Don't deduce a signature that makes a closure cyclic

[compiler-errors]

Sometimes when elaborating supertrait bounds for closure signature inference, we end up deducing a closure signature that is cyclical because either a parameter or the return type references a projection mentioning Self that also has escaping bound vars, which means that it's not eagerly replaced with an inference variable.

Interestingly, this is not just related to my PR that elaborates supertrait bounds for closure signature deduction. The committed test supertrait-hint-cycle-3.rs shows stable code that is fixed by this PR:

trait Foo<'a> {
    type Input;
}

impl<F: Fn(u32)> Foo<'_> for F {
    type Input = u32;
}

fn needs_super<F: for<'a> Fn(<F as Foo<'a>>::Input) + for<'a> Foo<'a>>(_: F) {}

fn main() {
    needs_super(|_: u32| {});
}

Fixes #105401
Fixes #105396

r? types

[compiler-errors]

This also doesn't fix #97680, but it makes it possible to compile with the standard "closure wasn't smart enough at inferring higher-ranked lifetime" trick:

use std::future::Future;

fn hrc<
    R,
    F: for<'a> AsyncClosure<'a, (), R>
        + for<'a> Fn(&'a ()) -> <F as AsyncClosure<'a, (), R>>::Fut,
>(
    f: F,
) -> F {
    f
}

fn main() {
-    hrc(|x| async { });
+    hrc(|x: &_| async { });
}

trait AsyncClosure<'a, I, R>: Fn(&'a I) -> Self::Fut
where
    I: 'a,
{
    type Fut: Future<Output = R> + Send + 'a;
}

impl<'a, I, R, Fut, F> AsyncClosure<'a, I, R> for F
where
    I: 'a,
    F: Fn(&'a I) -> Fut,
    Fut: Future<Output = R> + Send + 'a,
{
    type Fut = Fut;
}

Without this PR, no amount of fiddling with the closure makes this compile as far as I can tell.

[spastorino]

r? types

[bors]

☔ The latest upstream changes (presumably #104986) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #105657) made this pull request unmergeable. Please resolve the merge conflicts.

[jackh726]

@bors r+

[bors]

📌 Commit fa2f31b has been approved by jackh726

It is now in the queue for this repository.

[bors]

⌛ Testing commit fa2f31b with merge 03b9e1d...

[bors]

☀️ Test successful - checks-actions
Approved by: jackh726
Pushing 03b9e1d to master...

[rust-timer]

Finished benchmarking commit (03b9e1d): 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)	2.8%	[2.6%, 2.9%]	2
Regressions ❌ (secondary)	1.9%	[1.9%, 1.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.8%	[2.6%, 2.9%]	2

Cycles

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