Unhelpful error on mismatched types for anonymous futures about rust HOT 3 CLOSED

For reference, here's the error message for mismatched impl Traits (playground). Perhaps borrowing from this phrasing could be helpful here?

   = note: expected opaque type `impl BorkBork` (`Cat`)
              found opaque type `impl BorkBork` (`Dog`)
   = note: distinct uses of `impl Trait` result in different opaque types

from rust.

This seems like a good suggestion. I'd suggest tweaking the wording a bit, since I don't know that the implications of async blocks having unique types are immediately obvious. Maybe something like this?

  = note: the futures produced by async blocks each have unique types, so every async
          block has a different type even if they have the same output type.

from rust.

@rustbot labels +AsyncAwait-Triaged

We discussed this in our async call and agreed that some improvement along these lines would be good. We could wordsmith it in the PR of course.

(It'd be interesting too to add such diagnostics for closures and other similar cases.)

from rust.