Rollup merge of rust-lang#126675 - oli-obk:diagnostics_opaque, r=jackh726

Change a `DefineOpaqueTypes::No` to `Yes` in diagnostics code

Explanation in comments of the function.

r? ```@compiler-errors```

cc rust-lang#116652

Author: fee1-dead

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -1357,6 +1357,8 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         traits::SelectionContext::new(self).select(&obligation)
     }
 
+    /// Used for ambiguous method call error reporting. Uses probing that throws away the result internally,
+    /// so do not use to make a decision that may lead to a successful compilation.
     fn candidate_source(&self, candidate: &Candidate<'tcx>, self_ty: Ty<'tcx>) -> CandidateSource {
         match candidate.kind {
             InherentImplCandidate(_) => {
@@ -1370,8 +1372,10 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     self.instantiate_binder_with_fresh_vars(self.span, infer::FnCall, trait_ref);
                 let (xform_self_ty, _) =
                     self.xform_self_ty(candidate.item, trait_ref.self_ty(), trait_ref.args);
+                // Guide the trait selection to show impls that have methods whose type matches
+                // up with the `self` parameter of the method.
                 let _ = self.at(&ObligationCause::dummy(), self.param_env).sup(
-                    DefineOpaqueTypes::No,
+                    DefineOpaqueTypes::Yes,
                     xform_self_ty,
                     self_ty,
                 );