code will start running at this symbol which is placed at the start of the
 * image.
 */
ENTRY(entry)

MEMORY
{
	image : ORIGIN = 0x40080000, LENGTH = 2M
}

SECTIONS
{
	/*
	 * Collect together the code.
	 */
	.init : ALIGN(4096) {
		text_begin = .;
		*(.init.entry)
		*(.init.*)
	} >image
	.text : {
		*(.text.*)
	} >image
	text_end = .;

	/*
	 * Collect together read-only data.
	 */
	.rodata : ALIGN(4096) {
		rodata_begin = .;
		*(.rodata.*)
	} >image
	.got : {
		*(.got)
	} >image
	rodata_end = .;

	/*
	 * Collect together the read-write data including .bss at the end which
	 * will be zero'd by the entry code.
	 */
	.data : ALIGN(4096) {
		data_begin = .;
		*(.data.*)
		/*
		 * The entry point code assumes that .data is a multiple of 32
		 * bytes long.
		 */
		. = ALIGN(32);
		data_end = .;
	} >image

	/* Everything beyond this point will not be included in the binary. */
	bin_end = .;

	/* The entry point code assumes that .bss is 16-byte aligned. */
	.bss : ALIGN(16)  {
		bss_begin = .;
		*(.bss.*)
		*(COMMON)
		. = ALIGN(16);
		bss_end = .;
	} >image

	.stack (NOLOAD) : ALIGN(4096) {
		boot_stack_begin = .;
		. += 40 * 4096;
		. = ALIGN(4096);
		boot_stack_end = .;
	} >image

	. = ALIGN(4K);
	PROVIDE(dma_region = .);

	/*
	 * Remove unused sections from the image.
	 */
	/DISCARD/ : {
		/* The image loads itself so doesn't need these sections. */
		*(.gnu.hash)
		*(.hash)
		*(.interp)
		*(.eh_frame_hdr)
		*(.eh_frame)
		*(.note.gnu.build-id)
	}
}

Makefile (you shouldn't need to change this):

# Copyright 2023 Google LLC
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

UNAME := $(shell uname -s)
ifeq ($(UNAME),Linux)
	TARGET = aarch64-linux-gnu
else
	TARGET = aarch64-none-elf
endif
OBJCOPY = $(TARGET)-objcopy

.PHONY: build qemu_minimal qemu qemu_logger

all: rtc.bin

build:
	cargo build

rtc.bin: build
	$(OBJCOPY) -O binary target/aarch64-unknown-none/debug/rtc $@

qemu: rtc.bin
	qemu-system-aarch64 -machine virt,gic-version=3 -cpu max -serial mon:stdio -display none -kernel $< -s

clean:
	cargo clean
	rm -f *.bin

.cargo/config.toml (you shouldn't need to change this):

[build]
target = "aarch64-unknown-none"
rustflags = ["-C", "link-arg=-Timage.ld"]

Run the code in QEMU with make qemu.

Pomeriggio su Bare Metal Rust

RTC driver

(back to exercise)

main.rs:

#![no_main]
#![no_std]

mod exceptions;
mod logger;
mod pl011;
mod pl031;

use crate::pl031::Rtc;
use arm_gic::gicv3::{IntId, Trigger};
use arm_gic::{irq_enable, wfi};
use chrono::{TimeZone, Utc};
use core::hint::spin_loop;
use crate::pl011::Uart;
use arm_gic::gicv3::GicV3;
use core::panic::PanicInfo;
use log::{error, info, trace, LevelFilter};
use smccc::psci::system_off;
use smccc::Hvc;

/// Base addresses of the GICv3.
const GICD_BASE_ADDRESS: *mut u64 = 0x800_0000 as _;
const GICR_BASE_ADDRESS: *mut u64 = 0x80A_0000 as _;

/// Base address of the primary PL011 UART.
const PL011_BASE_ADDRESS: *mut u32 = 0x900_0000 as _;

/// Base address of the PL031 RTC.
const PL031_BASE_ADDRESS: *mut u32 = 0x901_0000 as _;
/// The IRQ used by the PL031 RTC.
const PL031_IRQ: IntId = IntId::spi(2);

#[no_mangle]
extern "C" fn main(x0: u64, x1: u64, x2: u64, x3: u64) {
    // Safe because `PL011_BASE_ADDRESS` is the base address of a PL011 device,
    // and nothing else accesses that address range.
    let uart = unsafe { Uart::new(PL011_BASE_ADDRESS) };
    logger::init(uart, LevelFilter::Trace).unwrap();

    info!("main({:#x}, {:#x}, {:#x}, {:#x})", x0, x1, x2, x3);

    // Safe because `GICD_BASE_ADDRESS` and `GICR_BASE_ADDRESS` are the base
    // addresses of a GICv3 distributor and redistributor respectively, and
    // nothing else accesses those address ranges.
    let mut gic = unsafe { GicV3::new(GICD_BASE_ADDRESS, GICR_BASE_ADDRESS) };
    gic.setup();

    // Safe because `PL031_BASE_ADDRESS` is the base address of a PL031 device,
    // and nothing else accesses that address range.
    let mut rtc = unsafe { Rtc::new(PL031_BASE_ADDRESS) };
    let timestamp = rtc.read();
    let time = Utc.timestamp_opt(timestamp.into(), 0).unwrap();
    info!("RTC: {time}");

    GicV3::set_priority_mask(0xff);
    gic.set_interrupt_priority(PL031_IRQ, 0x80);
    gic.set_trigger(PL031_IRQ, Trigger::Level);
    irq_enable();
    gic.enable_interrupt(PL031_IRQ, true);

    // Wait for 3 seconds, without interrupts.
    let target = timestamp + 3;
    rtc.set_match(target);
    info!("Waiting for {}", Utc.timestamp_opt(target.into(), 0).unwrap());
    trace!(
        "matched={}, interrupt_pending={}",
        rtc.matched(),
        rtc.interrupt_pending()
    );
    while !rtc.matched() {
        spin_loop();
    }
    trace!(
        "matched={}, interrupt_pending={}",
        rtc.matched(),
        rtc.interrupt_pending()
    );
    info!("Finished waiting");

    // Wait another 3 seconds for an interrupt.
    let target = timestamp + 6;
    info!("Waiting for {}", Utc.timestamp_opt(target.into(), 0).unwrap());
    rtc.set_match(target);
    rtc.clear_interrupt();
    rtc.enable_interrupt(true);
    trace!(
        "matched={}, interrupt_pending={}",
        rtc.matched(),
        rtc.interrupt_pending()
    );
    while !rtc.interrupt_pending() {
        wfi();
    }
    trace!(
        "matched={}, interrupt_pending={}",
        rtc.matched(),
        rtc.interrupt_pending()
    );
    info!("Finished waiting");

    system_off::<Hvc>().unwrap();
}

#[panic_handler]
fn panic(info: &PanicInfo) -> ! {
    error!("{info}");
    system_off::<Hvc>().unwrap();
    loop {}
}

pl031.rs:

#![allow(unused)]
fn main() {
use core::ptr::{addr_of, addr_of_mut};

#[repr(C, align(4))]
struct Registers {
    /// Data register
    dr: u32,
    /// Match register
    mr: u32,
    /// Load register
    lr: u32,
    /// Control register
    cr: u8,
    _reserved0: [u8; 3],
    /// Interrupt Mask Set or Clear register
    imsc: u8,
    _reserved1: [u8; 3],
    /// Raw Interrupt Status
    ris: u8,
    _reserved2: [u8; 3],
    /// Masked Interrupt Status
    mis: u8,
    _reserved3: [u8; 3],
    /// Interrupt Clear Register
    icr: u8,
    _reserved4: [u8; 3],
}

/// Driver for a PL031 real-time clock.
#[derive(Debug)]
pub struct Rtc {
    registers: *mut Registers,
}

impl Rtc {
    /// Constructs a new instance of the RTC driver for a PL031 device at the
    /// given base address.
    ///
    /// # Safety
    ///
    /// The given base address must point to the MMIO control registers of a
    /// PL031 device, which must be mapped into the address space of the process
    /// as device memory and not have any other aliases.
    pub unsafe fn new(base_address: *mut u32) -> Self {
        Self { registers: base_address as *mut Registers }
    }

    /// Reads the current RTC value.
    pub fn read(&self) -> u32 {
        // Safe because we know that self.registers points to the control
        // registers of a PL031 device which is appropriately mapped.
        unsafe { addr_of!((*self.registers).dr).read_volatile() }
    }

    /// Writes a match value. When the RTC value matches this then an interrupt
    /// will be generated (if it is enabled).
    pub fn set_match(&mut self, value: u32) {
        // Safe because we know that self.registers points to the control
        // registers of a PL031 device which is appropriately mapped.
        unsafe { addr_of_mut!((*self.registers).mr).write_volatile(value) }
    }

    /// Returns whether the match register matches the RTC value, whether or not
    /// the interrupt is enabled.
    pub fn matched(&self) -> bool {
        // Safe because we know that self.registers points to the control
        // registers of a PL031 device which is appropriately mapped.
        let ris = unsafe { addr_of!((*self.registers).ris).read_volatile() };
        (ris & 0x01) != 0
    }

    /// Returns whether there is currently an interrupt pending.
    ///
    /// This should be true if and only if `matched` returns true and the
    /// interrupt is masked.
    pub fn interrupt_pending(&self) -> bool {
        // Safe because we know that self.registers points to the control
        // registers of a PL031 device which is appropriately mapped.
        let ris = unsafe { addr_of!((*self.registers).mis).read_volatile() };
        (ris & 0x01) != 0
    }

    /// Sets or clears the interrupt mask.
    ///
    /// When the mask is true the interrupt is enabled; when it is false the
    /// interrupt is disabled.
    pub fn enable_interrupt(&mut self, mask: bool) {
        let imsc = if mask { 0x01 } else { 0x00 };
        // Safe because we know that self.registers points to the control
        // registers of a PL031 device which is appropriately mapped.
        unsafe { addr_of_mut!((*self.registers).imsc).write_volatile(imsc) }
    }

    /// Clears a pending interrupt, if any.
    pub fn clear_interrupt(&mut self) {
        // Safe because we know that self.registers points to the control
        // registers of a PL031 device which is appropriately mapped.
        unsafe { addr_of_mut!((*self.registers).icr).write_volatile(0x01) }
    }
}

// Safe because it just contains a pointer to device memory, which can be
// accessed from any context.
unsafe impl Send for Rtc {}
}

Welcome to Concurrency in Rust

Rust has full support for concurrency using OS threads with mutexes and channels.

