Rollup merge of #132612 - compiler-errors:async-trait-bounds, r=lcnr

Gate async fn trait bound modifier on `async_trait_bounds`

This PR moves `async Fn()` trait bounds into a new feature gate: `feature(async_trait_bounds)`. The general vibe is that we will most likely stabilize the `feature(async_closure)` *without* the `async Fn()` trait bound modifier, so we need to gate that separately.

We're trying to work on the general vision of `async` trait bound modifier general in: rust-lang/rfcs#3710, however that RFC still needs more time for consensus to converge, and we've decided that the value that users get from calling the bound `async Fn()` is *not really* worth blocking landing async closures in general.

Author: matthiaskrgr

tests/pass/async-closure-captures.rs

@@ -1,6 +1,6 @@
 // Same as rustc's `tests/ui/async-await/async-closures/captures.rs`, keep in sync
 
-#![feature(async_closure, noop_waker)]
+#![feature(async_closure, noop_waker, async_trait_bounds)]
 
 use std::future::Future;
 use std::pin::pin;

tests/pass/async-closure-drop.rs

@@ -1,4 +1,4 @@
-#![feature(async_closure, noop_waker, async_fn_traits)]
+#![feature(async_closure, noop_waker, async_trait_bounds)]
 
 use std::future::Future;
 use std::pin::pin;