Rollup merge of #127482 - compiler-errors:closure-two-par-sig-inference, r=oli-obk

Infer async closure signature from (old-style) two-part `Fn` + `Future` bounds

When an async closure is passed to a function that has a "two-part" `Fn` and `Future` trait bound, like:

```rust
use std::future::Future;

fn not_exactly_an_async_closure(_f: F)
where
    F: FnOnce(String) -> Fut,
    Fut: Future<Output = ()>,
{}
```

The we want to be able to extract the signature to guide inference in the async closure, like:

```rust
not_exactly_an_async_closure(async |string| {
    for x in string.split('\n') { ... }
    //~^ We need to know that the type of `string` is `String` to call methods on it.
})
```

Closure signature inference will see two bounds: `<?F as FnOnce<Args>>::Output = ?Fut`, `<?Fut as Future>::Output = String`. We need to extract the signature by looking through both projections.

### Why?

I expect the ecosystem's move onto `async Fn` trait bounds (which are not affected by this PR, and already do signature inference fine) to be slow. In the mean time, I don't see major overhead to supporting this "old–style" of trait bounds that were used to model async closures.

r? oli-obk
Fixes #127468
Fixes #127425

0 files changed, 0 insertions, 0 deletions