lazily reevaluate nested goals

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

@@ -450,12 +450,15 @@ where
         let (orig_values, canonical_goal) = self.canonicalize_goal(goal);
         let mut goal_evaluation =
             self.inspect.new_goal_evaluation(goal, &orig_values, goal_evaluation_kind);
-        let canonical_result = self.search_graph.evaluate_goal(
-            self.cx(),
-            canonical_goal,
-            self.step_kind_for_source(source),
-            &mut goal_evaluation,
-        );
+        let canonical_result = self
+            .search_graph
+            .evaluate_goal(
+                self.cx(),
+                canonical_goal,
+                self.step_kind_for_source(source),
+                &mut goal_evaluation,
+            )
+            .1;
         goal_evaluation.query_result(canonical_result);
         self.inspect.goal_evaluation(goal_evaluation);
         let response = match canonical_result {

compiler/rustc_next_trait_solver/src/solve/search_graph.rs

@@ -48,7 +48,9 @@ where
     ) -> QueryResult<I> {
         match kind {
             PathKind::Coinductive => response_no_constraints(cx, input, Certainty::Yes),
-            PathKind::Unknown => response_no_constraints(cx, input, Certainty::overflow(false)),
+            PathKind::Unknown | PathKind::ForcedAmbiguity => {
+                response_no_constraints(cx, input, Certainty::overflow(false))
+            }
             // Even though we know these cycles to be unproductive, we still return
             // overflow during coherence. This is both as we are not 100% confident in
             // the implementation yet and any incorrect errors would be unsound there.

compiler/rustc_type_ir/src/search_graph/global_cache.rs

@@ -47,8 +47,14 @@ impl<X: Cx> GlobalCache<X> {
         evaluation_result: EvaluationResult<X>,
         dep_node: X::DepNodeIndex,
     ) {
-        let EvaluationResult { encountered_overflow, required_depth, heads, nested_goals, result } =
-            evaluation_result;
+        let EvaluationResult {
+            node_id: _,
+            encountered_overflow,
+            required_depth,
+            heads,
+            nested_goals,
+            result,
+        } = evaluation_result;
         debug_assert!(heads.is_empty());
         let result = cx.mk_tracked(result, dep_node);
         let entry = self.map.entry(input).or_default();