diff -r 000000000000 -r 6474c204b198 xpcom/string/public/nsTSubstringTuple.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/string/public/nsTSubstringTuple.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,78 @@
+//* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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/. */
+// IWYU pragma: private, include "nsString.h"
+
+  /**
+   * nsTSubstringTuple_CharT
+   *
+   * Represents a tuple of string fragments.  Built as a recursive binary tree.
+   * It is used to implement the concatenation of two or more string objects.
+   *
+   * NOTE: This class is a private implementation detail and should never be 
+   * referenced outside the string code.
+   */
+class nsTSubstringTuple_CharT
+  {
+    public:
+
+      typedef CharT                      char_type;
+      typedef nsCharTraits<char_type>    char_traits;
+
+      typedef nsTSubstringTuple_CharT    self_type;
+      typedef nsTSubstring_CharT         substring_type;
+      typedef nsTSubstring_CharT         base_string_type;
+      typedef uint32_t                   size_type;
+
+    public:
+
+      nsTSubstringTuple_CharT(const base_string_type* a, const base_string_type* b)
+        : mHead(nullptr)
+        , mFragA(a)
+        , mFragB(b) {}
+
+      nsTSubstringTuple_CharT(const self_type& head, const base_string_type* b)
+        : mHead(&head)
+        , mFragA(nullptr) // this fragment is ignored when head != nullptr
+        , mFragB(b) {}
+
+        /**
+         * computes the aggregate string length
+         */
+      size_type Length() const;
+
+        /**
+         * writes the aggregate string to the given buffer.  bufLen is assumed
+         * to be equal to or greater than the value returned by the Length()
+         * method.  the string written to |buf| is not null-terminated.
+         */
+      void WriteTo(char_type *buf, uint32_t bufLen) const;
+
+        /**
+         * returns true if this tuple is dependent on (i.e., overlapping with)
+         * the given char sequence.
+         */
+      bool IsDependentOn(const char_type *start, const char_type *end) const;
+
+    private:
+
+      const self_type*        mHead;
+      const base_string_type* mFragA;
+      const base_string_type* mFragB;
+  };
+
+inline
+const nsTSubstringTuple_CharT
+operator+(const nsTSubstringTuple_CharT::base_string_type& a, const nsTSubstringTuple_CharT::base_string_type& b)
+  {
+    return nsTSubstringTuple_CharT(&a, &b);
+  }
+
+inline
+const nsTSubstringTuple_CharT
+operator+(const nsTSubstringTuple_CharT& head, const nsTSubstringTuple_CharT::base_string_type& b)
+  {
+    return nsTSubstringTuple_CharT(head, &b);
+  }