Return nested obligations from canonical response var unification

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

@@ -99,20 +99,20 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         param_env: ty::ParamEnv<'tcx>,
         original_values: Vec<ty::GenericArg<'tcx>>,
         response: CanonicalResponse<'tcx>,
-    ) -> Result<Certainty, NoSolution> {
+    ) -> Result<(Certainty, Vec<Goal<'tcx, ty::Predicate<'tcx>>>), NoSolution> {
         let substitution = self.compute_query_response_substitution(&original_values, &response);
 
         let Response { var_values, external_constraints, certainty } =
             response.substitute(self.tcx(), &substitution);
 
-        self.unify_query_var_values(param_env, &original_values, var_values)?;
+        let nested_goals = self.unify_query_var_values(param_env, &original_values, var_values)?;
 
         // FIXME: implement external constraints.
         let ExternalConstraintsData { region_constraints, opaque_types: _ } =
             external_constraints.deref();
         self.register_region_constraints(region_constraints);
 
-        Ok(certainty)
+        Ok((certainty, nested_goals))
     }
 
     /// This returns the substitutions to instantiate the bound variables of
@@ -205,21 +205,15 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         param_env: ty::ParamEnv<'tcx>,
         original_values: &[ty::GenericArg<'tcx>],
         var_values: CanonicalVarValues<'tcx>,
-    ) -> Result<(), NoSolution> {
+    ) -> Result<Vec<Goal<'tcx, ty::Predicate<'tcx>>>, NoSolution> {
         assert_eq!(original_values.len(), var_values.len());
+
+        let mut nested_goals = vec![];
         for (&orig, response) in iter::zip(original_values, var_values.var_values) {
-            // This can fail due to the occurs check, see
-            // `tests/ui/typeck/lazy-norm/equating-projection-cyclically.rs` for an example
-            // where that can happen.
-            //
-            // FIXME: To deal with #105787 I also expect us to emit nested obligations here at
-            // some point. We can figure out how to deal with this once we actually have
-            // an ICE.
-            let nested_goals = self.eq_and_get_goals(param_env, orig, response)?;
-            assert!(nested_goals.is_empty(), "{nested_goals:?}");
+            nested_goals.extend(self.eq_and_get_goals(param_env, orig, response)?);
         }
 
-        Ok(())
+        Ok(nested_goals)
     }
 
     fn register_region_constraints(&mut self, region_constraints: &QueryRegionConstraints<'tcx>) {

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -70,17 +70,16 @@ pub trait InferCtxtEvalExt<'tcx> {
     fn evaluate_root_goal(
         &self,
         goal: Goal<'tcx, ty::Predicate<'tcx>>,
-    ) -> Result<(bool, Certainty), NoSolution>;
+    ) -> Result<(bool, Certainty, Vec<Goal<'tcx, ty::Predicate<'tcx>>>), NoSolution>;
 }
 
 impl<'tcx> InferCtxtEvalExt<'tcx> for InferCtxt<'tcx> {
     #[instrument(level = "debug", skip(self))]
     fn evaluate_root_goal(
         &self,
         goal: Goal<'tcx, ty::Predicate<'tcx>>,
-    ) -> Result<(bool, Certainty), NoSolution> {
+    ) -> Result<(bool, Certainty, Vec<Goal<'tcx, ty::Predicate<'tcx>>>), NoSolution> {
         let mode = if self.intercrate { SolverMode::Coherence } else { SolverMode::Normal };
-
         let mut search_graph = search_graph::SearchGraph::new(self.tcx, mode);
 
         let mut ecx = EvalCtxt {
@@ -152,13 +151,13 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         &mut self,
         is_normalizes_to_hack: IsNormalizesToHack,
         goal: Goal<'tcx, ty::Predicate<'tcx>>,
-    ) -> Result<(bool, Certainty), NoSolution> {
+    ) -> Result<(bool, Certainty, Vec<Goal<'tcx, ty::Predicate<'tcx>>>), NoSolution> {
         let (orig_values, canonical_goal) = self.canonicalize_goal(goal);
         let canonical_response =
             EvalCtxt::evaluate_canonical_goal(self.tcx(), self.search_graph, canonical_goal)?;
 
         let has_changed = !canonical_response.value.var_values.is_identity();
-        let certainty = self.instantiate_and_apply_query_response(
+        let (certainty, nested_goals) = self.instantiate_and_apply_query_response(
             goal.param_env,
             orig_values,
             canonical_response,
@@ -186,7 +185,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             assert_eq!(certainty, canonical_response.value.certainty);
         }
 
