Duplicate error messages in async code

[eholk]

We often get multiple copies of the same error message in async code. For example, see unresolved_type_param.stderr. It would be good to only get this error message once.

[eholk]

@rustbot label +A-async-await +AsyncAwait-Polish

[eholk]

We discussed this issue in the async triage meeting meeting today. It seems like this has something to do with the .await desugaring, and probably something to do with generator_interior.rs. @estebank has a PR out that will add some more metadata that might be helpful here as well.

@rustbot label +AsyncAwait-Triaged

[lqd]

I don't think the example linked above is something that users will actually experience: rustc and cargo deduplicate error messages, but the test suite runs with this deduplication turned off.

Here's how this unresolved_type_param test looks on nightly, for example.

We should probably find another example to link here, with slightly different errors that are thus not deduplicated ?

[eholk]

Ah, I didn't realize there was a further deduplication step. It'd be nice to not generate duplicate errors in the first place, but the fact that they get filtered out normally means we can consider this issue to be pretty low priority. If we find a case where they aren't exactly the same but should still be deduplicated, then that would be worth capturing here too.