rustdoc: support for async Fn* trait bounds

[fmease]

Source:

#![feature(async_closure)]

pub fn demo(_: impl async Fn() -> i32) {}

Current rendered output (local, etc. — more generally: HIR-based) below (issue: dropping the trait bound modifier async).

pub fn demo(_: impl Fn() -> i32)

Current rendered output (inlined cross-crate re-export, etc. — more generally: middle-based) below (issue: leaking the internal desugaring):

pub fn demo(_: impl AsyncFn<(), Output = i32>)

Tracking:

• Tracking issue for #![feature(async_closure)] (RFC 2394 / RFC 3668) #62290

[fmease]

I consider this issue to be partially blocked: Please don't try to add async Fn* support for inlined cross-crate reexports — more generally: middle-based cleaning — before I fix #113015 (which should happen very soon!).

[fmease]

Blocking this on for<> async vs. async for<>: #127054 (in lang-FCP-merge)
@rustbot blocked