Auto merge of #117278 - lcnr:try-normalize-ty, r=compiler-errors

new solver normalization improvements

cool beans

At the core of this PR is a `try_normalize_ty` which stops for rigid aliases by using `commit_if_ok`.

Reworks alias-relate to fully normalize both the lhs and rhs and then equate the resulting rigid (or inference) types. This fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/68 by avoiding the exponential blowup. Also supersedes #116369 by only defining opaque types if the hidden type is rigid.

I removed the stability check in `EvalCtxt::evaluate_goal` due to https://github.com/rust-lang/trait-system-refactor-initiative/issues/75. While I personally have opinions on how to fix it, that still requires further t-types/`@nikomatsakis` buy-in, so I removed that for now. Once we've decided on our approach there, we can revert this commit.

r? `@compiler-errors`

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -469,7 +469,7 @@ 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 destructured as a `dyn Trait`.
             let Some(b_ty) =
-                ecx.try_normalize_ty(goal.param_env, goal.predicate.trait_ref.args.type_at(1))?
+                ecx.try_normalize_ty(goal.param_env, goal.predicate.trait_ref.args.type_at(1))
             else {
                 return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::OVERFLOW);
             };
@@ -536,9 +536,8 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
             let b_ty = match ecx
                 .try_normalize_ty(goal.param_env, goal.predicate.trait_ref.args.type_at(1))
             {
-                Ok(Some(b_ty)) => b_ty,
-                Ok(None) => return vec![misc_candidate(ecx, Certainty::OVERFLOW)],
-                Err(_) => return vec![],
+                Some(b_ty) => b_ty,
+                None => return vec![misc_candidate(ecx, Certainty::OVERFLOW)],
             };
 
             let goal = goal.with(ecx.tcx(), (a_ty, b_ty));