suggest `await` for future-related type errors

[nikomatsakis]

A common problem when using async-await is to forget to invoke .await. This often shows up as a type error, e.g. in a case like this (playground):

#![feature(async_await)]

async fn make_u32() -> u32 {
    22
}

fn take_u32(x: u32) {

}

async fn foo() {
    let x = make_u32();
    take_u32(x);
}

fn main() { }

currently we give a rather generic error:

error[E0308]: mismatched types
  --> src/main.rs:13:14
   |
13 |     take_u32(x);
   |              ^ expected u32, found opaque type
   |
   = note: expected type `u32`
              found type `impl std::future::Future`

it'd be nice if we could suggest adding .await somewhere.

[nikomatsakis]

cc @rust-lang/wg-diagnostics @estebank

[nikomatsakis]

@wycats was reporting a much worse error, but I'm quite sure what he did to encounter it:

Probably has something to do with streams?

[estebank]

This will be harder to do for chained expessions future.field_or_resolved_future.execute_new_future()? that should have been future.await.field_or_resolved_future.execute_new_future().await?.

[cramertj]

@nikomatsakis Looks like @wycats's error was the same, it's just that the output type of the future is more complicated (Pin<Box<Stream<Item = Value> + Send + 'static>>).

[cofibrant]

Hi, I'd be really interested in working on this

[davidtwco]

@doctorn awesome! Sending @rustbot claim should assign you to the issue and feel free to start a topic in the #wg-async-foundations stream if you need a hand.

[cofibrant]

@rustbot claim