Return Expressions

A return expression exits the current function and provides its return value.

return_expr = "return" expression? ;

If the expression is omitted, it is equivalent to return ().

The expression following return (or the implicit ()) MUST have a type compatible with the function's declared return type.

A return expression has the never type ! because it never produces a local value.

fn abs(x: i32) -> i32 {
    if x < 0 {
        return 0 - x;
    }
    x
}

fn main() -> i32 {
    abs(-5)  // 5
}

When a return expression is evaluated, the function immediately returns the value of the expression. No further code in the function is executed.

Because return has type !, it can appear in contexts that expect any type.

fn test(x: i32) -> i32 {
    // `return 100` has type !, which coerces to i32
    let y = if x > 5 { return 100 } else { x };
    y * 2
}

fn main() -> i32 {
    test(3) + test(10)  // 6 + 100 = 106
}

fn do_nothing() {
    return;  // equivalent to return ()
}

fn explicit_return() {
    return ();  // explicit unit return
}

fn main() -> i32 {
    do_nothing();
    explicit_return();
    0
}