The Rust type system plays an important role in making many concurrency bugs compile time bugs. This is often referred to as fearless concurrency since you can rely on the compiler to ensure correctness at runtime.

Threads

Rust threads work similarly to threads in other languages:

use std::thread;
use std::time::Duration;

fn main() {
    thread::spawn(|| {
        for i in 1..10 {
            println!("Count in thread: {i}!");
            thread::sleep(Duration::from_millis(5));
        }
    });

    for i in 1..5 {
        println!("Main thread: {i}");
        thread::sleep(Duration::from_millis(5));
    }
}

• Threads are all daemon threads, the main thread does not wait for them.
• Thread panics are independent of each other.
  • Panics can carry a payload, which can be unpacked with downcast_ref.

Punti chiave:

• Notice that the thread is stopped before it reaches 10 --- the main thread is not waiting.

• Use let handle = thread::spawn(...) and later handle.join() to wait for the thread to finish.

• Trigger a panic in the thread, notice how this doesn't affect main.

• Use the Result return value from handle.join() to get access to the panic payload. This is a good time to talk about Any.

Thread con Scope (Scoped Threads)

Normal threads cannot borrow from their environment:

use std::thread;

fn foo() {
    let s = String::from("Hello");
    thread::spawn(|| {
        println!("Length: {}", s.len());
    });
}

fn main() {
    foo();
}

However, you can use a scoped thread for this:

use std::thread;

fn main() {
    let s = String::from("Hello");

    thread::scope(|scope| {
        scope.spawn(|| {
            println!("Length: {}", s.len());
        });
    });
}

• The reason for that is that when the thread::scope function completes, all the threads are guaranteed to be joined, so they can return borrowed data.
• Normal Rust borrowing rules apply: you can either borrow mutably by one thread, or immutably by any number of threads.

Canali (Channels)

Rust channels have two parts: a Sender<T> and a Receiver<T>. The two parts are connected via the channel, but you only see the end-points.

use std::sync::mpsc;

fn main() {
    let (tx, rx) = mpsc::channel();

    tx.send(10).unwrap();
    tx.send(20).unwrap();

    println!("Received: {:?}", rx.recv());
    println!("Received: {:?}", rx.recv());

    let tx2 = tx.clone();
    tx2.send(30).unwrap();
    println!("Received: {:?}", rx.recv());
}

• mpsc stands for Multi-Producer, Single-Consumer. Sender and SyncSender implement Clone (so you can make multiple producers) but Receiver does not.
• send() and recv() return Result. If they return Err, it means the counterpart Sender or Receiver is dropped and the channel is closed.

Canali illimitati (Unbounded Channels)

You get an unbounded and asynchronous channel with mpsc::channel():

use std::sync::mpsc;
use std::thread;
use std::time::Duration;

fn main() {
    let (tx, rx) = mpsc::channel();

    thread::spawn(move || {
        let thread_id = thread::current().id();
        for i in 1..10 {
            tx.send(format!("Message {i}")).unwrap();
            println!("{thread_id:?}: sent Message {i}");
        }
        println!("{thread_id:?}: done");
    });
    thread::sleep(Duration::from_millis(100));

    for msg in rx.iter() {
        println!("Main: got {msg}");
    }
}

Canali delimitati (Bounded Channels)

With bounded (synchronous) channels, send can block the current thread:

use std::sync::mpsc;
use std::thread;
use std::time::Duration;

fn main() {
    let (tx, rx) = mpsc::sync_channel(3);

    thread::spawn(move || {
        let thread_id = thread::current().id();
        for i in 1..10 {
            tx.send(format!("Message {i}")).unwrap();
            println!("{thread_id:?}: sent Message {i}");
        }
        println!("{thread_id:?}: done");
    });
    thread::sleep(Duration::from_millis(100));

    for msg in rx.iter() {
        println!("Main: got {msg}");
    }
}

• Calling send will block the current thread until there is space in the channel for the new message. The thread can be blocked indefinitely if there is nobody who reads from the channel.
• A call to send will abort with an error (that is why it returns Result) if the channel is closed. A channel is closed when the receiver is dropped.
• A bounded channel with a size of zero is called a "rendezvous channel". Every send will block the current thread until another thread calls read.

Send and Sync

How does Rust know to forbid shared access across threads? The answer is in two traits:

• Send: a type T is Send if it is safe to move a T across a thread boundary.
• Sync: a type T is Sync if it is safe to move a &T across a thread boundary.

Send and Sync are unsafe traits. The compiler will automatically derive them for your types as long as they only contain Send and Sync types. You can also implement them manually when you know it is valid.

• One can think of these traits as markers that the type has certain thread-safety properties.
• They can be used in the generic constraints as normal traits.

Send

A type T is Send if it is safe to move a T value to another thread.

The effect of moving ownership to another thread is that destructors will run in that thread. So the question is when you can allocate a value in one thread and deallocate it in another.

As an example, a connection to the SQLite library must only be accessed from a single thread.

Sync

A type T is Sync if it is safe to access a T value from multiple threads at the same time.

More precisely, the definition is:

T is Sync if and only if &T is Send

This statement is essentially a shorthand way of saying that if a type is thread-safe for shared use, it is also thread-safe to pass references of it across threads.

This is because if a type is Sync it means that it can be shared across multiple threads without the risk of data races or other synchronization issues, so it is safe to move it to another thread. A reference to the type is also safe to move to another thread, because the data it references can be accessed from any thread safely.

Esempi

Send + Sync

Most types you come across are Send + Sync:

• i8, f32, bool, char, &str, ...
• (T1, T2), [T; N], &[T], struct { x: T }, ...
• String, Option<T>, Vec<T>, Box<T>, ...
• Arc<T>: Explicitly thread-safe via atomic reference count.
• Mutex<T>: Explicitly thread-safe via internal locking.
• AtomicBool, AtomicU8, ...: Uses special atomic instructions.

The generic types are typically Send + Sync when the type parameters are Send + Sync.

Send + !Sync

These types can be moved to other threads, but they're not thread-safe. Typically because of interior mutability:

• mpsc::Sender<T>
• mpsc::Receiver<T>
• Cell<T>
• RefCell<T>

!Send + Sync

These types are thread-safe, but they cannot be moved to another thread:

• MutexGuard<T: Sync>: Uses OS level primitives which must be deallocated on the thread which created them.

!Send + !Sync

These types are not thread-safe and cannot be moved to other threads:

• Rc<T>: each Rc<T> has a reference to an RcBox<T>, which contains a non-atomic reference count.
• *const T, *mut T: Rust assumes raw pointers may have special concurrency considerations.

Stato Condiviso

Rust uses the type system to enforce synchronization of shared data. This is primarily done via two types:

• Arc<T>, atomic reference counted T: handles sharing between threads and takes care to deallocate T when the last reference is dropped,
• Mutex<T>: ensures mutually exclusive access to the T value.

Arc

Arc<T> allows shared read-only access via Arc::clone:

use std::sync::Arc;
use std::thread;

fn main() {
    let v = Arc::new(vec![10, 20, 30]);
    let mut handles = Vec::new();
    for _ in 1..5 {
        let v = Arc::clone(&v);
        handles.push(thread::spawn(move || {
            let thread_id = thread::current().id();
            println!("{thread_id:?}: {v:?}");
        }));
    }

    handles.into_iter().for_each(|h| h.join().unwrap());
    println!("v: {v:?}");
}

• Arc stands for "Atomic Reference Counted", a thread safe version of Rc that uses atomic operations.
• Arc<T> implements Clone whether or not T does. It implements Send and Sync if and only if T implements them both.
• Arc::clone() has the cost of atomic operations that get executed, but after that the use of the T is free.
• Beware of reference cycles, Arc does not use a garbage collector to detect them.
  • std::sync::Weak can help.

