Rollup merge of #112443 - compiler-errors:next-solver-opportunistically-resolve-regions, r=lcnr

Opportunistically resolve regions in new solver

Use `opportunistic_resolve_var` during canonicalization to collapse some regions.

We have to start using `CanonicalVarValues::is_identity_modulo_regions`. We also have to modify that function to consider responses like `['static, ^0, '^1, ^2]` to be an "identity" response, since because we opportunistically resolve regions, there's no longer a 1:1 mapping between canonical var values and bound var indices in the response...

There's one nasty side-effect -- one test (`tests/ui/dyn-star/param-env-infer.rs`) starts to ICE because the certainty goes from `Yes` to `Maybe(Overflow)`... Not exactly sure why, though? Putting this up for discussion/investigation.

r? ```@lcnr```

compiler/rustc_trait_selection/src/solve/canonicalize.rs

@@ -208,8 +208,25 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for Canonicalizer<'_, 'tcx> {
         t
     }
 
-    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
-        let r = self.infcx.shallow_resolve(r);
+    fn fold_region(&mut self, mut r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        match self.canonicalize_mode {
+            CanonicalizeMode::Input => {
+                // Don't resolve infer vars in input, since it affects
+                // caching and may cause trait selection bugs which rely
+                // on regions to be equal.
+            }
+            CanonicalizeMode::Response { .. } => {
+                if let ty::ReVar(vid) = *r {
+                    r = self
+                        .infcx
+                        .inner
+                        .borrow_mut()
+                        .unwrap_region_constraints()
+                        .opportunistic_resolve_var(self.infcx.tcx, vid);
+                }
+            }
+        }
+
         let kind = match *r {
             ty::ReLateBound(..) => return r,
 

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -263,7 +263,10 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             let (_orig_values, canonical_goal) = self.canonicalize_goal(goal);
             let new_canonical_response =
                 EvalCtxt::evaluate_canonical_goal(self.tcx(), self.search_graph, canonical_goal)?;
-            if !new_canonical_response.value.var_values.is_identity() {
+            // We only check for modulo regions as we convert all regions in
+            // the input to new existentials, even if they're expected to be
+            // `'static` or a placeholder region.
+            if !new_canonical_response.value.var_values.is_identity_modulo_regions() {
                 bug!(
                     "unstable result: re-canonicalized goal={canonical_goal:#?} \
                     first_response={canonical_response:#?} \