Suggest making `fn` `async` if future found in return expr

[estebank]

Code

async fn number() -> i32 {
    42
}

fn wrapped_number() -> i32 {
    number()
}

Current output

error[E0308]: mismatched types
 --> src/lib.rs:6:5
  |
5 | fn wrapped_number() -> i32 {
  |                        --- expected `i32` because of return type
6 |     number()
  |     ^^^^^^^^ expected `i32`, found future

Desired output

error[E0308]: mismatched types
 --> src/lib.rs:6:5
  |
5 | fn wrapped_number() -> i32 {
  |                        --- expected `i32` because of return type
6 |     number()
  |     ^^^^^^^^ expected `i32`, found future
help: consider making the function `async`
  |
5 | async fn wrapped_number() -> i32 {
  | +++++
help: if you make the function `async`, you can `await` on the future
6 |     number().await
  |             ++++++

Rationale and extra context

No response

Other cases

Rust Version

1.97

Anything else?

No response

[lsunsi]

@chenyukang are you still working on this? I'm asking because it's not claimed via rustbot. If you're not I could take a shot at it!

[chenyukang]

@chenyukang are you still working on this? I'm asking because it's not claimed via rustbot. If you're not I could take a shot at it!

I already had a branch for it, will open a PR for it.