Slightly incorrect help message with mismatched typs in async code

[jfrimmel]

The following works, since foo and bar return (), so the expression is simply returned:

fn foo() {}
fn bar() {
    foo()
}

The "equivalent" code in the async context yields a compiler error:

async fn foo() {}
async fn bar() {
    foo()
}

error[E0308]: mismatched types
  --> src/main.rs:62:5
   |
62 |     foo()
   |     ^^^^^- help: try adding a semicolon: `;`
   |     |
   |     expected (), found opaque type
   |
   = note: expected type `()`
              found type `impl core::future::future::Future`

The help message would be correct for non-async-code with mismatched types, but in the async case a .await should be suggested.

Technically the current help message is correct, but leads to another warning (that can be even overlooked):

warning: unused implementer of `core::future::future::Future` that must be used
  --> src/main.rs:62:5
   |
62 |     foo();
   |     ^^^^^^
   |
   = note: `#[warn(unused_must_use)]` on by default
   = note: futures do nothing unless you `.await` or poll them

So you have to do a second compile iteration.

I'd suggest to spawn a help message like that:

error[E0308]: mismatched types
  --> src/main.rs:62:5
   |
62 |     foo()
   |     ^^^^^- help: try to await the future: `.await`
   |     |
   |     expected (), found opaque type
   |
   = note: expected type `()`
              found type `impl core::future::future::Future`

[jfrimmel]

Note: this is the message with the current nightly (rustc 1.41.0-nightly (25d8a94 2019-11-29)).
Rust 1.39 does not even spawn the help message

[nikomatsakis]

Async await traige: marking as on-deck, as suggesting the use of .await seems pretty useful and general

[Aaron1011]

I started working on this, then realized that it already exists: #62067

However, it looks like we don't currently attempt to emit suggestions for certain coercions (including the return type coercion), which is why it doesn't show up here. I'm working on emitting coercion suggestions in more places.

[Aaron1011]

@rustbot claim