new file mode 100644
--- /dev/null
+++ b/mfbt/Char16.h
@@ -0,0 +1,46 @@
+/* -*- 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/. */
+
+/* Implements a UTF-16 character type. */
+
+#ifndef mozilla_Char16_h_
+#define mozilla_Char16_h_
+
+#include "mozilla/Assertions.h"
+
+/*
+ * C11 and C++11 introduce a char16_t type and support for UTF-16 string and
+ * character literals. C++11's char16_t is a distinct builtin type. C11's
+ * char16_t is a typedef for uint_least16_t. Technically, char16_t is a 16-bit
+ * code unit of a Unicode code point, not a "character".
+ *
+ * For now, Char16.h only supports C++ because we don't want mix different C
+ * and C++ definitions of char16_t in the same code base.
+ */
+
+#ifdef _MSC_VER
+   /*
+    * C++11 says char16_t is a distinct builtin type, but Windows's yvals.h
+    * typedefs char16_t as an unsigned short. We would like to alias char16_t
+    * to Windows's 16-bit wchar_t so we can declare UTF-16 literals as constant
+    * expressions (and pass char16_t pointers to Windows APIs). We #define our
+    * char16_t as a macro to override yval.h's typedef of the same name.
+    */
+#  define MOZ_UTF16(s) L##s
+#  include <yvals.h>
+#  define char16_t wchar_t
+#elif defined(__cplusplus) && \
+      (__cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__))
+   /* C++11 has a builtin char16_t type. */
+#  define MOZ_UTF16(s) u##s
+#else
+#  error "Char16.h requires C++11 (or something like it) for UTF-16 support."
+#endif
+
+MOZ_STATIC_ASSERT(sizeof(char16_t) == 2, "Is char16_t type 16 bits?");
+MOZ_STATIC_ASSERT(sizeof(MOZ_UTF16('A')) == 2, "Is char literal 16 bits?");
+MOZ_STATIC_ASSERT(sizeof(MOZ_UTF16("")[0]) == 2, "Is string char 16 bits?");
+
+#endif /* mozilla_Char16_h_ */

--- a/mfbt/exported_headers.mk
+++ b/mfbt/exported_headers.mk
@@ -7,16 +7,17 @@
 # mfbt's exported headers itself.
 
 EXPORTS_NAMESPACES += mozilla
 
 EXPORTS_mozilla += \
   Assertions.h \
   Attributes.h \
   BloomFilter.h \
+  Char16.h \
   CheckedInt.h \
   Constants.h \
   DebugOnly.h \
   EnumSet.h \
   FloatingPoint.h \
   GuardObjects.h \
   HashFunctions.h \
   Likely.h \