E0004 lists incorrect secondary witnesses

[jfirebaugh]

#31020 added display of multiple witnesses to the error message for non-exhaustive match patterns. However, witnesses beyond the first one may have inappropriate types. For example:

enum Direction {
    North,
    East,
    South,
    West,
}

fn main() {
    match Some(Direction::North) {
        Some(Direction::North) => (),
        None => ()
    }
}

Output:

error[E0004]: non-exhaustive patterns: `Some(East)`, `South` and `West` not covered
 --> <anon>:9:11
  |
9 |     match Some(Direction::North) {
  |           ^^^^^^^^^^^^^^^^^^^^^^ patterns `Some(East)`, `South` and `West` not covered

error: aborting due to previous error

Here, "South and West not covered" should read "Some(South) and Some(West) not covered".

[TimNN]

Duplicate of / related to #35609, I think.

[Cobrand]

Ok, so I tried to "fix" this issue to dirty my hands a little with the rust compiler, thinking that it would be easy, cause it was probably a very simple issue to fix. How wrong I was. This is simply not a bug of my level, I'm sorry; but I'll try to explain what I've understood while investigating.

This issue is actually quite messed up, here is some code that might make you understand : https://play.rust-lang.org/?gist=5ca5977ed3c44048518e808e8d821bb8&version=nightly&backtrace=0

So what would we expect in that case ? Certainly not
patterns Double(Chocolate, Pistachio)andStrawberry not covered. It makes no sense.

This problem comes actually from here : https://github.com/rust-lang/rust/blob/master/src/librustc_const_eval/check_match.rs#L726-L735

Number of patterns left and arity are mixed together, in this code. Guess what happens If you have an enum where the number of fields possible is greater than the number of variants in the enum ?
This happens :
https://play.rust-lang.org/?gist=863ccc9cecc4932a28532011127f7d40&version=nightly&backtrace=0

That's right, it just displays "_" for the fields that can't be "bound" to a variant.

Now you might be wondering, but if the arity and the number of patterns left are mixed together, what happens if you mix different enums in the patterns ? I'll show you what happens 👍

https://play.rust-lang.org/?gist=3175b98130b9ba03bd9f2f6d85631a16&version=nightly&backtrace=0

We should at least see a mention of the other enum in this code, but we don't. Again, this makes absolutely no sense at all, but I'd say this behavior is "expected" from the bug.

Now I think I'm going to leave this issue as it is, cause this is probably way out of my league for newcomer like myself :( .

[arielb1]

duplicate of #35609.