Give indication of missing enum variants in empty match

[aidanhs]

enum X {
    A,
    B,
    C,
}

fn main() {
    let x = X::A;
    match x {
    };
    match x {
        X::A => (),
    };
}

https://is.gd/FpEj86

The first match does not tell you which variants are missing, the second does. Seems like a minor inconsistency that could be handy to fix.