Improve diagnostic for: AsyncFn is not implemented for async block

Code

fn main() {
    handler(async {});
}

fn handler(_f: impl AsyncFnOnce()) {}

Current output

error[E0277]: the trait bound `{async block@src/main.rs:2:13: 2:18}: AsyncFnOnce()` is not satisfied
 --> src/main.rs:2:13
  |
2 |     handler(async {});
  |     ------- ^^^^^^^^ the trait `AsyncFnOnce()` is not implemented for `{async block@src/main.rs:2:13: 2:18}`
  |     |
  |     required by a bound introduced by this call
  |
note: required by a bound in `handler`
 --> src/main.rs:5:21
  |
5 | fn handler(_f: impl AsyncFnOnce()) {}
  |                     ^^^^^^^^^^^^^ required by this bound in `handler`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Desired output

error[E0277]: the trait bound `{async block@src/main.rs:2:13: 2:18}: AsyncFnOnce()` is not satisfied
 --> src/main.rs:2:13
  |
2 |     handler(async {});
  |     ------- ^^^^^^^^ the trait `AsyncFnOnce()` is not implemented for `{async block@src/main.rs:2:13: 2:18}`
  |     |
  |     required by a bound introduced by this call
  |
  = help: the trait `AsyncFnOnce()` is not implemented for `()`
  = note: wrap the `()` in a closure with no arguments: `async || { /* code */ }`note: required by a bound in `handler`
 --> src/main.rs:5:21
  |
5 | fn handler(_f: impl AsyncFnOnce()) {}
  |                     ^^^^^^^^^^^^^ required by this bound in `handler`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Rationale and extra context

The additional note and help text are already present for the non-async function counterparts (FnOnce, and others). This issue refers to all AsyncFn variants.

Other cases

Rust Version

Rust playground: 1.100.0-nightly (2026-09-20 bba531001d4de6d7f496)

Anything else?

Playground with an example of both async and non-async code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=84d4f70ed2b310bc5fbc4f39aa63dba5