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

[compiler-errors]

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

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:

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

[compiler-errors]

oh lol let me fix the doc comment

[compiler-errors]

OK this is ready :3

[oli-obk]

[bors]

📌 Commit f4f678f has been approved by oli-obk

It is now in the queue for this repository.