Mutex

Mutex<T> ensures mutual exclusion and allows mutable access to T behind a read-only interface (another form of interior mutability):

use std::sync::Mutex;

fn main() {
    let v = Mutex::new(vec![10, 20, 30]);
    println!("v: {:?}", v.lock().unwrap());

    {
        let mut guard = v.lock().unwrap();
        guard.push(40);
    }

    println!("v: {:?}", v.lock().unwrap());
}

Notice how we have a impl<T: Send> Sync for Mutex<T> blanket implementation.

• Mutex in Rust looks like a collection with just one element --- the protected data.
  • It is not possible to forget to acquire the mutex before accessing the protected data.
• You can get an &mut T from an &Mutex<T> by taking the lock. The MutexGuard ensures that the &mut T doesn't outlive the lock being held.
• Mutex<T> implements both Send and Sync iff (if and only if) T implements Send.
• A read-write lock counterpart: RwLock.
• Why does lock() return a Result?
  • If the thread that held the Mutex panicked, the Mutex becomes "poisoned" to signal that the data it protected might be in an inconsistent state. Calling lock() on a poisoned mutex fails with a PoisonError. You can call into_inner() on the error to recover the data regardless.

Esempio

Let us see Arc and Mutex in action:

use std::thread;
// use std::sync::{Arc, Mutex};

fn main() {
    let v = vec![10, 20, 30];
    let handle = thread::spawn(|| {
        v.push(10);
    });
    v.push(1000);

    handle.join().unwrap();
    println!("v: {v:?}");
}

Possible solution:

use std::sync::{Arc, Mutex};
use std::thread;

fn main() {
    let v = Arc::new(Mutex::new(vec![10, 20, 30]));

    let v2 = Arc::clone(&v);
    let handle = thread::spawn(move || {
        let mut v2 = v2.lock().unwrap();
        v2.push(10);
    });

    {
        let mut v = v.lock().unwrap();
        v.push(1000);
    }

    handle.join().unwrap();

    println!("v: {v:?}");
}

Notable parts:

• v is wrapped in both Arc and Mutex, because their concerns are orthogonal.
  • Wrapping a Mutex in an Arc is a common pattern to share mutable state between threads.
• v: Arc<_> needs to be cloned as v2 before it can be moved into another thread. Note move was added to the lambda signature.
• Blocks are introduced to narrow the scope of the LockGuard as much as possible.

Esercizi

Let us practice our new concurrency skills with

• Dining philosophers: a classic problem in concurrency.

• Multi-threaded link checker: a larger project where you'll use Cargo to download dependencies and then check links in parallel.

After looking at the exercises, you can look at the solutions provided.

Filosofi a tavola

The dining philosophers problem is a classic problem in concurrency:

Five philosophers dine together at the same table. Each philosopher has their own place at the table. There is a fork between each plate. The dish served is a kind of spaghetti which has to be eaten with two forks. Each philosopher can only alternately think and eat. Moreover, a philosopher can only eat their spaghetti when they have both a left and right fork. Thus two forks will only be available when their two nearest neighbors are thinking, not eating. After an individual philosopher finishes eating, they will put down both forks.

You will need a local Cargo installation for this exercise. Copy the code below to a file called src/main.rs, fill out the blanks, and test that cargo run does not deadlock:

use std::sync::{mpsc, Arc, Mutex};
use std::thread;
use std::time::Duration;

struct Fork;

struct Philosopher {
    name: String,
    // left_fork: ...
    // right_fork: ...
    // thoughts: ...
}

impl Philosopher {
    fn think(&self) {
        self.thoughts
            .send(format!("Eureka! {} has a new idea!", &self.name))
            .unwrap();
    }

    fn eat(&self) {
        // Pick up forks...
        println!("{} is eating...", &self.name);
        thread::sleep(Duration::from_millis(10));
    }
}

static PHILOSOPHERS: &[&str] =
    &["Socrates", "Hypatia", "Plato", "Aristotle", "Pythagoras"];

fn main() {
    // Create forks

    // Create philosophers

    // Make each of them think and eat 100 times

    // Output their thoughts
}

You can use the following Cargo.toml:

[package]
name = "dining-philosophers"
version = "0.1.0"
edition = "2021"

Correttore di Link a Thread multipli

Let us use our new knowledge to create a multi-threaded link checker. It should start at a webpage and check that links on the page are valid. It should recursively check other pages on the same domain and keep doing this until all pages have been validated.

For this, you will need an HTTP client such as reqwest. Create a new Cargo project and reqwest it as a dependency with:

cargo new link-checker
cd link-checker
cargo add --features blocking,rustls-tls reqwest

If cargo add fails with error: no such subcommand, then please edit the Cargo.toml file by hand. Add the dependencies listed below.

You will also need a way to find links. We can use scraper for that:

cargo add scraper

Finally, we'll need some way of handling errors. We use thiserror for that:

cargo add thiserror

The cargo add calls will update the Cargo.toml file to look like this:

[package]
name = "link-checker"
version = "0.1.0"
edition = "2021"
publish = false

[dependencies]
reqwest = { version = "0.11.12", features = ["blocking", "rustls-tls"] }
scraper = "0.13.0"
thiserror = "1.0.37"

You can now download the start page. Try with a small site such as https://www.google.org/.

Your src/main.rs file should look something like this:

use reqwest::blocking::Client;
use reqwest::Url;
use scraper::{Html, Selector};
use thiserror::Error;

#[derive(Error, Debug)]
enum Error {
    #[error("request error: {0}")]
    ReqwestError(#[from] reqwest::Error),
    #[error("bad http response: {0}")]
    BadResponse(String),
}

#[derive(Debug)]
struct CrawlCommand {
    url: Url,
    extract_links: bool,
}

fn visit_page(client: &Client, command: &CrawlCommand) -> Result<Vec<Url>, Error> {
    println!("Checking {:#}", command.url);
    let response = client.get(command.url.clone()).send()?;
    if !response.status().is_success() {
        return Err(Error::BadResponse(response.status().to_string()));
    }

    let mut link_urls = Vec::new();
    if !command.extract_links {
        return Ok(link_urls);
    }

    let base_url = response.url().to_owned();
    let body_text = response.text()?;
    let document = Html::parse_document(&body_text);

    let selector = Selector::parse("a").unwrap();
    let href_values = document
        .select(&selector)
        .filter_map(|element| element.value().attr("href"));
    for href in href_values {
        match base_url.join(href) {
            Ok(link_url) => {
                link_urls.push(link_url);
            }
            Err(err) => {
                println!("On {base_url:#}: ignored unparsable {href:?}: {err}");
            }
        }
    }
    Ok(link_urls)
}

fn main() {
    let client = Client::new();
    let start_url = Url::parse("https://www.google.org").unwrap();
    let crawl_command = CrawlCommand{ url: start_url, extract_links: true };
    match visit_page(&client, &crawl_command) {
        Ok(links) => println!("Links: {links:#?}"),
        Err(err) => println!("Could not extract links: {err:#}"),
    }
}

Run the code in src/main.rs with

cargo run

Compiti (Tasks)

• Use threads to check the links in parallel: send the URLs to be checked to a channel and let a few threads check the URLs in parallel.
• Extend this to recursively extract links from all pages on the www.google.org domain. Put an upper limit of 100 pages or so so that you don't end up being blocked by the site.

Concurrency Morning Exercise

Filosofi a tavola

(back to exercise)

use std::sync::{mpsc, Arc, Mutex};
use std::thread;
use std::time::Duration;

struct Fork;

struct Philosopher {
    name: String,
    left_fork: Arc<Mutex<Fork>>,
    right_fork: Arc<Mutex<Fork>>,
    thoughts: mpsc::SyncSender<String>,
}

impl Philosopher {
    fn think(&self) {
        self.thoughts
            .send(format!("Eureka! {} has a new idea!", &self.name))
            .unwrap();
    }

    fn eat(&self) {
        println!("{} is trying to eat", &self.name);
        let _left = self.left_fork.lock().unwrap();
        let _right = self.right_fork.lock().unwrap();

        println!("{} is eating...", &self.name);
        thread::sleep(Duration::from_millis(10));
    }
}

static PHILOSOPHERS: &[&str] =
    &["Socrates", "Hypatia", "Plato", "Aristotle", "Pythagoras"];

fn main() {
    let (tx, rx) = mpsc::sync_channel(10);

    let forks = (0..PHILOSOPHERS.len())
        .map(|_| Arc::new(Mutex::new(Fork)))
        .collect::<Vec<_>>();

    for i in 0..forks.len() {
        let tx = tx.clone();
        let mut left_fork = Arc::clone(&forks[i]);
        let mut right_fork = Arc::clone(&forks[(i + 1) % forks.len()]);

        // To avoid a deadlock, we have to break the symmetry
        // somewhere. This will swap the forks without deinitializing
        // either of them.
        if i == forks.len() - 1 {
            std::mem::swap(&mut left_fork, &mut right_fork);
        }

        let philosopher = Philosopher {
            name: PHILOSOPHERS[i].to_string(),
            thoughts: tx,
            left_fork,
            right_fork,
        };

        thread::spawn(move || {
            for _ in 0..100 {
                philosopher.eat();
                philosopher.think();
            }
        });
    }

    drop(tx);
    for thought in rx {
        println!("{thought}");
    }
}

Link Checker

(back to exercise)

use std::sync::{mpsc, Arc, Mutex};
use std::thread;

use reqwest::blocking::Client;
use reqwest::Url;
use scraper::{Html, Selector};
use thiserror::Error;

