diff -r 000000000000 -r 6474c204b198 xpcom/glue/nsJSThingHashtable.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/glue/nsJSThingHashtable.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,58 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsJSThingHashtable_h__
+#define nsJSThingHashtable_h__
+
+#include "nsHashKeys.h"
+#include "nsBaseHashtable.h"
+
+namespace JS {
+template <class T>
+class Heap;
+} /* namespace JS */
+
+/**
+ * A wrapper for hash keys that sets ALLOW_MEMMOVE to false.
+ *
+ * This is used in the implementation of nsJSThingHashtable and is not intended
+ * to be used directly.
+ *
+ * It is necessary for hash tables containing JS::Heap<T> values as these must
+ * be copied rather than memmoved.
+ */
+template<class T>
+class nsHashKeyDisallowMemmove : public T
+{
+ public:
+  nsHashKeyDisallowMemmove(const T& key) : T(key) {}
+  enum { ALLOW_MEMMOVE = false };
+};
+
+
+/**
+ * Templated hashtable class for use on the heap where the values are JS GC things.
+ *
+ * Storing JS GC thing pointers on the heap requires wrapping them in a
+ * JS::Heap<T>, and this class takes care of that while presenting an interface
+ * in terms of the wrapped type T.
+ *
+ * For example, to store a hashtable mapping strings to JSObject pointers, you
+ * can declare a data member like this:
+ *
+ *   nsJSThingHashtable<nsStringHashKey, JSObject*> mStringToObjectMap;
+ *
+ * See nsBaseHashtable for complete declaration
+ * @param KeyClass a wrapper-class for the hashtable key, see nsHashKeys.h
+ *   for a complete specification.
+ * @param DataType the datatype being wrapped, must be a JS GC thing.
+ * @see nsInterfaceHashtable, nsClassHashtable
+ */
+template<class KeyClass,class DataType>
+class nsJSThingHashtable :
+  public nsBaseHashtable<nsHashKeyDisallowMemmove<KeyClass>, JS::Heap<DataType>, DataType>
+{ };
+
+#endif // nsJSThingHashtable_h__