Auto merge of #105409 - compiler-errors:closure-infer-cycle, r=jackh726

Don't deduce a signature that makes a closure cyclic

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:

```rust
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

1 files changed, 6 insertions, 1 deletions

diff --git a/src/test/ui/issues/issue-25439.stderr b/src/test/ui/issues/issue-25439.stderr
index 325c28c15e2..dadae23fdf3 100644
--- a/src/test/ui/issues/issue-25439.stderr
+++ b/src/test/ui/issues/issue-25439.stderr
@@ -2,12 +2,17 @@ error[E0644]: closure/generator type that references itself
   --> $DIR/issue-25439.rs:8:9
    |
 LL |     fix(|_, x| x);
-   |         ^^^^^^^^ cyclic type of infinite size
+   |         ^^^^^^ cyclic type of infinite size
    |
    = note: closures cannot capture themselves or take themselves as argument;
            this error may be the result of a recent compiler bug-fix,
            see issue #46062 <https://github.com/rust-lang/rust/issues/46062>
            for more information
+note: required by a bound in `fix`
+  --> $DIR/issue-25439.rs:3:33
+   |
+LL | fn fix<F>(f: F) -> i32 where F: Fn(Helper<F>, i32) -> i32 {
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `fix`
 
 error: aborting due to previous error