#[derive(Error, Debug)]
enum Error {
    #[error("request error: {0}")]
    ReqwestError(#[from] reqwest::Error),
    #[error("bad http response: {0}")]
    BadResponse(String),
}

#[derive(Debug)]
struct CrawlCommand {
    url: Url,
    extract_links: bool,
}

fn visit_page(client: &Client, command: &CrawlCommand) -> Result<Vec<Url>, Error> {
    println!("Checking {:#}", command.url);
    let response = client.get(command.url.clone()).send()?;
    if !response.status().is_success() {
        return Err(Error::BadResponse(response.status().to_string()));
    }

    let mut link_urls = Vec::new();
    if !command.extract_links {
        return Ok(link_urls);
    }

    let base_url = response.url().to_owned();
    let body_text = response.text()?;
    let document = Html::parse_document(&body_text);

    let selector = Selector::parse("a").unwrap();
    let href_values = document
        .select(&selector)
        .filter_map(|element| element.value().attr("href"));
    for href in href_values {
        match base_url.join(href) {
            Ok(link_url) => {
                link_urls.push(link_url);
            }
            Err(err) => {
                println!("On {base_url:#}: ignored unparsable {href:?}: {err}");
            }
        }
    }
    Ok(link_urls)
}

struct CrawlState {
    domain: String,
    visited_pages: std::collections::HashSet<String>,
}

impl CrawlState {
    fn new(start_url: &Url) -> CrawlState {
        let mut visited_pages = std::collections::HashSet::new();
        visited_pages.insert(start_url.as_str().to_string());
        CrawlState { domain: start_url.domain().unwrap().to_string(), visited_pages }
    }

    /// Determine whether links within the given page should be extracted.
    fn should_extract_links(&self, url: &Url) -> bool {
        let Some(url_domain) = url.domain() else {
            return false;
        };
        url_domain == self.domain
    }

    /// Mark the given page as visited, returning false if it had already
    /// been visited.
    fn mark_visited(&mut self, url: &Url) -> bool {
        self.visited_pages.insert(url.as_str().to_string())
    }
}

type CrawlResult = Result<Vec<Url>, (Url, Error)>;
fn spawn_crawler_threads(
    command_receiver: mpsc::Receiver<CrawlCommand>,
    result_sender: mpsc::Sender<CrawlResult>,
    thread_count: u32,
) {
    let command_receiver = Arc::new(Mutex::new(command_receiver));

    for _ in 0..thread_count {
        let result_sender = result_sender.clone();
        let command_receiver = command_receiver.clone();
        thread::spawn(move || {
            let client = Client::new();
            loop {
                let command_result = {
                    let receiver_guard = command_receiver.lock().unwrap();
                    receiver_guard.recv()
                };
                let Ok(crawl_command) = command_result else {
                    // The sender got dropped. No more commands coming in.
                    break;
                };
                let crawl_result = match visit_page(&client, &crawl_command) {
                    Ok(link_urls) => Ok(link_urls),
                    Err(error) => Err((crawl_command.url, error)),
                };
                result_sender.send(crawl_result).unwrap();
            }
        });
    }
}

fn control_crawl(
    start_url: Url,
    command_sender: mpsc::Sender<CrawlCommand>,
    result_receiver: mpsc::Receiver<CrawlResult>,
) -> Vec<Url> {
    let mut crawl_state = CrawlState::new(&start_url);
    let start_command = CrawlCommand { url: start_url, extract_links: true };
    command_sender.send(start_command).unwrap();
    let mut pending_urls = 1;

    let mut bad_urls = Vec::new();
    while pending_urls > 0 {
        let crawl_result = result_receiver.recv().unwrap();
        pending_urls -= 1;

        match crawl_result {
            Ok(link_urls) => {
                for url in link_urls {
                    if crawl_state.mark_visited(&url) {
                        let extract_links = crawl_state.should_extract_links(&url);
                        let crawl_command = CrawlCommand { url, extract_links };
                        command_sender.send(crawl_command).unwrap();
                        pending_urls += 1;
                    }
                }
            }
            Err((url, error)) => {
                bad_urls.push(url);
                println!("Got crawling error: {:#}", error);
                continue;
            }
        }
    }
    bad_urls
}

fn check_links(start_url: Url) -> Vec<Url> {
    let (result_sender, result_receiver) = mpsc::channel::<CrawlResult>();
    let (command_sender, command_receiver) = mpsc::channel::<CrawlCommand>();
    spawn_crawler_threads(command_receiver, result_sender, 16);
    control_crawl(start_url, command_sender, result_receiver)
}

fn main() {
    let start_url = reqwest::Url::parse("https://www.google.org").unwrap();
    let bad_urls = check_links(start_url);
    println!("Bad URLs: {:#?}", bad_urls);
}

Async Rust

"Async" is a concurrency model where multiple tasks are executed concurrently by executing each task until it would block, then switching to another task that is ready to make progress. The model allows running a larger number of tasks on a limited number of threads. This is because the per-task overhead is typically very low and operating systems provide primitives for efficiently identifying I/O that is able to proceed.

Rust's asynchronous operation is based on "futures", which represent work that may be completed in the future. Futures are "polled" until they signal that they are complete.

Futures are polled by an async runtime, and several different runtimes are available.

Comparazione

• Python has a similar model in its asyncio. However, its Future type is callback-based, and not polled. Async Python programs require a "loop", similar to a runtime in Rust.

• JavaScript's Promise is similar, but again callback-based. The language runtime implements the event loop, so many of the details of Promise resolution are hidden.

async/await

At a high level, async Rust code looks very much like "normal" sequential code:

use futures::executor::block_on;

async fn count_to(count: i32) {
    for i in 1..=count {
        println!("Count is: {i}!");
    }
}

async fn async_main(count: i32) {
    count_to(count).await;
}

fn main() {
    block_on(async_main(10));
}

Punti chiave:

• Note that this is a simplified example to show the syntax. There is no long running operation or any real concurrency in it!

• What is the return type of an async call?

  • Use let future: () = async_main(10); in main to see the type.

• The "async" keyword is syntactic sugar. The compiler replaces the return type with a future.

• You cannot make main async, without additional instructions to the compiler on how to use the returned future.

• You need an executor to run async code. block_on blocks the current thread until the provided future has run to completion.

• .await asynchronously waits for the completion of another operation. Unlike block_on, .await doesn't block the current thread.

• .await can only be used inside an async function (or block; these are introduced later).

Futures

Future is a trait, implemented by objects that represent an operation that may not be complete yet. A future can be polled, and poll returns a Poll.

#![allow(unused)]
fn main() {
use std::pin::Pin;
use std::task::Context;

pub trait Future {
    type Output;
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;
}

pub enum Poll<T> {
    Ready(T),
    Pending,
}
}

An async function returns an impl Future. It's also possible (but uncommon) to implement Future for your own types. For example, the JoinHandle returned from tokio::spawn implements Future to allow joining to it.

The .await keyword, applied to a Future, causes the current async function to pause until that Future is ready, and then evaluates to its output.

• The Future and Poll types are implemented exactly as shown; click the links to show the implementations in the docs.

• We will not get to Pin and Context, as we will focus on writing async code, rather than building new async primitives. Briefly:

  • Context allows a Future to schedule itself to be polled again when an event occurs.

  • Pin ensures that the Future isn't moved in memory, so that pointers into that future remain valid. This is required to allow references to remain valid after an .await.

Esecutori

A runtime provides support for performing operations asynchronously (a reactor) and is responsible for executing futures (an executor). Rust does not have a "built-in" runtime, but several options are available:

• Tokio: performant, with a well-developed ecosystem of functionality like Hyper for HTTP or Tonic for gRPC.
• async-std: aims to be a "std for async", and includes a basic runtime in async::task.
• smol: simple and lightweight

Several larger applications have their own runtimes. For example, Fuchsia already has one.

• Note that of the listed runtimes, only Tokio is supported in the Rust playground. The playground also does not permit any I/O, so most interesting async things can't run in the playground.

• Futures are "inert" in that they do not do anything (not even start an I/O operation) unless there is an executor polling them. This differs from JS Promises, for example, which will run to completion even if they are never used.

Tokio

Tokio provides:

• A multi-threaded runtime for executing asynchronous code.
• An asynchronous version of the standard library.
• A large ecosystem of libraries.

use tokio::time;

async fn count_to(count: i32) {
    for i in 1..=count {
        println!("Count in task: {i}!");
        time::sleep(time::Duration::from_millis(5)).await;
    }
}

#[tokio::main]
async fn main() {
    tokio::spawn(count_to(10));

    for i in 1..5 {
        println!("Main task: {i}");
        time::sleep(time::Duration::from_millis(5)).await;
    }
}

• With the tokio::main macro we can now make main async.

• The spawn function creates a new, concurrent "task".

• Note: spawn takes a Future, you don't call .await on count_to.

Further exploration:

• Why does count_to not (usually) get to 10? This is an example of async cancellation. tokio::spawn returns a handle which can be awaited to wait until it finishes.

• Try count_to(10).await instead of spawning.

