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Auto merge of #87244 - jackh726:issue-71883, r=estebank

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Better diagnostics with mismatched types due to implicit static lifetime

Fixes #78113

I think this is my first diagnostics PR...definitely happy to hear thoughts on the direction/implementation here.

I was originally just trying to solve the error above, where the lifetime on a GAT was causing a cryptic "mismatched types" error. But as I was writing this, I realized that this (unintentionally) also applied to a different case: `wf-in-foreign-fn-decls-issue-80468.rs`. I'm not sure if this diagnostic should get a new error code, or even reuse an existing one. And, there might be some ways to make this even more generalized. Also, the error is a bit more lengthy and verbose than probably needed. So thoughts there are welcome too.

This PR essentially ended up adding a new nice region error pass that triggers if a type doesn't match the self type of an impl which is selected because of a predicate because of an implicit static bound on that self type.

r? `@estebank`
This commit is contained in:
bors 2021-07-20 10:56:08 +00:00
commit da7d405357
16 changed files with 299 additions and 38 deletions
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3
compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
View file

@ -1903,7 +1903,8 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
| ObligationCauseCode::UnifyReceiver(..)
| ObligationCauseCode::OpaqueType
| ObligationCauseCode::MiscObligation
| ObligationCauseCode::WellFormed(..) => {}
| ObligationCauseCode::WellFormed(..)
| ObligationCauseCode::MatchImpl(..) => {}
ObligationCauseCode::SliceOrArrayElem => {
err.note("slice and array elements must have `Sized` type");
}

8
compiler/rustc_trait_selection/src/traits/select/mod.rs
View file

@ -1903,9 +1903,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
debug!(?impl_trait_ref, ?placeholder_obligation_trait_ref);
let cause = ObligationCause::new(
obligation.cause.span,
obligation.cause.body_id,
ObligationCauseCode::MatchImpl(Lrc::new(obligation.cause.code.clone()), impl_def_id),
);
let InferOk { obligations, .. } = self
.infcx
.at(&obligation.cause, obligation.param_env)
.at(&cause, obligation.param_env)
.eq(placeholder_obligation_trait_ref, impl_trait_ref)
.map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?;
nested_obligations.extend(obligations);