Gate `AsyncFn*` under `async_closure` feature T-lang has not come to a consensus on the naming of async closure callable bounds, and as part of allowing the async closures RFC merge, we agreed to place `AsyncFn` under the same gate as `async Fn` so that these syntaxes can be evaluated in parallel. See https://github.com/rust-lang/rfcs/pull/3668#issuecomment-2246435537 r? oli-obk