diff --git a/js/src/vm/Shape-inl.h b/js/src/vm/Shape-inl.h
index 1846ec819cf..4617d3395b9 100644
--- a/js/src/vm/Shape-inl.h
+++ b/js/src/vm/Shape-inl.h
@@ -234,14 +234,6 @@ AutoRooterGetterSetter::AutoRooterGetterSetter(ThreadSafeContext *cx, uint8_t at
     MOZ_GUARD_OBJECT_NOTIFIER_INIT;
 }
 
-inline
-StackBaseShape::AutoRooter::AutoRooter(ThreadSafeContext *cx, const StackBaseShape *base_
-                                       MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL)
-  : CustomAutoRooter(cx), base(base_), skip(cx, base_)
-{
-    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
-}
-
 inline
 StackShape::AutoRooter::AutoRooter(ThreadSafeContext *cx, const StackShape *shape_
                                    MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL)
diff --git a/js/src/vm/Shape.cpp b/js/src/vm/Shape.cpp
index 41c465fc8c9..531bf1ad006 100644
--- a/js/src/vm/Shape.cpp
+++ b/js/src/vm/Shape.cpp
@@ -306,7 +306,7 @@ ShapeTable::grow(ThreadSafeContext *cx)
 }
 
 /* static */ Shape *
-Shape::replaceLastProperty(ExclusiveContext *cx, const StackBaseShape &base,
+Shape::replaceLastProperty(ExclusiveContext *cx, StackBaseShape &base,
                            TaggedProto proto, HandleShape shape)
 {
     JS_ASSERT(!shape->inDictionary());
@@ -546,7 +546,7 @@ ShouldConvertToDictionary(JSObject *obj)
 template <ExecutionMode mode>
 static inline UnownedBaseShape *
 GetOrLookupUnownedBaseShape(typename ExecutionModeTraits<mode>::ExclusiveContextType cx,
-                            const StackBaseShape &base)
+                            StackBaseShape &base)
 {
     if (mode == ParallelExecution)
         return BaseShape::lookupUnowned(cx, base);
@@ -1439,28 +1439,28 @@ StackBaseShape::match(UnownedBaseShape *key, const StackBaseShape *lookup)
 }
 
 void
-StackBaseShape::AutoRooter::trace(JSTracer *trc)
+StackBaseShape::trace(JSTracer *trc)
 {
-    if (base->parent) {
-        gc::MarkObjectRoot(trc, (JSObject**)&base->parent,
-                           "StackBaseShape::AutoRooter parent");
+    if (parent) {
+        gc::MarkObjectRoot(trc, (JSObject**)&parent,
+                           "StackBaseShape parent");
     }
-    if (base->metadata) {
-        gc::MarkObjectRoot(trc, (JSObject**)&base->metadata,
-                           "StackBaseShape::AutoRooter metadata");
+    if (metadata) {
+        gc::MarkObjectRoot(trc, (JSObject**)&metadata,
+                           "StackBaseShape metadata");
     }
-    if ((base->flags & BaseShape::HAS_GETTER_OBJECT) && base->rawGetter) {
-        gc::MarkObjectRoot(trc, (JSObject**)&base->rawGetter,
-                           "StackBaseShape::AutoRooter getter");
+    if ((flags & BaseShape::HAS_GETTER_OBJECT) && rawGetter) {
+        gc::MarkObjectRoot(trc, (JSObject**)&rawGetter,
+                           "StackBaseShape getter");
     }
-    if ((base->flags & BaseShape::HAS_SETTER_OBJECT) && base->rawSetter) {
-        gc::MarkObjectRoot(trc, (JSObject**)&base->rawSetter,
-                           "StackBaseShape::AutoRooter setter");
+    if ((flags & BaseShape::HAS_SETTER_OBJECT) && rawSetter) {
+        gc::MarkObjectRoot(trc, (JSObject**)&rawSetter,
+                           "StackBaseShape setter");
     }
 }
 
 /* static */ UnownedBaseShape*
-BaseShape::getUnowned(ExclusiveContext *cx, const StackBaseShape &base)
+BaseShape::getUnowned(ExclusiveContext *cx, StackBaseShape &base)
 {
     BaseShapeSet &table = cx->compartment()->baseShapes;
 
@@ -1471,17 +1471,17 @@ BaseShape::getUnowned(ExclusiveContext *cx, const StackBaseShape &base)
     if (p)
         return *p;
 
-    StackBaseShape::AutoRooter root(cx, &base);
+    RootedGeneric<StackBaseShape*> root(cx, &base);
 
     BaseShape *nbase_ = js_NewGCBaseShape<CanGC>(cx);
     if (!nbase_)
         return nullptr;
 
-    new (nbase_) BaseShape(base);
+    new (nbase_) BaseShape(*root);
 
     UnownedBaseShape *nbase = static_cast<UnownedBaseShape *>(nbase_);
 
-    if (!p.add(cx, table, &base, nbase))
+    if (!p.add(cx, table, root, nbase))
         return nullptr;
 
     return nbase;
diff --git a/js/src/vm/Shape.h b/js/src/vm/Shape.h
index d81da1d0af3..227b00f829b 100644
--- a/js/src/vm/Shape.h
+++ b/js/src/vm/Shape.h
@@ -715,7 +715,7 @@ class BaseShape : public gc::BarrieredCell<BaseShape>
      * Lookup base shapes from the compartment's baseShapes table, adding if
      * not already found.
      */
-    static UnownedBaseShape* getUnowned(ExclusiveContext *cx, const StackBaseShape &base);
+    static UnownedBaseShape* getUnowned(ExclusiveContext *cx, StackBaseShape &base);
 
     /*
      * Lookup base shapes from the compartment's baseShapes table, returning
@@ -847,19 +847,9 @@ struct StackBaseShape
     static inline HashNumber hash(const StackBaseShape *lookup);
     static inline bool match(UnownedBaseShape *key, const StackBaseShape *lookup);
 
-    class AutoRooter : private JS::CustomAutoRooter
-    {
-      public:
-        inline AutoRooter(ThreadSafeContext *cx, const StackBaseShape *base_
-                          MOZ_GUARD_OBJECT_NOTIFIER_PARAM);
-
-      private:
-        virtual void trace(JSTracer *trc);
-
-        const StackBaseShape *base;
-        SkipRoot skip;
-        MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
-    };
+    // For RootedGeneric<StackBaseShape*>
+    static inline js::ThingRootKind rootKind() { return js::THING_ROOT_CUSTOM; }
+    void trace(JSTracer *trc);
 };
 
 inline
@@ -961,7 +951,7 @@ class Shape : public gc::BarrieredCell<Shape>
     }
 
     /* Replace the base shape of the last shape in a non-dictionary lineage with base. */
-    static Shape *replaceLastProperty(ExclusiveContext *cx, const StackBaseShape &base,
+    static Shape *replaceLastProperty(ExclusiveContext *cx, StackBaseShape &base,
                                       TaggedProto proto, HandleShape shape);
 
     /*