• Try awaiting the task returned from tokio::spawn.

Compiti (Tasks)

Rust has a task system, which is a form of lightweight threading.

A task has a single top-level future which the executor polls to make progress. That future may have one or more nested futures that its poll method polls, corresponding loosely to a call stack. Concurrency within a task is possible by polling multiple child futures, such as racing a timer and an I/O operation.

use tokio::io::{self, AsyncReadExt, AsyncWriteExt};
use tokio::net::TcpListener;

#[tokio::main]
async fn main() -> io::Result<()> {
    let listener = TcpListener::bind("127.0.0.1:0").await?;
    println!("listening on port {}", listener.local_addr()?.port());

    loop {
        let (mut socket, addr) = listener.accept().await?;

        println!("connection from {addr:?}");

        tokio::spawn(async move {
            socket.write_all(b"Who are you?\n").await.expect("socket error");

            let mut buf = vec![0; 1024];
            let name_size = socket.read(&mut buf).await.expect("socket error");
            let name = std::str::from_utf8(&buf[..name_size]).unwrap().trim();
            let reply = format!("Thanks for dialing in, {name}!\n");
            socket.write_all(reply.as_bytes()).await.expect("socket error");
        });
    }
}

Copy this example into your prepared src/main.rs and run it from there.

Try connecting to it with a TCP connection tool like nc or telnet.

• Ask students to visualize what the state of the example server would be with a few connected clients. What tasks exist? What are their Futures?

• This is the first time we've seen an async block. This is similar to a closure, but does not take any arguments. Its return value is a Future, similar to an async fn.

• Refactor the async block into a function, and improve the error handling using ?.

Canali asincroni (Async Channels)

Several crates have support for asynchronous channels. For instance tokio:

use tokio::sync::mpsc::{self, Receiver};

async fn ping_handler(mut input: Receiver<()>) {
    let mut count: usize = 0;

    while let Some(_) = input.recv().await {
        count += 1;
        println!("Received {count} pings so far.");
    }

    println!("ping_handler complete");
}

#[tokio::main]
async fn main() {
    let (sender, receiver) = mpsc::channel(32);
    let ping_handler_task = tokio::spawn(ping_handler(receiver));
    for i in 0..10 {
        sender.send(()).await.expect("Failed to send ping.");
        println!("Sent {} pings so far.", i + 1);
    }

    drop(sender);
    ping_handler_task.await.expect("Something went wrong in ping handler task.");
}

• Change the channel size to 3 and see how it affects the execution.

• Overall, the interface is similar to the sync channels as seen in the morning class.

• Try removing the std::mem::drop call. What happens? Why?

• The Flume crate has channels that implement both sync and async send and recv. This can be convenient for complex applications with both IO and heavy CPU processing tasks.

• What makes working with async channels preferable is the ability to combine them with other futures to combine them and create complex control flow.

Futures Control Flow

Futures can be combined together to produce concurrent compute flow graphs. We have already seen tasks, that function as independent threads of execution.

• Join
• Select

Giunzione (Join)

A join operation waits until all of a set of futures are ready, and returns a collection of their results. This is similar to Promise.all in JavaScript or asyncio.gather in Python.

use anyhow::Result;
use futures::future;
use reqwest;
use std::collections::HashMap;

async fn size_of_page(url: &str) -> Result<usize> {
    let resp = reqwest::get(url).await?;
    Ok(resp.text().await?.len())
}

#[tokio::main]
async fn main() {
    let urls: [&str; 4] = [
        "https://google.com",
        "https://httpbin.org/ip",
        "https://play.rust-lang.org/",
        "BAD_URL",
    ];
    let futures_iter = urls.into_iter().map(size_of_page);
    let results = future::join_all(futures_iter).await;
    let page_sizes_dict: HashMap<&str, Result<usize>> =
        urls.into_iter().zip(results.into_iter()).collect();
    println!("{:?}", page_sizes_dict);
}

Copy this example into your prepared src/main.rs and run it from there.

• For multiple futures of disjoint types, you can use std::future::join! but you must know how many futures you will have at compile time. This is currently in the futures crate, soon to be stabilised in std::future.

• The risk of join is that one of the futures may never resolve, this would cause your program to stall.

• You can also combine join_all with join! for instance to join all requests to an http service as well as a database query. Try adding a tokio::time::sleep to the future, using futures::join!. This is not a timeout (that requires select!, explained in the next chapter), but demonstrates join!.

Selezione (Select)

A select operation waits until any of a set of futures is ready, and responds to that future's result. In JavaScript, this is similar to Promise.race. In Python, it compares to asyncio.wait(task_set, return_when=asyncio.FIRST_COMPLETED).

Similar to a match statement, the body of select! has a number of arms, each of the form pattern = future => statement. When a future is ready, its return value is destructured by the pattern. The statement is then run with the resulting variables. The statement result becomes the result of the select! macro.

use tokio::sync::mpsc::{self, Receiver};
use tokio::time::{sleep, Duration};

#[derive(Debug, PartialEq)]
enum Animal {
    Cat { name: String },
    Dog { name: String },
}

async fn first_animal_to_finish_race(
    mut cat_rcv: Receiver<String>,
    mut dog_rcv: Receiver<String>,
) -> Option<Animal> {
    tokio::select! {
        cat_name = cat_rcv.recv() => Some(Animal::Cat { name: cat_name? }),
        dog_name = dog_rcv.recv() => Some(Animal::Dog { name: dog_name? })
    }
}

#[tokio::main]
async fn main() {
    let (cat_sender, cat_receiver) = mpsc::channel(32);
    let (dog_sender, dog_receiver) = mpsc::channel(32);
    tokio::spawn(async move {
        sleep(Duration::from_millis(500)).await;
        cat_sender.send(String::from("Felix")).await.expect("Failed to send cat.");
    });
    tokio::spawn(async move {
        sleep(Duration::from_millis(50)).await;
        dog_sender.send(String::from("Rex")).await.expect("Failed to send dog.");
    });

    let winner = first_animal_to_finish_race(cat_receiver, dog_receiver)
        .await
        .expect("Failed to receive winner");

    println!("Winner is {winner:?}");
}

• In this example, we have a race between a cat and a dog. first_animal_to_finish_race listens to both channels and will pick whichever arrives first. Since the dog takes 50ms, it wins against the cat that take 500ms.

• You can use oneshot channels in this example as the channels are supposed to receive only one send.

• Try adding a deadline to the race, demonstrating selecting different sorts of futures.

• Note that select! drops unmatched branches, which cancels their futures. It is easiest to use when every execution of select! creates new futures.

  • An alternative is to pass &mut future instead of the future itself, but this can lead to issues, further discussed in the pinning slide.

Pitfalls of async/await

Async / await provides convenient and efficient abstraction for concurrent asynchronous programming. However, the async/await model in Rust also comes with its share of pitfalls and footguns. We illustrate some of them in this chapter:

• Blocking the Executor
• Pin
• Async Traits
• Cancellation

Blocking the executor

Most async runtimes only allow IO tasks to run concurrently. This means that CPU blocking tasks will block the executor and prevent other tasks from being executed. An easy workaround is to use async equivalent methods where possible.

use futures::future::join_all;
use std::time::Instant;

async fn sleep_ms(start: &Instant, id: u64, duration_ms: u64) {
    std::thread::sleep(std::time::Duration::from_millis(duration_ms));
    println!(
        "future {id} slept for {duration_ms}ms, finished after {}ms",
        start.elapsed().as_millis()
    );
}

#[tokio::main(flavor = "current_thread")]
async fn main() {
    let start = Instant::now();
    let sleep_futures = (1..=10).map(|t| sleep_ms(&start, t, t * 10));
    join_all(sleep_futures).await;
}

• Run the code and see that the sleeps happen consecutively rather than concurrently.

• The "current_thread" flavor puts all tasks on a single thread. This makes the effect more obvious, but the bug is still present in the multi-threaded flavor.

• Switch the std::thread::sleep to tokio::time::sleep and await its result.

• Another fix would be to tokio::task::spawn_blocking which spawns an actual thread and transforms its handle into a future without blocking the executor.

• You should not think of tasks as OS threads. They do not map 1 to 1 and most executors will allow many tasks to run on a single OS thread. This is particularly problematic when interacting with other libraries via FFI, where that library might depend on thread-local storage or map to specific OS threads (e.g., CUDA). Prefer tokio::task::spawn_blocking in such situations.

• Use sync mutexes with care. Holding a mutex over an .await may cause another task to block, and that task may be running on the same thread.

Pin

Async blocks and functions return types implementing the Future trait. The type returned is the result of a compiler transformation which turns local variables into data stored inside the future.

Some of those variables can hold pointers to other local variables. Because of that, the future should never be moved to a different memory location, as it would invalidate those pointers.

