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A block in Rust contains a sequence of expressions. Each block has a value and a type, which are those of 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}");
}

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

同样的规则也适用于函数：函数主体的值 是返回值：

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

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