diff -r 000000000000 -r 6474c204b198 xpcom/string/public/nsTDependentSubstring.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/string/public/nsTDependentSubstring.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,110 @@
+/* -*- 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"
+
+  /**
+   * nsTDependentSubstring_CharT
+   *
+   * A string class which wraps an external array of string characters. It
+   * is the client code's responsibility to ensure that the external buffer
+   * remains valid for a long as the string is alive.
+   *
+   * NAMES:
+   *   nsDependentSubstring for wide characters
+   *   nsDependentCSubstring for narrow characters
+   */
+class nsTDependentSubstring_CharT : public nsTSubstring_CharT
+  {
+    public:
+
+      typedef nsTDependentSubstring_CharT    self_type;
+
+    public:
+
+      void Rebind( const substring_type&, uint32_t startPos, uint32_t length = size_type(-1) );
+
+      void Rebind( const char_type* data, size_type length );
+
+      void Rebind( const char_type* start, const char_type* end )
+        {
+          Rebind(start, size_type(end - start));
+        }
+
+      nsTDependentSubstring_CharT( const substring_type& str, uint32_t startPos, uint32_t length = size_type(-1) )
+        : substring_type()
+        {
+          Rebind(str, startPos, length);
+        }
+
+      nsTDependentSubstring_CharT( const char_type* data, size_type length )
+        : substring_type(const_cast<char_type*>(data), length, F_NONE) {}
+
+      nsTDependentSubstring_CharT( const char_type* start, const char_type* end )
+        : substring_type(const_cast<char_type*>(start), uint32_t(end - start), F_NONE) {}
+
+#if defined(CharT_is_PRUnichar) && defined(MOZ_USE_CHAR16_WRAPPER)
+      nsTDependentSubstring_CharT( char16ptr_t data, size_type length )
+        : nsTDependentSubstring_CharT(static_cast<const char16_t*>(data), length) {}
+
+      nsTDependentSubstring_CharT( char16ptr_t start, char16ptr_t end )
+        : nsTDependentSubstring_CharT(static_cast<const char16_t*>(start), static_cast<const char16_t*>(end)) {}
+#endif
+
+      nsTDependentSubstring_CharT( const const_iterator& start, const const_iterator& end )
+        : substring_type(const_cast<char_type*>(start.get()), uint32_t(end.get() - start.get()), F_NONE) {}
+
+      // Create a nsTDependentSubstring to be bound later
+      nsTDependentSubstring_CharT()
+        : substring_type() {}
+
+      // auto-generated copy-constructor OK (XXX really?? what about base class copy-ctor?)
+
+    private:
+        // NOT USED
+      void operator=( const self_type& );        // we're immutable, you can't assign into a substring
+  };
+
+inline
+const nsTDependentSubstring_CharT
+Substring( const nsTSubstring_CharT& str, uint32_t startPos, uint32_t length = uint32_t(-1) )
+  {
+    return nsTDependentSubstring_CharT(str, startPos, length);
+  }
+
+inline
+const nsTDependentSubstring_CharT
+Substring( const nsReadingIterator<CharT>& start, const nsReadingIterator<CharT>& end )
+  {
+    return nsTDependentSubstring_CharT(start.get(), end.get());
+  }
+
+inline
+const nsTDependentSubstring_CharT
+Substring( const CharT* data, uint32_t length )
+  {
+    return nsTDependentSubstring_CharT(data, length);
+  }
+
+inline
+const nsTDependentSubstring_CharT
+Substring( const CharT* start, const CharT* end )
+  {
+    return nsTDependentSubstring_CharT(start, end);
+  }
+
+inline
+const nsTDependentSubstring_CharT
+StringHead( const nsTSubstring_CharT& str, uint32_t count )
+  {
+    return nsTDependentSubstring_CharT(str, 0, count);
+  }
+
+inline
+const nsTDependentSubstring_CharT
+StringTail( const nsTSubstring_CharT& str, uint32_t count )
+  {
+    return nsTDependentSubstring_CharT(str, str.Length() - count, count);
+  }