Macro fallthrough::fallthrough

macro_rules! fallthrough {
    ($scrutinee:expr $(,)?) => { ... };
    ($scrutinee:expr, $first_pat:pat $(if $first_guard:expr)? => $first_body:expr $(, $label:lifetime $(: $pat:pat $(if $guard:expr)? => $body:expr)?)* $(,)?) => { ... };
}

Accpets a match scrutinee, followed by a comma-separated list of zero or more pattern match arms. All arms but the first must be preceded with a 'label: . Only the first arm can acccess identifiers bound by the pattern match. Inside the match arms, calling break 'label; will jump to the label’s corresponding match arm (you can only jump downwards). The list of arms can optionally be followed by a final trailing label, which can be used to jump out of the fallthrough entirely.

Example

use fallthrough::fallthrough;

fn fall(scrutinee: u32) -> u32 {
    let mut ret: u32 = 0;

    fallthrough!(scrutinee,
        val @ (0 | 63..) => ret = val + 7,
        'one: 1 => ret += 8,
        'two: 2 => ret += 9,
        'three: 3 if true => { ret += 10; break 'end },
        'four: 4 => ret = 42,
        'five: 5 => { ret += 1; break 'seven },
        'six: 6 => ret = 3,
        'seven: _ => ret *= 2,
        'end
    );
    ret
}

fn main() {
    assert_eq!(fall(0), 34);
    assert_eq!(fall(1), 27);
    assert_eq!(fall(2), 19);
    assert_eq!(fall(3), 10);
    assert_eq!(fall(4), 86);
    assert_eq!(fall(5), 2);
    assert_eq!(fall(6), 6);
    assert_eq!(fall(7), 0);
    assert_eq!(fall(64), 98);
}