diff -r 000000000000 -r 6474c204b198 content/base/src/nsAttrValueOrString.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/content/base/src/nsAttrValueOrString.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,62 @@
+/* -*- 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/. */
+
+/*
+ * A wrapper to contain either an nsAttrValue or an nsAString. This is useful
+ * because constructing an nsAttrValue from an nsAString can be expensive when
+ * the buffer of the string is not shared.
+ *
+ * Since a raw pointer to the passed-in string is kept, this class should only
+ * be used on the stack.
+ */
+
+#ifndef nsAttrValueOrString_h___
+#define nsAttrValueOrString_h___
+
+#include "nsString.h"
+#include "nsAttrValue.h"
+
+class MOZ_STACK_CLASS nsAttrValueOrString
+{
+public:
+  nsAttrValueOrString(const nsAString& aValue)
+    : mAttrValue(nullptr)
+    , mStringPtr(&aValue)
+    , mCheapString(nullptr)
+  { }
+  nsAttrValueOrString(const nsAttrValue& aValue)
+    : mAttrValue(&aValue)
+    , mStringPtr(nullptr)
+    , mCheapString(nullptr)
+  { }
+
+  /**
+   * Returns a reference to the string value of the contents of this object.
+   *
+   * When this object points to a string or an nsAttrValue of string or atom
+   * type this should be fairly cheap. Other nsAttrValue types will be
+   * serialized the first time this is called and cached from thereon.
+   */
+  const nsAString& String() const;
+
+  /**
+   * Compares the string representation of this object with the string
+   * representation of an nsAttrValue.
+   */
+  bool EqualsAsStrings(const nsAttrValue& aOther) const
+  {
+    if (mStringPtr) {
+      return aOther.Equals(*mStringPtr, eCaseMatters);
+    }
+    return aOther.EqualsAsStrings(*mAttrValue);
+  }
+
+protected:
+  const nsAttrValue*       mAttrValue;
+  mutable const nsAString* mStringPtr;
+  mutable nsCheapString    mCheapString;
+};
+
+#endif // nsAttrValueOrString_h___