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More accurate suggestion for missing binop trait bounds #96442

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estebank opened this issue Apr 26, 2022 · 1 comment
Open

More accurate suggestion for missing binop trait bounds #96442

estebank opened this issue Apr 26, 2022 · 1 comment
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@willcrichton
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A-diagnostics Area: Messages for errors, warnings, and lints A-suggestion-diagnostics Area: Suggestions generated by the compiler applied by `cargo fix`. D-papercut Diagnostics: An error or lint that needs small tweaks. D-terse Diagnostics: An error or lint that doesn't give enough information about the problem at hand. P-low Low priority T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@estebank
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estebank commented Apr 26, 2022 •
edited by rustbot
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#94034 (comment)

We currently emit

LL | impl<B: std::ops::Add> Add for D<B> {
   |       +++++++++++++++

but it should be

LL | impl<B: std::ops::Add<Output = B>> Add for D<B> {
   |       +++++++++++++++++++++++++++
@estebank estebank added A-diagnostics Area: Messages for errors, warnings, and lints P-low Low priority T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. A-suggestion-diagnostics Area: Suggestions generated by the compiler applied by `cargo fix`. D-papercut Diagnostics: An error or lint that needs small tweaks. D-terse Diagnostics: An error or lint that doesn't give enough information about the problem at hand. labels Apr 26, 2022
@willcrichton
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I will take a stab at this soon. I am fairly certain I know how to do it -- there's an Expectation type that describes the expected return type, and if that expectation exists, we can use it to bound Output=<Expectation>.

I tried implementing that today. This is the core change: willcrichton@e652939#diff-ea6f9d55e90333a1f5fc5e202d46862a6a031410831f2df42317ac024f821162R519-R539

This change seems to introduce some noise in other tests involving binops that will require manual fixing. But maybe next weekend I will try to finish this.

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@willcrichton willcrichton mentioned this issue Apr 27, 2022
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bors added a commit to rust-lang-ci/rust that referenced this issue Jul 16, 2022
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Auto merge of rust-lang#96482 - willcrichton:fix-trait-suggestion-for…
d695a49 
…-binops, r=estebank

Add Output = expected type trait obligation for known binary operators

This PR is a follow-on to rust-lang#94034 that addresses rust-lang#96442. That is, after replacing the trait-suggestion logic in `op.rs` with a more generic path that analyzes a general set of `Obligation`s, then we lost some specificity in the suggestions where the bounds on the associated type `Output=` would not get suggested.

This PR fixes this issue by changing `FnCtxt::construct_obligation_for_trait` to include a new `ProjectionPredicate` obligation for binary operators that obliges that `Output` is the same as the expected type of the expression. Additionally, to get the expected type of the expression, this PR threads the `Expectation<'tcx>` structure throughout several functions.

See src/test/ui/generic-associated-types/missing-bounds.stderr for an example of how this works.

One side effect of this change is it causes type-check failures with binops to include additional information. Specifically, many now say

```
error: type mismatch resolving `<Lhs as TheBinop>::Output == ExpectedTy`
```

It's up for discussion whether this added context is worth it to the user.

r? `@estebank`
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@willcrichton willcrichton

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A-diagnostics Area: Messages for errors, warnings, and lints A-suggestion-diagnostics Area: Suggestions generated by the compiler applied by `cargo fix`. D-papercut Diagnostics: An error or lint that needs small tweaks. D-terse Diagnostics: An error or lint that doesn't give enough information about the problem at hand. P-low Low priority T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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