Blocks and Scopes


A block in Rust contains a sequence of expressions, enclosed by braces {}. Each block has a value and a type, which are those of the last expression of the block:

fn main() {
    let z = 13;
    let x = {
        let y = 10;
        println!("y: {y}");
        z - y
    println!("x: {x}");

If the last expression ends with ;, then the resulting value and type is ().

GĂŒltigkeitsbereiche und Verschattungen

A variable’s scope is limited to the enclosing block.

Sie können Variablen verschatten (shadow), sowohl solche aus Ă€ußeren GĂŒltigkeitsbereichen (outer scopes) als auch Variablen aus dem gleichen GĂŒltigkeitsbereich:

fn main() {
    let a = 10;
    println!("before: {a}");
        let a = "hello";
        println!("inner scope: {a}");

        let a = true;
        println!("shadowed in inner scope: {a}");

    println!("after: {a}");
This slide should take about 10 minutes.
  • 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 sieht zunĂ€chst obskur aus, ist aber praktisch, um Werte nach .unwrap() festzuhalten.