Auto merge of #105888 - skyzh:skyzh/suggest-lifetime-closure, r=compiler-errors

suggest lifetime for closure parameter type when mismatch

This is a draft PR, will add test cases later and be ready for review.

This PR fixes https://github.com/rust-lang/rust/issues/105675 by adding a diagnostics suggestion. Also a partial fix to https://github.com/rust-lang/rust/issues/105528.

The following code will have a compile error now:

```
fn const_if_unit(input: bool) -> impl for<'a> FnOnce(&'a ()) -> usize {
    let x = |_| 1;
    x
}
```

Before this PR:

```
error[E0308]: mismatched types
 --> src/lib.rs:3:5
  |
3 |     x
  |     ^ one type is more general than the other
  |
  = note: expected trait `for<'a> FnOnce<(&'a (),)>`
             found trait `FnOnce<(&(),)>`
note: this closure does not fulfill the lifetime requirements
 --> src/lib.rs:2:13
  |
2 |     let x = |_| 1;
  |             ^^^

error: implementation of `FnOnce` is not general enough
 --> src/lib.rs:3:5
  |
3 |     x
  |     ^ implementation of `FnOnce` is not general enough
  |
  = note: closure with signature `fn(&'2 ()) -> usize` must implement `FnOnce<(&'1 (),)>`, for any lifetime `'1`...
  = note: ...but it actually implements `FnOnce<(&'2 (),)>`, for some specific lifetime `'2`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `rust-test` due to 2 previous errors
```

After this PR:

```
error[E0308]: mismatched types
 --> src/lib.rs:3:5
  |
3 |     x
  |     ^ one type is more general than the other
  |
  = note: expected trait `for<'a> FnOnce<(&'a (),)>`
             found trait `FnOnce<(&(),)>`
note: this closure does not fulfill the lifetime requirements
 --> src/lib.rs:2:13
  |
2 |     let x = |_| 1;
  |             ^^^
help: consider changing the type of the closure parameters
  |
2 |     let x = |_: &_| 1;
  |             ~~~~~~~

error: implementation of `FnOnce` is not general enough
 --> src/lib.rs:3:5
  |
3 |     x
  |     ^ implementation of `FnOnce` is not general enough
  |
  = note: closure with signature `fn(&'2 ()) -> usize` must implement `FnOnce<(&'1 (),)>`, for any lifetime `'1`...
  = note: ...but it actually implements `FnOnce<(&'2 (),)>`, for some specific lifetime `'2`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `rust-test` due to 2 previous errors
```

After applying the suggestion, it compiles. The suggestion might not always be correct as the generation procedure of that suggestion is quite simple...

0 files changed, 0 insertions, 0 deletions