assert that solver results are stable

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -174,6 +174,7 @@ fn evaluate_root_goal(
             search_graph: &mut search_graph,
             infcx: self,
             var_values: CanonicalVarValues::dummy(),
+            in_projection_eq_hack: false,
         }
         .evaluate_goal(goal);
 
@@ -187,6 +188,10 @@ struct EvalCtxt<'a, 'tcx> {
     var_values: CanonicalVarValues<'tcx>,
 
     search_graph: &'a mut search_graph::SearchGraph<'tcx>,
+
+    /// This field is used by a debug assertion in [`EvalCtxt::evaluate_goal`],
+    /// see the comment in that method for more details.
+    in_projection_eq_hack: bool,
 }
 
 impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
@@ -213,7 +218,8 @@ fn evaluate_canonical_goal(
         loop {
             let (ref infcx, goal, var_values) =
                 tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &canonical_goal);
-            let mut ecx = EvalCtxt { infcx, var_values, search_graph };
+            let mut ecx =
+                EvalCtxt { infcx, var_values, search_graph, in_projection_eq_hack: false };
             let result = ecx.compute_goal(goal);
 
             // FIXME: `Response` should be `Copy`
@@ -243,10 +249,28 @@ fn evaluate_goal(
         let canonical_goal = self.infcx.canonicalize_query(goal, &mut orig_values);
         let canonical_response =
             EvalCtxt::evaluate_canonical_goal(self.tcx(), self.search_graph, canonical_goal)?;
-        Ok((
-            !canonical_response.value.var_values.is_identity(),
-            instantiate_canonical_query_response(self.infcx, &orig_values, canonical_response),
-        ))
+
+        let has_changed = !canonical_response.value.var_values.is_identity();
+        let certainty =
+            instantiate_canonical_query_response(self.infcx, &orig_values, canonical_response);
+
+        // Check that rerunning this query with its inference constraints applied
+        // doesn't result in new inference constraints and has the same result.
+        //
+        // If we have projection goals like `<T as Trait>::Assoc == u32` we recursively
+        // call `exists<U> <T as Trait>::Assoc == U` to enable better caching. This goal
+        // could constrain `U` to `u32` which would cause this check to result in a
+        // solver cycle.
+        if cfg!(debug_assertions) && has_changed && !self.in_projection_eq_hack {
+            let mut orig_values = OriginalQueryValues::default();
+            let canonical_goal = self.infcx.canonicalize_query(goal, &mut orig_values);
+            let canonical_response =
+                EvalCtxt::evaluate_canonical_goal(self.tcx(), self.search_graph, canonical_goal)?;
+            assert!(canonical_response.value.var_values.is_identity());
+            assert_eq!(certainty, canonical_response.value.certainty);
+        }
+
+        Ok((has_changed, certainty))
     }
 
     fn compute_goal(&mut self, goal: Goal<'tcx, ty::Predicate<'tcx>>) -> QueryResult<'tcx> {

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -45,8 +45,9 @@ pub(super) fn compute_projection_goal(
                 projection_ty: goal.predicate.projection_ty,
                 term: unconstrained_rhs,
             });
-            let (_has_changed, normalize_certainty) =
-                self.evaluate_goal(goal.with(self.tcx(), unconstrained_predicate))?;
+            let (_has_changed, normalize_certainty) = self.in_projection_eq_hack(|this| {
+                this.evaluate_goal(goal.with(this.tcx(), unconstrained_predicate))
+            })?;
 
             let nested_eq_goals =
                 self.infcx.eq(goal.param_env, unconstrained_rhs, predicate.term)?;
@@ -55,6 +56,15 @@ pub(super) fn compute_projection_goal(
         }
     }
 
+    /// This sets a flag used by a debug assert in [`EvalCtxt::evaluate_goal`],
+    /// see the comment in that method for more details.
+    fn in_projection_eq_hack<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
+        self.in_projection_eq_hack = true;
+        let result = f(self);
+        self.in_projection_eq_hack = false;
+        result
+    }
+
     /// Is the projection predicate is of the form `exists<T> <Ty as Trait>::Assoc = T`.
     ///
     /// This is the case if the `term` is an inference variable in the innermost universe

compiler/rustc_trait_selection/src/solve/search_graph/mod.rs

@@ -45,6 +45,7 @@ pub(super) fn is_empty(&self) -> bool {
     /// Tries putting the new goal on the stack, returning an error if it is already cached.
     ///
     /// This correctly updates the provisional cache if there is a cycle.
+    #[instrument(level = "debug", skip(self, tcx), ret)]
     pub(super) fn try_push_stack(
         &mut self,
         tcx: TyCtxt<'tcx>,
@@ -79,8 +80,10 @@ pub(super) fn try_push_stack(
             Entry::Occupied(entry_index) => {
                 let entry_index = *entry_index.get();
 
-                cache.add_dependency_of_leaf_on(entry_index);
                 let stack_depth = cache.depth(entry_index);
+                debug!("encountered cycle with depth {stack_depth:?}");
+
+                cache.add_dependency_of_leaf_on(entry_index);
 
                 self.stack[stack_depth].has_been_used = true;
                 // NOTE: The goals on the stack aren't the only goals involved in this cycle.
@@ -117,6 +120,7 @@ pub(super) fn try_push_stack(
     /// updated the provisional cache and we have to recompute the current goal.
     ///
     /// FIXME: Refer to the rustc-dev-guide entry once it exists.
+    #[instrument(level = "debug", skip(self, tcx, actual_goal), ret)]
     pub(super) fn try_finalize_goal(
         &mut self,
         tcx: TyCtxt<'tcx>,