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diff --git a/xpcom/ds/nsHashtablesFwd.h b/xpcom/ds/nsHashtablesFwd.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 XPCOM_DS_NSHASHTABLESFWD_H_
+#define XPCOM_DS_NSHASHTABLESFWD_H_
+
+#include "mozilla/Attributes.h"
+
+struct PLDHashEntryHdr;
+
+template <class T>
+class MOZ_IS_REFPTR nsCOMPtr;
+
+template <class T>
+class MOZ_IS_REFPTR RefPtr;
+
+template <class EntryType>
+class MOZ_NEEDS_NO_VTABLE_TYPE nsTHashtable;
+
+template <class DataType, class UserDataType>
+class nsDefaultConverter;
+
+template <class KeyClass, class DataType, class UserDataType,
+          class Converter = nsDefaultConverter<DataType, UserDataType>>
+class nsBaseHashtable;
+
+template <class KeyClass, class T>
+class nsClassHashtable;
+
+template <class KeyClass, class PtrType>
+class nsRefCountedHashtable;
+
+/**
+ * templated hashtable class maps keys to interface pointers.
+ * See nsBaseHashtable for complete declaration.
+ * @deprecated This is going to be removed. Use nsTHashMap instead.
+ * @param KeyClass a wrapper-class for the hashtable key, see nsHashKeys.h
+ *   for a complete specification.
+ * @param Interface the interface-type being wrapped
+ * @see nsClassHashtable, nsTHashMap
+ */
+template <class KeyClass, class Interface>
+using nsInterfaceHashtable =
+    nsRefCountedHashtable<KeyClass, nsCOMPtr<Interface>>;
+
+/**
+ * templated hashtable class maps keys to reference pointers.
+ * See nsBaseHashtable for complete declaration.
+ * @deprecated This is going to be removed. Use nsTHashMap instead.
+ * @param KeyClass a wrapper-class for the hashtable key, see nsHashKeys.h
+ *   for a complete specification.
+ * @param PtrType the reference-type being wrapped
+ * @see nsClassHashtable, nsTHashMap
+ */
+template <class KeyClass, class ClassType>
+using nsRefPtrHashtable = nsRefCountedHashtable<KeyClass, RefPtr<ClassType>>;
+
+namespace mozilla::detail {
+template <class KeyType, class = void>
+struct nsKeyClass;
+}  // namespace mozilla::detail
+
+/**
+ * A universal hash map that maps some KeyType to some DataType. It can be used
+ * for any DataType, including RefPtr<T>, nsCOMPtr<T> and UniquePtr<T>.
+ *
+ * For the default hash keys types, the appropriate hash key class is determined
+ * automatically, so you can just specify `nsTHashMap<uint32_t,
+ * RefPtr<Foo>>`, for example.
+ *
+ * If you require custom hash behaviour (e.g. case insensitive string handling),
+ * you can still specify a hash key class derived from PLDHashEntryHdr
+ * explicitly.
+ *
+ * If you need to use a custom UserDataType, use nsBaseHashtable (or
+ * nsTHashtable) directly. However, you should double-check if that's really
+ * necessary.
+ */
+template <class KeyType, class DataType>
+using nsTHashMap =
+    nsBaseHashtable<typename mozilla::detail::nsKeyClass<KeyType>::type,
+                    DataType, DataType>;
+
+template <class KeyClass>
+class nsTBaseHashSet;
+
+template <class KeyType>
+using nsTHashSet =
+    nsTBaseHashSet<typename mozilla::detail::nsKeyClass<KeyType>::type>;
+
+#endif  // XPCOM_DS_NSHASHTABLESFWD_H_