non non-wf alias detection into sub-fn

Author: lcnr

compiler/rustc_trait_selection/src/solve/fulfill/derive_errors.rs

@@ -290,6 +290,33 @@ impl<'tcx> BestObligation<'tcx> {
             ControlFlow::Continue(())
         }
     }
+
+    /// It is likely that `NormalizesTo` failed without any applicable candidates
+    /// because the alias is not well-formed.
+    ///
+    /// As we only enter `RigidAlias` candidates if the trait bound of the associated type
+    /// holds, we discard these candidates in `non_trivial_candidates` and always manually
+    /// check this here.
+    fn detect_non_well_formed_assoc_item(
+        &mut self,
+        goal: &inspect::InspectGoal<'_, 'tcx>,
+        alias: ty::AliasTerm<'tcx>,
+    ) -> ControlFlow<PredicateObligation<'tcx>> {
+        let tcx = goal.infcx().tcx;
+        let obligation = Obligation::new(
+            tcx,
+            self.obligation.cause.clone(),
+            goal.goal().param_env,
+            alias.trait_ref(tcx),
+        );
+        self.with_derived_obligation(obligation, |this| {
+            goal.infcx().visit_proof_tree_at_depth(
+                goal.goal().with(tcx, alias.trait_ref(tcx)),
+                goal.depth() + 1,
+                this,
+            )
+        })
+    }
 }
 
 impl<'tcx> ProofTreeVisitor<'tcx> for BestObligation<'tcx> {
@@ -321,23 +348,7 @@ impl<'tcx> ProofTreeVisitor<'tcx> for BestObligation<'tcx> {
                         | ty::AliasTermKind::ProjectionConst = pred.alias.kind(tcx) =>
                     {
                         self.detect_error_in_self_ty_normalization(goal, pred.alias.self_ty())?;
-                        // It is likely that `NormalizesTo` failed because the alias is not well-formed.
-                        // As we only enter `RigidAlias` candidates if the trait bound of the associated type
-                        // holds, we discard these candidates in `non_trivial_candidates` and always manually
-                        // check this here.
-                        let obligation = Obligation::new(
-                            tcx,
-                            self.obligation.cause.clone(),
-                            goal.goal().param_env,
-                            pred.alias.trait_ref(tcx),
-                        );
-                        self.with_derived_obligation(obligation, |this| {
-                            goal.infcx().visit_proof_tree_at_depth(
-                                goal.goal().with(tcx, pred.alias.trait_ref(tcx)),
-                                goal.depth() + 1,
-                                this,
-                            )
-                        })?;
+                        self.detect_non_well_formed_assoc_item(goal, pred.alias)?;
                     }
                     Some(_) | None => {}
                 }