diagnostics: Suggest fn binding type for unstable closure for<> binders

[Dnreikronos]

Fixes #160431

for<'a> |x: &'a T| -> U { ... } currently just says "consider removing for<...>", which idk is kinda useless when someone landed there while fighting HRTB errors. This rewrites the obvious case to let cl: for<'a> fn(...) = |x| { ... } instead.

Visitor emits the gate so we have AST context for the rewrite, and we still keep a pre-expansion fallback for #[cfg(false)] etc. MachineApplicable only when we're pretty sure: by-value bindings, lifetime-only unbounded binders, no _ in the sig, no macro expansion. Capture checking is a conservative single-segment free-path heuristic since this runs pre-resolution/pre-typeck; when that looks uncertain we fall back to MaybeIncorrect (or just "consider removing"). imo that's the right tradeoff here: better a maybe-wrong help than rustfix auto-applying into E0308. fyi free-fn / None / Some cases also hit MaybeIncorrect for the same reason.

ltm if the heuristic feels too conservative and y'all want it tightened later.

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[rustbot]

r? @JonathanBrouwer

rustbot has assigned @JonathanBrouwer.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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The reviewer was selected based on:

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[JonathanBrouwer]

This looks cool but I might not have time to review this in the next 2 weeks as it would require me to do some reading about how for<> binders are implemented
@rustbot reroll

[JonathanBrouwer]

I will do a perf run, just in case, since this does some extra work
@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 44fa895 (44fa895cdd61615981ffd0c4e535670c534b839a)
Base parent: ef20314 (ef20314466010b8b9259ec5f86230c530ca08661)

[rust-timer]

Finished benchmarking commit (44fa895): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (secondary -0.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.0%	[0.4%, 2.7%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.4%	[-5.7%, -0.4%]	9
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 2.6%, secondary -0.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	0.8%	[0.4%, 2.3%]	10
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.8%	[-3.0%, -0.4%]	17
All ❌✅ (primary)	2.6%	[2.6%, 2.6%]	1

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 455.478s -> 454.351s (-0.25%)
Artifact size: 398.50 MiB -> 398.45 MiB (-0.01%)