To prevent moving the future type in memory, it can only be polled through a pinned pointer. Pin is a wrapper around a reference that disallows all operations that would move the instance it points to into a different memory location.

use tokio::sync::{mpsc, oneshot};
use tokio::task::spawn;
use tokio::time::{sleep, Duration};

// A work item. In this case, just sleep for the given time and respond
// with a message on the `respond_on` channel.
#[derive(Debug)]
struct Work {
    input: u32,
    respond_on: oneshot::Sender<u32>,
}

// A worker which listens for work on a queue and performs it.
async fn worker(mut work_queue: mpsc::Receiver<Work>) {
    let mut iterations = 0;
    loop {
        tokio::select! {
            Some(work) = work_queue.recv() => {
                sleep(Duration::from_millis(10)).await; // Pretend to work.
                work.respond_on
                    .send(work.input * 1000)
                    .expect("failed to send response");
                iterations += 1;
            }
            // TODO: report number of iterations every 100ms
        }
    }
}

// A requester which requests work and waits for it to complete.
async fn do_work(work_queue: &mpsc::Sender<Work>, input: u32) -> u32 {
    let (tx, rx) = oneshot::channel();
    work_queue
        .send(Work { input, respond_on: tx })
        .await
        .expect("failed to send on work queue");
    rx.await.expect("failed waiting for response")
}

#[tokio::main]
async fn main() {
    let (tx, rx) = mpsc::channel(10);
    spawn(worker(rx));
    for i in 0..100 {
        let resp = do_work(&tx, i).await;
        println!("work result for iteration {i}: {resp}");
    }
}

• You may recognize this as an example of the actor pattern. Actors typically call select! in a loop.

• This serves as a summation of a few of the previous lessons, so take your time with it.

  • Naively add a _ = sleep(Duration::from_millis(100)) => { println!(..) } to the select!. This will never execute. Why?

  • Instead, add a timeout_fut containing that future outside of the loop:

    #![allow(unused)]
    fn main() {
    let mut timeout_fut = sleep(Duration::from_millis(100));
    loop {
        select! {
            ..,
            _ = timeout_fut => { println!(..); },
        }
    }
    }

  • This still doesn't work. Follow the compiler errors, adding &mut to the timeout_fut in the select! to work around the move, then using Box::pin:

    #![allow(unused)]
    fn main() {
    let mut timeout_fut = Box::pin(sleep(Duration::from_millis(100)));
    loop {
        select! {
            ..,
            _ = &mut timeout_fut => { println!(..); },
        }
    }
    }

  • This compiles, but once the timeout expires it is Poll::Ready on every iteration (a fused future would help with this). Update to reset timeout_fut every time it expires.

• Box allocates on the heap. In some cases, std::pin::pin! (only recently stabilized, with older code often using tokio::pin!) is also an option, but that is difficult to use for a future that is reassigned.

• Another alternative is to not use pin at all but spawn another task that will send to a oneshot channel every 100ms.

• Data that contains pointers to itself is called self-referential. Normally, the Rust borrow checker would prevent self-referential data from being moved, as the references cannot outlive the data they point to. However, the code transformation for async blocks and functions is not verified by the borrow checker.

• Pin is a wrapper around a reference. An object cannot be moved from its place using a pinned pointer. However, it can still be moved through an unpinned pointer.

• The poll method of the Future trait uses Pin<&mut Self> instead of &mut Self to refer to the instance. That's why it can only be called on a pinned pointer.

Trait asincroni (Async Trait)

Async methods in traits are not yet supported in the stable channel (An experimental feature exists in nightly and should be stabilized in the mid term.)

The crate async_trait provides a workaround through a macro:

use async_trait::async_trait;
use std::time::Instant;
use tokio::time::{sleep, Duration};

#[async_trait]
trait Sleeper {
    async fn sleep(&self);
}

struct FixedSleeper {
    sleep_ms: u64,
}

#[async_trait]
impl Sleeper for FixedSleeper {
    async fn sleep(&self) {
        sleep(Duration::from_millis(self.sleep_ms)).await;
    }
}

async fn run_all_sleepers_multiple_times(
    sleepers: Vec<Box<dyn Sleeper>>,
    n_times: usize,
) {
    for _ in 0..n_times {
        println!("running all sleepers..");
        for sleeper in &sleepers {
            let start = Instant::now();
            sleeper.sleep().await;
            println!("slept for {}ms", start.elapsed().as_millis());
        }
    }
}

#[tokio::main]
async fn main() {
    let sleepers: Vec<Box<dyn Sleeper>> = vec![
        Box::new(FixedSleeper { sleep_ms: 50 }),
        Box::new(FixedSleeper { sleep_ms: 100 }),
    ];
    run_all_sleepers_multiple_times(sleepers, 5).await;
}

• async_trait is easy to use, but note that it's using heap allocations to achieve this. This heap allocation has performance overhead.

• The challenges in language support for async trait are deep Rust and probably not worth describing in-depth. Niko Matsakis did a good job of explaining them in this post if you are interested in digging deeper.

• Try creating a new sleeper struct that will sleep for a random amount of time and adding it to the Vec.

Cancellazione (cancellation)

Dropping a future implies it can never be polled again. This is called cancellation and it can occur at any await point. Care is needed to ensure the system works correctly even when futures are cancelled. For example, it shouldn't deadlock or lose data.

use std::io::{self, ErrorKind};
use std::time::Duration;
use tokio::io::{AsyncReadExt, AsyncWriteExt, DuplexStream};

struct LinesReader {
    stream: DuplexStream,
}

impl LinesReader {
    fn new(stream: DuplexStream) -> Self {
        Self { stream }
    }

    async fn next(&mut self) -> io::Result<Option<String>> {
        let mut bytes = Vec::new();
        let mut buf = [0];
        while self.stream.read(&mut buf[..]).await? != 0 {
            bytes.push(buf[0]);
            if buf[0] == b'\n' {
                break;
            }
        }
        if bytes.is_empty() {
            return Ok(None);
        }
        let s = String::from_utf8(bytes)
            .map_err(|_| io::Error::new(ErrorKind::InvalidData, "not UTF-8"))?;
        Ok(Some(s))
    }
}

async fn slow_copy(source: String, mut dest: DuplexStream) -> std::io::Result<()> {
    for b in source.bytes() {
        dest.write_u8(b).await?;
        tokio::time::sleep(Duration::from_millis(10)).await
    }
    Ok(())
}

#[tokio::main]
async fn main() -> std::io::Result<()> {
    let (client, server) = tokio::io::duplex(5);
    let handle = tokio::spawn(slow_copy("hi\nthere\n".to_owned(), client));

    let mut lines = LinesReader::new(server);
    let mut interval = tokio::time::interval(Duration::from_millis(60));
    loop {
        tokio::select! {
            _ = interval.tick() => println!("tick!"),
            line = lines.next() => if let Some(l) = line? {
                print!("{}", l)
            } else {
                break
            },
        }
    }
    handle.await.unwrap()?;
    Ok(())
}

• The compiler doesn't help with cancellation-safety. You need to read API documentation and consider what state your async fn holds.

• Unlike panic and ?, cancellation is part of normal control flow (vs error-handling).

• The example loses parts of the string.

  • Whenever the tick() branch finishes first, next() and its buf are dropped.

  • LinesReader can be made cancellation-safe by making buf part of the struct:

    #![allow(unused)]
    fn main() {
    struct LinesReader {
        stream: DuplexStream,
        bytes: Vec<u8>,
        buf: [u8; 1],
    }
    
    impl LinesReader {
        fn new(stream: DuplexStream) -> Self {
            Self { stream, bytes: Vec::new(), buf: [0] }
        }
        async fn next(&mut self) -> io::Result<Option<String>> {
            // prefix buf and bytes with self.
            // ...
            let raw = std::mem::take(&mut self.bytes);
            let s = String::from_utf8(raw)
            // ...
        }
    }
    }

• Interval::tick is cancellation-safe because it keeps track of whether a tick has been 'delivered'.

• AsyncReadExt::read is cancellation-safe because it either returns or doesn't read data.

• AsyncBufReadExt::read_line is similar to the example and isn't cancellation-safe. See its documentation for details and alternatives.

Esercizi

To practice your Async Rust skills, we have again two exercises for you:

• Dining philosophers: we already saw this problem in the morning. This time you are going to implement it with Async Rust.

• A Broadcast Chat Application: this is a larger project that allows you experiment with more advanced Async Rust features.

After looking at the exercises, you can look at the solutions provided.

Dining Philosophers --- Async

See dining philosophers for a description of the problem.

As before, you will need a local Cargo installation for this exercise. Copy the code below to a file called src/main.rs, fill out the blanks, and test that cargo run does not deadlock:

use std::sync::Arc;
use tokio::sync::mpsc::{self, Sender};
use tokio::sync::Mutex;
use tokio::time;

struct Fork;

struct Philosopher {
    name: String,
    // left_fork: ...
    // right_fork: ...
    // thoughts: ...
}

impl Philosopher {
    async fn think(&self) {
        self.thoughts
            .send(format!("Eureka! {} has a new idea!", &self.name))
            .await
            .unwrap();
    }

    async fn eat(&self) {
        // Pick up forks...
        println!("{} is eating...", &self.name);
        time::sleep(time::Duration::from_millis(5)).await;
    }
}

static PHILOSOPHERS: &[&str] =
    &["Socrates", "Hypatia", "Plato", "Aristotle", "Pythagoras"];

#[tokio::main]
async fn main() {
    // Create forks

    // Create philosophers

    // Make them think and eat

    // Output their thoughts
}

Since this time you are using Async Rust, you'll need a tokio dependency. You can use the following Cargo.toml:

[package]
name = "dining-philosophers-async-dine"
version = "0.1.0"
edition = "2021"

[dependencies]
tokio = { version = "1.26.0", features = ["sync", "time", "macros", "rt-multi-thread"] }

Also note that this time you have to use the Mutex and the mpsc module from the tokio crate.

