Fix await suggestion on non-future type

[compiler-errors]

Remove a match block that would suggest to add .await in the case where the expected type's Future::Output equals the found type. We only want to suggest .awaiting in the opposite case (the found type's Future::Output equals the expected type).

The code sample is here: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6ba6b83d4dddda263553b79dca9f6bcb

Before:

➜  ~ rustc --edition=2021 --crate-type=lib test.rs
error[E0308]: `match` arms have incompatible types
 --> test.rs:4:14
  |
2 |       let x = match 1 {
  |  _____________-
3 | |         1 => other(),
  | |              ------- this is found to be of type `impl Future`
4 | |         2 => other().await,
  | |              ^^^^^^^^^^^^^ expected opaque type, found enum `Result`
5 | |     };
  | |_____- `match` arms have incompatible types
  |
  = note: expected type `impl Future`
             found enum `Result<(), ()>`
help: consider `await`ing on the `Future`
  |
4 |         2 => other().await.await,
  |                           ++++++

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.

After:

➜  ~ rustc +stage1 --edition=2021 --crate-type=lib test.rs
error[E0308]: `match` arms have incompatible types
 --> test.rs:4:14
  |
2 |       let x = match 1 {
  |  _____________-
3 | |         1 => other(),
  | |              ------- this is found to be of type `impl Future`
4 | |         2 => other().await,
  | |              ^^^^^^^^^^^^^ expected opaque type, found enum `Result`
5 | |     };
  | |_____- `match` arms have incompatible types
  |
  = note: expected type `impl Future`
             found enum `Result<(), ()>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.

Fixes #90931

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) soon.

Please see the contribution instructions for more information.

[compiler-errors]

Requesting review from Esteban K. since you introduced the code in c548511 and probably know best!

r? @estebank

[compiler-errors]

Fixed up the error to only suggest on ObligationCauseCode::Pattern.. I think that'll unbreak this test.

[estebank]

I would expect the output for the test case to be

error[E0308]: `match` arms have incompatible types
 --> test.rs:4:14
  |
2 |       let x = match 1 {
  |  _____________-
3 | |         1 => other(),
  | |              ------- this is found to be of type `impl Future`
4 | |         2 => other().await,
  | |              ^^^^^^^^^^^^^ expected opaque type, found enum `Result`
5 | |     };
  | |_____- `match` arms have incompatible types
  |
  = note: expected type `impl Future`
             found enum `Result<(), ()>`
help: consider `await`ing on the `Future`
  |
4 |         1 => other().await,
  |                     ++++++

Looking at the name of the bindings, it looks like this should have been working correctly, but it's possible that the span being passed in was incorrect in the first place.

I think that where you changed the code, you need to look to see if you can get a MatchExpressionArmCause.prior_arms, and use those spans to suggest where to .await.

rust/compiler/rustc_infer/src/infer/error_reporting/mod.rs

Lines 601 to 604 in 3d29b68

	ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
	semi_span,
	source,
	ref prior_arms,

rust/compiler/rustc_infer/src/infer/error_reporting/mod.rs

Lines 646 to 657 in 3d29b68

	if prior_arms.len() <= 4 {
	for sp in prior_arms {
	any_multiline_arm |= source_map.is_multiline(*sp);
	err.span_label(*sp, format!("this is found to be of type `{}`", t));
	}
	} else if let Some(sp) = prior_arms.last() {
	any_multiline_arm |= source_map.is_multiline(*sp);
	err.span_label(
	*sp,
	format!("this and all prior arms are found to be of type `{}`", t),
	);
	}

I would make it a multipart_suggestion_verbose, so that it always shows on its own, and you might have more than one arm that resolved to impl Future, and you would have to .await all of them for the resulting code to be correct.

Would you be able to try that out and see if I'm right?

[compiler-errors]

I applied the changes you asked for, I think! Let me know if I did it wrong.

I also removed the ObligationCauseCode::Pattern case from the branch corresponding to when exp_found.expected type equals exp_found.found's awaited type. I might be totally misunderstanding when we see ObligationCauseCode::Pattern, but I think that case would only ever happen if we could write a pattern matching only the opaque type? Let me know if I should add it back.

[estebank]

The changes look reasonable. Can you add test cases covering all the new codepaths? Adding them to src/test/ui/async-await/suggest-missing-await.rs should suffice.

[estebank]

@bors r+

[bors]

📌 Commit fc816c3 has been approved by estebank