Assert that obligations are empty before deeply normalizing

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -199,6 +199,10 @@ where
         errors
     }
 
+    fn has_pending_obligations(&self) -> bool {
+        !self.obligations.pending.is_empty() || !self.obligations.overflowed.is_empty()
+    }
+
     fn pending_obligations(&self) -> PredicateObligations<'tcx> {
         self.obligations.clone_pending()
     }

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -213,6 +213,10 @@ where
         }
     }
 
+    fn has_pending_obligations(&self) -> bool {
+        self.predicates.has_pending_obligations()
+    }
+
     fn pending_obligations(&self) -> PredicateObligations<'tcx> {
         self.predicates.map_pending_obligations(|o| o.obligation.clone())
     }

compiler/rustc_trait_selection/src/traits/normalize.rs

@@ -7,6 +7,7 @@ use rustc_infer::traits::{
     FromSolverError, Normalized, Obligation, PredicateObligations, TraitEngine,
 };
 use rustc_macros::extension;
+use rustc_middle::span_bug;
 use rustc_middle::traits::{ObligationCause, ObligationCauseCode};
 use rustc_middle::ty::{
     self, Ty, TyCtxt, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitable, TypeVisitableExt,
@@ -63,6 +64,14 @@ impl<'tcx> At<'_, 'tcx> {
         if self.infcx.next_trait_solver() {
             crate::solve::deeply_normalize(self, value)
         } else {
+            if fulfill_cx.has_pending_obligations() {
+                let pending_obligations = fulfill_cx.pending_obligations();
+                span_bug!(
+                    pending_obligations[0].cause.span,
+                    "deeply_normalize should not be called with pending obligations: \
+                    {pending_obligations:#?}"
+                );
+            }
             let value = self
                 .normalize(value)
                 .into_value_registering_obligations(self.infcx, &mut *fulfill_cx);

compiler/rustc_trait_selection/src/traits/query/type_op/implied_outlives_bounds.rs

@@ -60,6 +60,9 @@ pub fn compute_implied_outlives_bounds_inner<'tcx>(
     ty: Ty<'tcx>,
 ) -> Result<Vec<OutlivesBound<'tcx>>, NoSolution> {
     let normalize_op = |ty| -> Result<_, NoSolution> {
+        // We must normalize the type so we can compute the right outlives components.
+        // for example, if we have some constrained param type like `T: Trait<Out = U>`,
+        // and we know that `&'a T::Out` is WF, then we want to imply `U: 'a`.
         let ty = ocx
             .deeply_normalize(&ObligationCause::dummy(), param_env, ty)
             .map_err(|_| NoSolution)?;