• Can you make your implementation single-threaded?

Applicazione Chat-Broadcast

In this exercise, we want to use our new knowledge to implement a broadcast chat application. We have a chat server that the clients connect to and publish their messages. The client reads user messages from the standard input, and sends them to the server. The chat server broadcasts each message that it receives to all the clients.

For this, we use a broadcast channel on the server, and tokio_websockets for the communication between the client and the server.

Create a new Cargo project and add the following dependencies:

Cargo.toml:

[package]
name = "chat-async"
version = "0.1.0"
edition = "2021"

[dependencies]
futures-util = { version = "0.3.30", features = ["sink"] }
http = "1.0.0"
tokio = { version = "1.28.1", features = ["full"] }
tokio-websockets = { version = "0.5.1", features = ["client", "fastrand", "server", "sha1_smol"] }

The required APIs

You are going to need the following functions from tokio and tokio_websockets. Spend a few minutes to familiarize yourself with the API.

• StreamExt::next() implemented by WebSocketStream: for asynchronously reading messages from a Websocket Stream.
• SinkExt::send() implemented by WebSocketStream: for asynchronously sending messages on a Websocket Stream.
• Lines::next_line(): for asynchronously reading user messages from the standard input.
• Sender::subscribe(): for subscribing to a broadcast channel.

Two binaries

Normally in a Cargo project, you can have only one binary, and one src/main.rs file. In this project, we need two binaries. One for the client, and one for the server. You could potentially make them two separate Cargo projects, but we are going to put them in a single Cargo project with two binaries. For this to work, the client and the server code should go under src/bin (see the documentation).

Copy the following server and client code into src/bin/server.rs and src/bin/client.rs, respectively. Your task is to complete these files as described below.

src/bin/server.rs:

use futures_util::sink::SinkExt;
use futures_util::stream::StreamExt;
use std::error::Error;
use std::net::SocketAddr;
use tokio::net::{TcpListener, TcpStream};
use tokio::sync::broadcast::{channel, Sender};
use tokio_websockets::{Message, ServerBuilder, WebSocketStream};

async fn handle_connection(
    addr: SocketAddr,
    mut ws_stream: WebSocketStream<TcpStream>,
    bcast_tx: Sender<String>,
) -> Result<(), Box<dyn Error + Send + Sync>> {

    // TODO: For a hint, see the description of the task below.

}

#[tokio::main]
async fn main() -> Result<(), Box<dyn Error + Send + Sync>> {
    let (bcast_tx, _) = channel(16);

    let listener = TcpListener::bind("127.0.0.1:2000").await?;
    println!("listening on port 2000");

    loop {
        let (socket, addr) = listener.accept().await?;
        println!("New connection from {addr:?}");
        let bcast_tx = bcast_tx.clone();
        tokio::spawn(async move {
            // Wrap the raw TCP stream into a websocket.
            let ws_stream = ServerBuilder::new().accept(socket).await?;

            handle_connection(addr, ws_stream, bcast_tx).await
        });
    }
}

src/bin/client.rs:

use futures_util::stream::StreamExt;
use futures_util::SinkExt;
use http::Uri;
use tokio::io::{AsyncBufReadExt, BufReader};
use tokio_websockets::{ClientBuilder, Message};

#[tokio::main]
async fn main() -> Result<(), tokio_websockets::Error> {
    let (mut ws_stream, _) =
        ClientBuilder::from_uri(Uri::from_static("ws://127.0.0.1:2000"))
            .connect()
            .await?;

    let stdin = tokio::io::stdin();
    let mut stdin = BufReader::new(stdin).lines();


    // TODO: For a hint, see the description of the task below.

}

Running the binaries

Run the server with:

cargo run --bin server

and the client with:

cargo run --bin client

Compiti (Tasks)

• Implement the handle_connection function in src/bin/server.rs.
  • Hint: Use tokio::select! for concurrently performing two tasks in a continuous loop. One task receives messages from the client and broadcasts them. The other sends messages received by the server to the client.
• Complete the main function in src/bin/client.rs.
  • Hint: As before, use tokio::select! in a continuous loop for concurrently performing two tasks: (1) reading user messages from standard input and sending them to the server, and (2) receiving messages from the server, and displaying them for the user.
• Optional: Once you are done, change the code to broadcast messages to all clients, but the sender of the message.

Concurrency Afternoon Exercise

Dining Philosophers --- Async

(back to exercise)

use std::sync::Arc;
use tokio::sync::mpsc::{self, Sender};
use tokio::sync::Mutex;
use tokio::time;

struct Fork;

struct Philosopher {
    name: String,
    left_fork: Arc<Mutex<Fork>>,
    right_fork: Arc<Mutex<Fork>>,
    thoughts: Sender<String>,
}

impl Philosopher {
    async fn think(&self) {
        self.thoughts
            .send(format!("Eureka! {} has a new idea!", &self.name))
            .await
            .unwrap();
    }

    async fn eat(&self) {
        // Pick up forks...
        let _first_lock = self.left_fork.lock().await;
        // Add a delay before picking the second fork to allow the execution
        // to transfer to another task
        time::sleep(time::Duration::from_millis(1)).await;
        let _second_lock = self.right_fork.lock().await;

        println!("{} is eating...", &self.name);
        time::sleep(time::Duration::from_millis(5)).await;

        // The locks are dropped here
    }
}

static PHILOSOPHERS: &[&str] =
    &["Socrates", "Hypatia", "Plato", "Aristotle", "Pythagoras"];

#[tokio::main]
async fn main() {
    // Create forks
    let mut forks = vec![];
    (0..PHILOSOPHERS.len()).for_each(|_| forks.push(Arc::new(Mutex::new(Fork))));

    // Create philosophers
    let (philosophers, mut rx) = {
        let mut philosophers = vec![];
        let (tx, rx) = mpsc::channel(10);
        for (i, name) in PHILOSOPHERS.iter().enumerate() {
            let left_fork = Arc::clone(&forks[i]);
            let right_fork = Arc::clone(&forks[(i + 1) % PHILOSOPHERS.len()]);
            // To avoid a deadlock, we have to break the symmetry
            // somewhere. This will swap the forks without deinitializing
            // either of them.
            if i == 0 {
                std::mem::swap(&mut left_fork, &mut right_fork);
            }
            philosophers.push(Philosopher {
                name: name.to_string(),
                left_fork,
                right_fork,
                thoughts: tx.clone(),
            });
        }
        (philosophers, rx)
        // tx is dropped here, so we don't need to explicitly drop it later
    };

    // Make them think and eat
    for phil in philosophers {
        tokio::spawn(async move {
            for _ in 0..100 {
                phil.think().await;
                phil.eat().await;
            }
        });
    }

    // Output their thoughts
    while let Some(thought) = rx.recv().await {
        println!("Here is a thought: {thought}");
    }
}

Applicazione Chat-Broadcast

(back to exercise)

src/bin/server.rs:

use futures_util::sink::SinkExt;
use futures_util::stream::StreamExt;
use std::error::Error;
use std::net::SocketAddr;
use tokio::net::{TcpListener, TcpStream};
use tokio::sync::broadcast::{channel, Sender};
use tokio_websockets::{Message, ServerBuilder, WebSocketStream};

async fn handle_connection(
    addr: SocketAddr,
    mut ws_stream: WebSocketStream<TcpStream>,
    bcast_tx: Sender<String>,
) -> Result<(), Box<dyn Error + Send + Sync>> {

    ws_stream
        .send(Message::text("Welcome to chat! Type a message".to_string()))
        .await?;
    let mut bcast_rx = bcast_tx.subscribe();

    // A continuous loop for concurrently performing two tasks: (1) receiving
    // messages from `ws_stream` and broadcasting them, and (2) receiving
    // messages on `bcast_rx` and sending them to the client.
    loop {
        tokio::select! {
            incoming = ws_stream.next() => {
                match incoming {
                    Some(Ok(msg)) => {
                        if let Some(text) = msg.as_text() {
                            println!("From client {addr:?} {text:?}");
                            bcast_tx.send(text.into())?;
                        }
                    }
                    Some(Err(err)) => return Err(err.into()),
                    None => return Ok(()),
                }
            }
            msg = bcast_rx.recv() => {
                ws_stream.send(Message::text(msg?)).await?;
            }
        }
    }
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn Error + Send + Sync>> {
    let (bcast_tx, _) = channel(16);

    let listener = TcpListener::bind("127.0.0.1:2000").await?;
    println!("listening on port 2000");

    loop {
        let (socket, addr) = listener.accept().await?;
        println!("New connection from {addr:?}");
        let bcast_tx = bcast_tx.clone();
        tokio::spawn(async move {
            // Wrap the raw TCP stream into a websocket.
            let ws_stream = ServerBuilder::new().accept(socket).await?;

            handle_connection(addr, ws_stream, bcast_tx).await
        });
    }
}

src/bin/client.rs:

use futures_util::stream::StreamExt;
use futures_util::SinkExt;
use http::Uri;
use tokio::io::{AsyncBufReadExt, BufReader};
use tokio_websockets::{ClientBuilder, Message};

#[tokio::main]
async fn main() -> Result<(), tokio_websockets::Error> {
    let (mut ws_stream, _) =
        ClientBuilder::from_uri(Uri::from_static("ws://127.0.0.1:2000"))
            .connect()
            .await?;

    let stdin = tokio::io::stdin();
    let mut stdin = BufReader::new(stdin).lines();

    // Continuous loop for concurrently sending and receiving messages.
    loop {
        tokio::select! {
            incoming = ws_stream.next() => {
                match incoming {
                    Some(Ok(msg)) => {
                        if let Some(text) = msg.as_text() {
                            println!("From server: {}", text);
                        }
                    },
                    Some(Err(err)) => return Err(err.into()),
                    None => return Ok(()),
                }
            }
            res = stdin.next_line() => {
                match res {
                    Ok(None) => return Ok(()),
                    Ok(Some(line)) => ws_stream.send(Message::text(line.to_string())).await?,
                    Err(err) => return Err(err.into()),
                }
            }

        }
    }
}

Grazie!

