A block in Rust has a value and a type: the value is the last expression of the block:

fn main() {
    let x = {
        let y = 10;
        println!("y: {y}");
        let z = {
            let w = {
                3 + 4
            println!("w: {w}");
            y * w
        println!("z: {z}");
        z - y
    println!("x: {x}");

The same rule is used for functions: the value of the function body is the return value:

fn double(x: i32) -> i32 {
    x + x

fn main() {
    println!("doubled: {}", double(7));

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

Key Points:

  • The point of this slide is to show that blocks have a type and value in Rust.
  • 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.