Diagnostic involving the type of an async block refers to `generator`'s

[guswynn]

version 1.49:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6cf5c01427a274fed3395916ea86ab87

FuturesUnordered requires futures with the same type:

use futures::stream::FuturesUnordered;
fn main() {

    let u = FuturesUnordered::new();
    
    u.push(async {});
    u.push(async {});
    
}

results in:

error[E0308]: mismatched types
  --> src/main.rs:12:18
   |
11 |     u.push(async {});
   |                  -- the expected generator
12 |     u.push(async {});
   |                  ^^ expected generator, found a different generator
   |
   = note: expected generator `[static generator@src/main.rs:11:18: 11:20]`
              found generator `[static generator@src/main.rs:12:18: 12:20]`

As far as I know, generators have 2 types in their internal representation, so their display impl (its not actually display as far as I remember, but whatever creates the string to describe a type), should me more accurate about it being a future

also, these spans seem off?

@rustbot label +A-async-await +A-diagnostics +D-papercut

[estebank]

The spans are correct (or at least consistent with what I'd expect and what we do in similar cases not involving generators/futures).

I would be in favor of carrying metadata in the generator types (do we do that already?) to differentiate between non-futures and futures and use a different user visible textual representation.

[guswynn]

Yeah we do carry that metadata (in the hir::Ty). I have a local commit to clean this up but i have a bit more refactoring to do to make it look right (turns out TypeError is quite complicated haha)