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diff --git a/xpcom/ds/nsJSThingHashtable.h b/xpcom/ds/nsJSThingHashtable.h
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+++ b/xpcom/ds/nsJSThingHashtable.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "js/RootingAPI.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:
+  explicit nsHashKeyDisallowMemmove(const typename T::KeyTypePointer aKey)
+      : T(aKey) {}
+  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__