Rollup merge of #120549 - lcnr:errs-showcase, r=compiler-errors

modify alias-relate to also normalize ambiguous opaques

allows a bunch of further cleanups and generally simplifies the type system. To handle https://github.com/rust-lang/trait-system-refactor-initiative/issues/8 we'll have to add a some additional complexity to the `(Alias, Infer)` branches in alias-relate, so removing the opaque type special case here is really valuable.

It does worsen `deduce_closure_signature` and friends even more as they now receive an inference variable which is only constrained via an `AliasRelate` goal. These probably have to look into alias relate goals somehow. Leaving that for a future PR as this is something we'll have to tackle regardless.

r? `@compiler-errors`

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -584,11 +584,11 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
             let a_ty = goal.predicate.self_ty();
             // We need to normalize the b_ty since it's matched structurally
             // in the other functions below.
-            let b_ty = match ecx
-                .try_normalize_ty(goal.param_env, goal.predicate.trait_ref.args.type_at(1))
-            {
-                Some(b_ty) => b_ty,
-                None => return vec![misc_candidate(ecx, Certainty::OVERFLOW)],
+            let Ok(b_ty) = ecx.structurally_normalize_ty(
+                goal.param_env,
+                goal.predicate.trait_ref.args.type_at(1),
+            ) else {
+                return vec![];
             };
 
             let goal = goal.with(ecx.tcx(), (a_ty, b_ty));