-        Ok((has_changed, certainty))
+        Ok((has_changed, certainty, nested_goals))
     }
 
     fn compute_goal(&mut self, goal: Goal<'tcx, ty::Predicate<'tcx>>) -> QueryResult<'tcx> {
@@ -263,13 +262,14 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 let mut has_changed = Err(Certainty::Yes);
 
                 if let Some(goal) = goals.normalizes_to_hack_goal.take() {
-                    let (_, certainty) = match this.evaluate_goal(
+                    let (_, certainty, nested_goals) = match this.evaluate_goal(
                         IsNormalizesToHack::Yes,
                         goal.with(this.tcx(), ty::Binder::dummy(goal.predicate)),
                     ) {
                         Ok(r) => r,
                         Err(NoSolution) => return Some(Err(NoSolution)),
                     };
+                    new_goals.goals.extend(nested_goals);
 
                     if goal.predicate.projection_ty
                         != this.resolve_vars_if_possible(goal.predicate.projection_ty)
@@ -308,11 +308,12 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 }
 
                 for nested_goal in goals.goals.drain(..) {
-                    let (changed, certainty) =
+                    let (changed, certainty, nested_goals) =
                         match this.evaluate_goal(IsNormalizesToHack::No, nested_goal) {
                             Ok(result) => result,
                             Err(NoSolution) => return Some(Err(NoSolution)),
                         };
+                    new_goals.goals.extend(nested_goals);
 
                     if changed {
                         has_changed = Ok(());

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -1,6 +1,7 @@
 use std::mem;
 
 use rustc_infer::infer::InferCtxt;
+use rustc_infer::traits::Obligation;
 use rustc_infer::traits::{
     query::NoSolution, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes,
     PredicateObligation, SelectionError, TraitEngine,
@@ -61,7 +62,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
             let mut has_changed = false;
             for obligation in mem::take(&mut self.obligations) {
                 let goal = obligation.clone().into();
-                let (changed, certainty) = match infcx.evaluate_root_goal(goal) {
+                let (changed, certainty, nested_goals) = match infcx.evaluate_root_goal(goal) {
                     Ok(result) => result,
                     Err(NoSolution) => {
                         errors.push(FulfillmentError {
@@ -125,7 +126,16 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
                         continue;
                     }
                 };
-
+                // Push any nested goals that we get from unifying our canonical response
+                // with our obligation onto the fulfillment context.
+                self.obligations.extend(nested_goals.into_iter().map(|goal| {
+                    Obligation::new(
+                        infcx.tcx,
+                        obligation.cause.clone(),
+                        goal.param_env,
+                        goal.predicate,
+                    )
+                }));
                 has_changed |= changed;
                 match certainty {
                     Certainty::Yes => {}

compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs

@@ -81,10 +81,18 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
 
         if self.tcx.trait_solver_next() {
             self.probe(|snapshot| {
-                if let Ok((_, certainty)) =
+                if let Ok((_, certainty, nested_goals)) =
                     self.evaluate_root_goal(Goal::new(self.tcx, param_env, obligation.predicate))
                 {
                     match certainty {
+                        // If we have nested obligations from instantiating the canonical
+                        // response from this goal, just treat the response as ambiguous.
+                        //
+                        // FIXME(deferred_projection_equality): We need to process this
+                        // in a loop probably... can't be worse than an ICE though
+                        Certainty::Yes if !nested_goals.is_empty() => {
+                            Ok(EvaluationResult::EvaluatedToAmbig)
+                        }
                         Certainty::Yes => {
                             if self.opaque_types_added_in_snapshot(snapshot) {
                                 Ok(EvaluationResult::EvaluatedToOkModuloOpaqueTypes)

tests/ui/traits/new-solver/alias-eq-in-canonical-response.rs

@@ -0,0 +1,40 @@
+// check-pass
+// compile-flags: -Ztrait-solver=next
+
+trait Foo {
+    type Gat<'a>
+    where
+        Self: 'a;
+    fn bar(&self) -> Self::Gat<'_>;
+}
+
+enum Option<T> {
+    Some(T),
+    None,
+}
+
+impl<T> Option<T> {
+    fn as_ref(&self) -> Option<&T> {
+        match self {
+            Option::Some(t) => Option::Some(t),
+            Option::None => Option::None,
+        }
+    }
+
+    fn map<U>(self, f: impl FnOnce(T) -> U) -> Option<U> {
+        match self {
+            Option::Some(t) => Option::Some(f(t)),
+            Option::None => Option::None,
+        }
+    }
+}
+
+impl<T: Foo + 'static> Foo for Option<T> {
+    type Gat<'a> = Option<<T as Foo>::Gat<'a>> where Self: 'a;
+
+    fn bar(&self) -> Self::Gat<'_> {
+        self.as_ref().map(Foo::bar)
+    }
+}
+
+fn main() {}