Incorrect syntax on suggestion to make pub function async with E0277

[sciguy16]

Given the following code:
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=0db76b4ce2ff53636126640d313e2b91

fn main() {
   bar();
}

async fn bar() {
    a_mod::foo().await;
}

mod a_mod {
    pub fn foo() {}
}

The current output is:

error[E0277]: `()` is not a future
 --> a.rs:6:17
  |
6 |     a_mod::foo().await;
  |     ------------^^^^^^ `()` is not a future
  |     |
  |     this call returns `()`
  |
  = help: the trait `Future` is not implemented for `()`
  = note: () must be a future or must implement `IntoFuture` to be awaited
  = note: required because of the requirements on the impl of `IntoFuture` for `()`
help: remove the `.await`
  |
6 -     a_mod::foo().await;
6 +     a_mod::foo();
  |
help: alternatively, consider making `fn foo` asynchronous
  |
10|     async pub fn foo() {}
  |     +++++

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

Ideally the output should look like:

...
10|     pub async fn foo() {}
  |         +++++
...

Tested on rustc 1.60.0 stable and nightly-2022-04-29

[sciguy16]

This looks like the section of code responsible for the message:

rust/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

Lines 1100 to 1108 in 683c582

	err.span_suggestion_verbose(
	span.shrink_to_lo(),
	&format!(
	"alternatively, consider making `fn {}` asynchronous",
	ident
	),
	"async ".to_string(),
	Applicability::MaybeIncorrect,
	);

[ken-matsui]

@rustbot claim