Determining Preconditions

Where do you find the safety preconditions?

fn main() {
    let b: *mut i32 = std::ptr::null_mut();
    println!("{:?}", b.as_mut());
}
This slide and its sub-slides should take about 10 minutes.

Attempt to compile the program to trigger the compiler error (“error[E0133]: call to unsafe function …”).

Ask: “Where would you look if you wanted to know the preconditions for a function? Here we need to understand when it’s safe to convert from a null pointer to a mutable reference.”

Locations to look:

  • A function’s API documentation, especially its safety section
  • The source code and its internal safety comments
  • Module documentation
  • Rust Reference

Consult the documentation for the as_mut method.

Highlight Safety section.

Safety

When calling this method, you have to ensure that either the pointer is null or the pointer is convertible to a reference.

Click the “convertible to a reference” hyperlink to the “Pointer to reference conversion”

Track down the rules for converting a pointer to a reference, i.e., whether it is “dereferenceable”.

Consider the implications of this excerpt (Rust 1.90.0) “You must enforce Rust’s aliasing rules. The exact aliasing rules are not decided yet, …”