--- a/js/public/GCHashTable.h
+++ b/js/public/GCHashTable.h
@@ -87,23 +87,24 @@ class GCHashMap : public js::HashMap<Key
     for (; !e.empty(); e.popFront()) {
       if (MapSweepPolicy::needsSweep(&e.front().mutableKey(),
                                      &e.front().value())) {
         e.removeFront();
       }
     }
   }
 
-  void traceWeak(JSTracer* trc) {
+  bool traceWeak(JSTracer* trc) {
     for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
       if (!MapSweepPolicy::traceWeak(trc, &e.front().mutableKey(),
                                      &e.front().value())) {
         e.removeFront();
       }
     }
+    return !this->empty();
   }
 
   // GCHashMap is movable
   GCHashMap(GCHashMap&& rhs) : Base(std::move(rhs)) {}
   void operator=(GCHashMap&& rhs) {
     MOZ_ASSERT(this != &rhs, "self-move assignment is prohibited");
     Base::operator=(std::move(rhs));
   }
@@ -144,25 +145,26 @@ class GCRekeyableHashMap : public JS::GC
       if (MapSweepPolicy::needsSweep(&key, &e.front().value())) {
         e.removeFront();
       } else if (!HashPolicy::match(key, e.front().key())) {
         e.rekeyFront(key);
       }
     }
   }
 
-  void traceWeak(JSTracer* trc) {
+  bool traceWeak(JSTracer* trc) {
     for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
       Key key(e.front().key());
       if (!MapSweepPolicy::traceWeak(trc, &key, &e.front().value())) {
         e.removeFront();
       } else if (!HashPolicy::match(key, e.front().key())) {
         e.rekeyFront(key);
       }
     }
+    return !this->empty();
   }
 
   // GCRekeyableHashMap is movable
   GCRekeyableHashMap(GCRekeyableHashMap&& rhs) : Base(std::move(rhs)) {}
   void operator=(GCRekeyableHashMap&& rhs) {
     MOZ_ASSERT(this != &rhs, "self-move assignment is prohibited");
     Base::operator=(std::move(rhs));
   }
@@ -283,22 +285,23 @@ class GCHashSet : public js::HashSet<T, 
   void sweepEntries(typename Base::Enum& e) {
     for (; !e.empty(); e.popFront()) {
       if (GCPolicy<T>::needsSweep(&e.mutableFront())) {
         e.removeFront();
       }
     }
   }
 
-  void traceWeak(JSTracer* trc) {
+  bool traceWeak(JSTracer* trc) {
     for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
       if (!GCPolicy<T>::traceWeak(trc, &e.mutableFront())) {
         e.removeFront();
       }
     }
+    return !this->empty();
   }
 
   // GCHashSet is movable
   GCHashSet(GCHashSet&& rhs) : Base(std::move(rhs)) {}
   void operator=(GCHashSet&& rhs) {
     MOZ_ASSERT(this != &rhs, "self-move assignment is prohibited");
     Base::operator=(std::move(rhs));
   }