diff --git a/src/librustc/dep_graph/dep_tracking_map.rs b/src/librustc/dep_graph/dep_tracking_map.rs
index 103f8ef653c1e..ad6c93bf8daac 100644
--- a/src/librustc/dep_graph/dep_tracking_map.rs
+++ b/src/librustc/dep_graph/dep_tracking_map.rs
@@ -63,8 +63,8 @@ impl<M: DepTrackingMapConfig> DepTrackingMap<M> {
 
     pub fn insert(&mut self, k: M::Key, v: M::Value) {
         self.write(&k);
-        let _old_value = self.map.insert(k, v);
-        // assert!(old_value.is_none());
+        let old_value = self.map.insert(k, v);
+        assert!(old_value.is_none());
     }
 
     pub fn contains_key(&self, k: &M::Key) -> bool {
diff --git a/src/librustc_typeck/collect.rs b/src/librustc_typeck/collect.rs
index cb0a6cccb03d4..266975994ec3a 100644
--- a/src/librustc_typeck/collect.rs
+++ b/src/librustc_typeck/collect.rs
@@ -848,9 +848,10 @@ fn convert_trait_item(ccx: &CrateCtxt, trait_item: &hir::TraitItem) {
             let const_def_id = ccx.tcx.hir.local_def_id(trait_item.id);
             generics_of_def_id(ccx, const_def_id);
             let ty = ccx.icx(&trait_predicates).to_ty(&ty);
-            tcx.item_types.borrow_mut().insert(const_def_id, ty);
-            convert_associated_const(ccx, TraitContainer(trait_def_id),
-                                     trait_item.id, ty);
+            convert_associated_const(ccx,
+                                     TraitContainer(trait_def_id),
+                                     trait_item.id,
+                                     ty);
         }
 
         hir::TraitItemKind::Type(_, ref opt_ty) => {
@@ -884,9 +885,10 @@ fn convert_impl_item(ccx: &CrateCtxt, impl_item: &hir::ImplItem) {
             let const_def_id = ccx.tcx.hir.local_def_id(impl_item.id);
             generics_of_def_id(ccx, const_def_id);
             let ty = ccx.icx(&impl_predicates).to_ty(&ty);
-            tcx.item_types.borrow_mut().insert(const_def_id, ty);
-            convert_associated_const(ccx, ImplContainer(impl_def_id),
-                                     impl_item.id, ty);
+            convert_associated_const(ccx,
+                                     ImplContainer(impl_def_id),
+                                     impl_item.id,
+                                     ty);
         }
 
         hir::ImplItemKind::Type(ref ty) => {