improve error for missing await

[nikomatsakis]

Given the following code:

use futures::executor::block_on;

async fn return_number() -> usize {
    return 1;
}

async fn return_call_async() -> usize {
    // return return_number().await;
    return return_number();
}

fn main() {
    let s = block_on(return_call_async());
    print!("{}", &s);
}

The current (pretty decent) output is:

error[E0308]: mismatched types
 --> src/main.rs:8:12
  |
3 | async fn return_number() -> usize {
  |                             ----- the `Output` of this `async fn`'s found opaque type
...
8 |     return return_number();
  |            ^^^^^^^^^^^^^^^ expected `usize`, found opaque type
  |
  = note:     expected type `usize`
          found opaque type `impl futures::Future`
help: consider `await`ing on the `Future`
  |
8 |     return return_number().await;
  |                           ^^^^^^

error: aborting due to previous error

[nikomatsakis]

This comes from @jlkiri's comment here

[nikomatsakis]

I think the output should be more aggressive. For example:

error[E0308]: missing await on future
 --> src/main.rs:8:12
  |
3 | async fn return_number() -> usize {
  | ----- calling an async function returns a future
...
8 |     return return_number();
  |            ^^^^^^^^^^^^^^^ expected `usize`, found future
  |
help: consider `await`ing on the `Future`
  |
8 |     return return_number().await;
  |                           ^^^^^^

error: aborting due to previous error

[estebank]

Another alternative would be to explicitly state what the fns desugared type is:

  |
3 | async fn return_number() -> usize {
  | ----- by annotating this function with `async` makes it `fn() -> impl Future<Output = usize>` instead of `fn() -> usize`
...

[SNCPlay42]

Related, if not a duplicate, to #80658

[SNCPlay42]

The note has changed on nightly/beta, from the PR linked to that isssue:

3 | async fn return_number() -> usize {
  |                             ----- checked the `Output` of this `async fn`, found opaque type

[nikomatsakis]

I agree this is something of a duplicate. I can move the conversation over there.