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

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`

1 files changed, 3 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index 221dac9ca03..f951e43fbfa 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -333,6 +333,9 @@ pub enum ObligationCauseCode<'tcx> {
     /// This is purely for diagnostic purposes - it is always
     /// correct to use `MiscObligation` instead
     WellFormed(Option<hir::HirId>),
+
+    /// From `match_impl`. The cause for us having to match an impl, and the DefId we are matching against.
+    MatchImpl(Lrc<ObligationCauseCode<'tcx>>, DefId),
 }
 
 impl ObligationCauseCode<'_> {