Display all witnesses for non-exhaustive patterns

Right now it's only one.

enum Direction {
    North,
    East,
    South,
    West
}

fn reverse(dir: Direction) -> Direction {
    match dir {
        North => South,
        East => West
    }
}

fn main() { }

<anon>:9:5: 12:6 error: non-exhaustive patterns: `South` not covered [E0004]
<anon>:9     match dir {
<anon>:10         North => South,
<anon>:11         East => West
<anon>:12     }
error: aborting due to previous error

Ideally, it'd be with some reasonable limit (10?).

[regexident]

Just pushed a PR for this: #31020

Hope I'm doing this right in regards to the PR workflow?

[jonas-schievink]

This can be closed:

<anon>:11:5: 14:6 error: non-exhaustive patterns: `South` and `West` not covered [E0004]
<anon>:11     match dir {
<anon>:12         North => South,
<anon>:13         East => West
<anon>:14     }
<anon>:11:5: 14:6 help: see the detailed explanation for E0004
error: aborting due to previous error