Thank you for taking Comprehensive Rust 🦀! We hope you enjoyed it and that it was useful.

We've had a lot of fun putting the course together. The course is not perfect, so if you spotted any mistakes or have ideas for improvements, please get in contact with us on GitHub. We would love to hear from you.

Glossario

The following is a glossary which aims to give a short definition of many Rust terms. For translations, this also serves to connect the term back to the English original.

• allocate:
  Dynamic memory allocation on the heap.
• argument:
  Information that is passed into a function or method.
• Bare-metal Rust:
  Low-level Rust development, often deployed to a system without an operating system. See Bare-metal Rust.
• block:
  See Blocks and scope.
• borrow:
  See Borrowing.
• borrow checker:
  The part of the Rust compiler which checks that all borrows are valid.
• brace:
  { and }. Also called curly brace, they delimit blocks.
• build:
  The process of converting source code into executable code or a usable program.
• call:
  To invoke or execute a function or method.
• channel:
  Used to safely pass messages between threads.
• Comprehensive Rust 🦀:
  The courses here are jointly called Comprehensive Rust 🦀.
• concurrency:
  The execution of multiple tasks or processes at the same time.
• Concurrency in Rust:
  See Concurrency in Rust.
• constant:
  A value that does not change during the execution of a program.
• control flow:
  The order in which the individual statements or instructions are executed in a program.
• crash:
  An unexpected and unhandled failure or termination of a program.
• enumeration:
  A data type that holds one of several named constants, possibly with an associated tuple or struct.
• error:
  An unexpected condition or result that deviates from the expected behavior.
• error handling:
  The process of managing and responding to errors that occur during program execution.
• exercise:
  A task or problem designed to practice and test programming skills.
• function:
  A reusable block of code that performs a specific task.
• garbage collector:
  A mechanism that automatically frees up memory occupied by objects that are no longer in use.
• generics:
  A feature that allows writing code with placeholders for types, enabling code reuse with different data types.
• immutable:
  Unable to be changed after creation.
• integration test:
  A type of test that verifies the interactions between different parts or components of a system.
• keyword:
  A reserved word in a programming language that has a specific meaning and cannot be used as an identifier.
• library:
  A collection of precompiled routines or code that can be used by programs.
• macro:
  Rust macros can be recognized by a ! in the name. Macros are used when normal functions are not enough. A typical example is format!, which takes a variable number of arguments, which isn't supported by Rust functions.
• main function:
  Rust programs start executing with the main function.
• match:
  A control flow construct in Rust that allows for pattern matching on the value of an expression.
• memory leak:
  A situation where a program fails to release memory that is no longer needed, leading to a gradual increase in memory usage.
• method:
  A function associated with an object or a type in Rust.
• module:
  A namespace that contains definitions, such as functions, types, or traits, to organize code in Rust.
• move:
  The transfer of ownership of a value from one variable to another in Rust.
• mutable:
  A property in Rust that allows variables to be modified after they have been declared.
• ownership:
  The concept in Rust that defines which part of the code is responsible for managing the memory associated with a value.
• panic:
  An unrecoverable error condition in Rust that results in the termination of the program.
• parameter:
  A value that is passed into a function or method when it is called.
• pattern:
  A combination of values, literals, or structures that can be matched against an expression in Rust.
• payload:
  The data or information carried by a message, event, or data structure.
• program:
  A set of instructions that a computer can execute to perform a specific task or solve a particular problem.
• programming language:
  A formal system used to communicate instructions to a computer, such as Rust.
• receiver:
  The first parameter in a Rust method that represents the instance on which the method is called.
• reference counting:
  A memory management technique in which the number of references to an object is tracked, and the object is deallocated when the count reaches zero.
• return:
  A keyword in Rust used to indicate the value to be returned from a function.
• Rust:
  A systems programming language that focuses on safety, performance, and concurrency.
• Fondamenti di Rust:
  Giorni da 1 a 3 di questo corso.
• Rust in Android:
  See Rust in Android.
• Rust in Chromium:
  See Rust in Chromium.
• safe:
  Refers to code that adheres to Rust's ownership and borrowing rules, preventing memory-related errors.
• scope:
  The region of a program where a variable is valid and can be used.
• standard library:
  A collection of modules providing essential functionality in Rust.
• static:
  A keyword in Rust used to define static variables or items with a 'static lifetime.
• string:
  A data type storing textual data. See String vs str for more.
• struct:
  A composite data type in Rust that groups together variables of different types under a single name.
• test:
  A Rust module containing functions that test the correctness of other functions.
• thread:
  A separate sequence of execution in a program, allowing concurrent execution.
• thread safety:
  The property of a program that ensures correct behavior in a multithreaded environment.
• trait:
  A collection of methods defined for an unknown type, providing a way to achieve polymorphism in Rust.
• trait bound:
  An abstraction where you can require types to implement some traits of your interest.
• tuple:
  A composite data type that contains variables of different types. Tuple fields have no names, and are accessed by their ordinal numbers.
• type:
  A classification that specifies which operations can be performed on values of a particular kind in Rust.
• type inference:
  The ability of the Rust compiler to deduce the type of a variable or expression.
• undefined behavior:
  Actions or conditions in Rust that have no specified result, often leading to unpredictable program behavior.
• union:
  A data type that can hold values of different types but only one at a time.
• unit test:
  Rust comes with built-in support for running small unit tests and larger integration tests. See Unit Tests.
• unit type:
  Type that holds no data, written as a tuple with no members.
• unsafe:
  The subset of Rust which allows you to trigger undefined behavior. See Unsafe Rust.
• variable:
  A memory location storing data. Variables are valid in a scope.

Other Rust Resources

The Rust community has created a wealth of high-quality and free resources online.

Official Documentation

The Rust project hosts many resources. These cover Rust in general:

• The Rust Programming Language: the canonical free book about Rust. Covers the language in detail and includes a few projects for people to build.
• Rust By Example: covers the Rust syntax via a series of examples which showcase different constructs. Sometimes includes small exercises where you are asked to expand on the code in the examples.
• Rust Standard Library: full documentation of the standard library for Rust.
• The Rust Reference: an incomplete book which describes the Rust grammar and memory model.

More specialized guides hosted on the official Rust site:

• The Rustonomicon: covers unsafe Rust, including working with raw pointers and interfacing with other languages (FFI).
• Asynchronous Programming in Rust: covers the new asynchronous programming model which was introduced after the Rust Book was written.
• The Embedded Rust Book: an introduction to using Rust on embedded devices without an operating system.

Unofficial Learning Material

A small selection of other guides and tutorial for Rust:

• Learn Rust the Dangerous Way: covers Rust from the perspective of low-level C programmers.
• Rust for Embedded C Programmers: covers Rust from the perspective of developers who write firmware in C.
• Rust for professionals: covers the syntax of Rust using side-by-side comparisons with other languages such as C, C++, Java, JavaScript, and Python.
• Rust on Exercism: 100+ exercises to help you learn Rust.
• Ferrous Teaching Material: a series of small presentations covering both basic and advanced part of the Rust language. Other topics such as WebAssembly, and async/await are also covered.
• Beginner's Series to Rust and Take your first steps with Rust: two Rust guides aimed at new developers. The first is a set of 35 videos and the second is a set of 11 modules which covers Rust syntax and basic constructs.
• Learn Rust With Entirely Too Many Linked Lists: in-depth exploration of Rust's memory management rules, through implementing a few different types of list structures.

Please see the Little Book of Rust Books for even more Rust books.

Crediti

The material here builds on top of the many great sources of Rust documentation. See the page on other resources for a full list of useful resources.

The material of Comprehensive Rust is licensed under the terms of the Apache 2.0 license, please see LICENSE for details.

Rust by Example

Some examples and exercises have been copied and adapted from Rust by Example. Please see the third_party/rust-by-example/ directory for details, including the license terms.

Rust on Exercism

Some exercises have been copied and adapted from Rust on Exercism. Please see the third_party/rust-on-exercism/ directory for details, including the license terms.

CXX

The Interoperability with C++ section uses an image from CXX. Please see the third_party/cxx/ directory for details, including the license terms.