suggest lifetime for closure parameter type when mismatch

[skyzh]

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

This PR fixes #105675 by adding a diagnostics suggestion. Also a partial fix to #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...

[rustbot]

r? @TaKO8Ki

(rustbot has picked a reviewer for you, use r? to override)

[skyzh]

@rustbot label +A-suggestion-diagnostics +T-compiler

[skyzh]

Originally I thought we should fix this bug and make the original code compiles. But it seems that we will need to decide whether _ is for <'a> or some specific lifetime in advance before going into the borrow checker. Therefore making a diagnostics suggestion might be a better fix... I guess?

[skyzh]

Test case updated. This suggestion may also fix some previous issues. e.g, for src/test/ui/lifetimes/issue-79187-2.stderr, the suggested fix actually works.

[apiraino]

I assume the last round of review is finished and the review flag can be switched. @skyzh Feel free to request a review with @rustbot ready, thanks!

@rustbot author

[skyzh]

@rustbot ready

[skyzh]

r? @compiler-errors

[bors]

☔ The latest upstream changes (presumably #107290) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

This needs a rebase. Marking this as waiting-on-author. Please use @rustbot ready to mark this as ready.

@rustbot author

[compiler-errors]

@bors r+

[bors]

📌 Commit c025483 has been approved by compiler-errors

It is now in the queue for this repository.

[Dylan-DPC]

@bors r-

failed in rollup

[skyzh]

@rustbot ready

Looks like a merge conflict 🙈 Fixed

[compiler-errors]

@bors r+

[bors]

📌 Commit 90dc6fe has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit 90dc6fe with merge 2a71115...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing 2a71115 to master...

[rust-timer]

Finished benchmarking commit (2a71115): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.5%	[0.8%, 4.8%]	6
Regressions ❌ (secondary)	2.3%	[1.4%, 2.8%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.5%	[0.8%, 4.8%]	6

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-3.4%, -1.2%]	9
All ❌✅ (primary)	-	-	0