Auto merge of #80104 - Nadrieril:usefulness-merging, r=varkor

Improve and fix diagnostics of exhaustiveness checking

Primarily, this fixes https://github.com/rust-lang/rust/issues/56379. This also fixes incorrect interactions between or-patterns and slice patterns that I discovered while working on #56379. Those two examples show the incorrect diagnostics:

```rust
match &[][..] {
    [true] => {}
    [true // detected as unreachable but that's not true
        | false, ..] => {}
    _ => {}
}
match (true, None) {
    (true, Some(_)) => {}
    (false, Some(true)) => {}
    (true | false, None | Some(true // should be detected as unreachable
                               | false)) => {}
}
```

I did not measure any perf impact. However, I suspect that [`616ba9f`](https://github.com/rust-lang/rust/pull/80104/commits/616ba9f9f7f5845777a36e1a41a515e6c33a8776) should have a negative impact on large or-patterns. I'll see what the perf run says; I have optimization ideas up my sleeve if needed.

EDIT: I initially had a noticeable perf impact that I thought unavoidable. I then proceeded to avoid it x)

r? `@varkor`
`@rustbot` label +A-exhaustiveness-checking

0 files changed, 0 insertions, 0 deletions