Elaborate Future::Output when printing opaque impl Future type

[compiler-errors]

I would love to see the Output = type when printing type errors involving opaque impl Future.

Test code

Before (cut relevant part of output):

note: while checking the return type of the `async fn`
--> /home/michael/test.rs:5:19
|
5 | async fn bar() -> usize {
| ^^^^^ checked the `Output` of this `async fn`, found opaque type
= note: expected type `usize`
found opaque type `impl Future`

After:

note: while checking the return type of the `async fn`
--> /home/michael/test.rs:5:19
|
5 | async fn bar() -> usize {
| ^^^^^ checked the `Output` of this `async fn`, found opaque type
= note: expected type `usize`
found opaque type `impl Future<Output = usize>`

Note the "found opaque type impl Future<Output = usize>" in the new output.

---

Questions:

1. We skip printing the output type when it's a projection, since I have been seeing some types like impl Future<Output = <[static generator@/home/michael/test.rs:2:11: 2:21] as Generator<ResumeTy>>::Return> which are not particularly helpful and leak implementation detail.
   • Am I able to normalize this type within rustc_middle::ty::print::pretty? Alternatively, can we normalize it when creating the diagnostic? Otherwise, I'm fine with skipping it and falling back to the old output.
   • Should I suppress any other types? I didn't encounter anything other than this generator projection type.
2. Not sure what the formatting of this should be. Do I include spaces in Output = ?

[rust-highfive]

r? @estebank

(rust-highfive has picked a reviewer for you, use r? to override)

[compiler-errors]

Had to re-bless test outputs

[compiler-errors]

A note: copied most of this from get_impl_future_output_ty in rustc_infer::infer::error_reporting. Didn't know how to factor those two together when we're already iterating thru the predicates here, so pardon the code duplication.

[estebank]

Ideally we'd generalize this for all associated types, but I'm more than happy to special case Future, given how critical it is. I could see, for example how this would be useful for impl Iterator as well, but doing this for a handful of traits would be an exercise in futility. I want to land this as is, would you be interested in generalizing this? Something like collecting a HashMap<DefId /* of trait */, Vec<ProjectionPredicate>> and only printing everything afterwards would work well, I think, and then we could remove the special casing. Would you be interested in doing that?

@bors r+

[bors]

📌 Commit f6392a1 has been approved by estebank

[estebank]

We skip printing the output type when it's a projection, since I have been seeing some types like impl Future<Output = <[static generator@/home/michael/test.rs:2:11: 2:21] as Generator>::Return> which are not particularly helpful and leak implementation detail.

 Am I able to normalize this type within rustc_middle::ty::print::pretty? Alternatively, can we normalize it when creating the diagnostic? Otherwise, I'm fine with skipping it and falling back to the old output.

I would try normalizing it, but wouldn't be surprised if you get ICEs from it. Also I think that they are resolve_vars_if_possible already before being passed to print::pretty where the error is being emitted, but if they aren't, that would be a safer place to do that.

 Should I suppress any other types? I didn't encounter anything other than this generator projection type.

I think it's ok to land the PR as is and leak these, at least for now. I prefer to have too much info than too little and we can improve it piecemeal.

Not sure what the formatting of this should be. Do I include spaces in Output = ?

Spaces around assoc type =s is my preferred style :)

[estebank]

Something like collecting a HashMap<DefId /* of trait */, Vec> and only printing everything afterwards would work well, I think, and then we could remove the special casing. Would you be interested in doing that?

One more thing: if you do that, you need to make sure that the following prints correctly impl Trait<Type, Assoc = Type, Assoc = ()>.

[compiler-errors]

I'd love to do this in general for impl Trait<Generic, Assoc = Ty>. I'll put up a draft this weekend -- I can totally see how to refactor this code to make that happen.