Rollup merge of #126404 - compiler-errors:alias-relate-terms, r=lcnr

Check that alias-relate terms are WF if reporting an error in alias-relate

Check that each of the left/right term is WF when deriving a best error obligation for an alias-relate goal. This will make sure that given `<i32 as NotImplemented>::Assoc = ()` will drill down into `i32: NotImplemented` since we currently treat the projection as rigid.

r? lcnr

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -515,6 +515,32 @@ impl<'tcx> ProofTreeVisitor<'tcx> for BestObligation<'tcx> {
             self.with_derived_obligation(obligation, |this| nested_goal.visit_with(this))?;
         }
 
+        // alias-relate may fail because the lhs or rhs can't be normalized,
+        // and therefore is treated as rigid.
+        if let Some(ty::PredicateKind::AliasRelate(lhs, rhs, _)) = pred_kind.no_bound_vars() {
+            if let Some(obligation) = goal
+                .infcx()
+                .visit_proof_tree_at_depth(
+                    goal.goal().with(goal.infcx().tcx, ty::ClauseKind::WellFormed(lhs.into())),
+                    goal.depth() + 1,
+                    self,
+                )
+                .break_value()
+            {
+                return ControlFlow::Break(obligation);
+            } else if let Some(obligation) = goal
+                .infcx()
+                .visit_proof_tree_at_depth(
+                    goal.goal().with(goal.infcx().tcx, ty::ClauseKind::WellFormed(rhs.into())),
+                    goal.depth() + 1,
+                    self,
+                )
+                .break_value()
+            {
+                return ControlFlow::Break(obligation);
+            }
+        }
+
         ControlFlow::Break(self.obligation.clone())
     }
 }

compiler/rustc_trait_selection/src/solve/inspect/analyse.rs

@@ -278,6 +278,10 @@ impl<'a, 'tcx> InspectGoal<'a, 'tcx> {
         self.source
     }
 
+    pub fn depth(&self) -> usize {
+        self.depth
+    }
+
     fn candidates_recur(
         &'a self,
         candidates: &mut Vec<InspectCandidate<'a, 'tcx>>,
@@ -435,9 +439,18 @@ impl<'tcx> InferCtxt<'tcx> {
         &self,
         goal: Goal<'tcx, ty::Predicate<'tcx>>,
         visitor: &mut V,
+    ) -> V::Result {
+        self.visit_proof_tree_at_depth(goal, 0, visitor)
+    }
+
+    fn visit_proof_tree_at_depth<V: ProofTreeVisitor<'tcx>>(
+        &self,
+        goal: Goal<'tcx, ty::Predicate<'tcx>>,
+        depth: usize,
+        visitor: &mut V,
     ) -> V::Result {
         let (_, proof_tree) = self.evaluate_root_goal(goal, GenerateProofTree::Yes);
         let proof_tree = proof_tree.unwrap();
-        visitor.visit_goal(&InspectGoal::new(self, 0, proof_tree, None, GoalSource::Misc))
+        visitor.visit_goal(&InspectGoal::new(self, depth, proof_tree, None, GoalSource::Misc))
     }
 }