diff -r 000000000000 -r 6474c204b198 xpcom/tests/TestEncoding.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/tests/TestEncoding.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,203 @@
+/* -*- 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/. */
+
+#include "TestHarness.h"
+
+nsresult TestGoodSurrogatePair()
+{
+  // When this string is decoded, the surrogate pair is U+10302 and the rest of
+  // the string is specified by indexes 2 onward.
+  const char16_t goodPairData[] = {  0xD800, 0xDF02, 0x65, 0x78, 0x0 };
+  nsDependentString goodPair16(goodPairData);
+
+  uint32_t byteCount = 0;
+  char* goodPair8 = ToNewUTF8String(goodPair16, &byteCount);
+  if (!goodPair8)
+  {
+    fail("out of memory creating goodPair8");
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  if (byteCount != 6)
+  {
+    fail("wrong number of bytes; expected 6, got %lu", byteCount);
+    return NS_ERROR_FAILURE;
+  }
+
+  const unsigned char expected8[] =
+    { 0xF0, 0x90, 0x8C, 0x82, 0x65, 0x78, 0x0 };
+  if (0 != memcmp(expected8, goodPair8, sizeof(expected8)))
+  {
+    fail("wrong translation to UTF8");
+    return NS_ERROR_FAILURE;
+  }
+
+  // This takes a different code path from the above, so test it to make sure
+  // the UTF-16 enumeration remains in sync with the UTF-8 enumeration.
+  nsDependentCString expected((const char*)expected8);
+  if (0 != CompareUTF8toUTF16(expected, goodPair16))
+  {
+    fail("bad comparison between UTF-8 and equivalent UTF-16");
+    return NS_ERROR_FAILURE;
+  }
+
+  NS_Free(goodPair8);
+
+  passed("TestGoodSurrogatePair");
+  return NS_OK;
+}
+
+nsresult TestBackwardsSurrogatePair()
+{
+  // When this string is decoded, the two surrogates are wrongly ordered and
+  // must each be interpreted as U+FFFD.
+  const char16_t backwardsPairData[] = { 0xDDDD, 0xD863, 0x65, 0x78, 0x0 };
+  nsDependentString backwardsPair16(backwardsPairData);
+
+  uint32_t byteCount = 0;
+  char* backwardsPair8 = ToNewUTF8String(backwardsPair16, &byteCount);
+  if (!backwardsPair8)
+  {
+    fail("out of memory creating backwardsPair8");
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  if (byteCount != 8)
+  {
+    fail("wrong number of bytes; expected 8, got %lu", byteCount);
+    return NS_ERROR_FAILURE;
+  }
+
+  const unsigned char expected8[] =
+    { 0xEF, 0xBF, 0xBD, 0xEF, 0xBF, 0xBD, 0x65, 0x78, 0x0 };
+  if (0 != memcmp(expected8, backwardsPair8, sizeof(expected8)))
+  {
+    fail("wrong translation to UTF8");
+    return NS_ERROR_FAILURE;
+  }
+
+  // This takes a different code path from the above, so test it to make sure
+  // the UTF-16 enumeration remains in sync with the UTF-8 enumeration.
+  nsDependentCString expected((const char*)expected8);
+  if (0 != CompareUTF8toUTF16(expected, backwardsPair16))
+  {
+    fail("bad comparison between UTF-8 and malformed but equivalent UTF-16");
+    return NS_ERROR_FAILURE;
+  }
+
+  NS_Free(backwardsPair8);
+
+  passed("TestBackwardsSurrogatePair");
+  return NS_OK;
+}
+
+nsresult TestMalformedUTF16OrphanHighSurrogate()
+{
+  // When this string is decoded, the high surrogate should be replaced and the
+  // rest of the string is specified by indexes 1 onward.
+  const char16_t highSurrogateData[] = { 0xD863, 0x74, 0x65, 0x78, 0x74, 0x0 };
+  nsDependentString highSurrogate16(highSurrogateData);
+
+  uint32_t byteCount = 0;
+  char* highSurrogate8 = ToNewUTF8String(highSurrogate16, &byteCount);
+  if (!highSurrogate8)
+  {
+    fail("out of memory creating highSurrogate8");
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  if (byteCount != 7)
+  {
+    fail("wrong number of bytes; expected 7, got %lu", byteCount);
+    return NS_ERROR_FAILURE;
+  }
+
+  const unsigned char expected8[] =
+    { 0xEF, 0xBF, 0xBD, 0x74, 0x65, 0x78, 0x74, 0x0 };
+  if (0 != memcmp(expected8, highSurrogate8, sizeof(expected8)))
+  {
+    fail("wrong translation to UTF8");
+    return NS_ERROR_FAILURE;
+  }
+
+  // This takes a different code path from the above, so test it to make sure
+  // the UTF-16 enumeration remains in sync with the UTF-8 enumeration.
+  nsDependentCString expected((const char*)expected8);
+  if (0 != CompareUTF8toUTF16(expected, highSurrogate16))
+  {
+    fail("bad comparison between UTF-8 and malformed but equivalent UTF-16");
+    return NS_ERROR_FAILURE;
+  }
+
+  NS_Free(highSurrogate8);
+
+  passed("TestMalformedUTF16OrphanHighSurrogate");
+  return NS_OK;
+}
+
+nsresult TestMalformedUTF16OrphanLowSurrogate()
+{
+  // When this string is decoded, the low surrogate should be replaced and the
+  // rest of the string is specified by indexes 1 onward.
+  const char16_t lowSurrogateData[] = { 0xDDDD, 0x74, 0x65, 0x78, 0x74, 0x0 };
+  nsDependentString lowSurrogate16(lowSurrogateData);
+
+  uint32_t byteCount = 0;
+  char* lowSurrogate8 = ToNewUTF8String(lowSurrogate16, &byteCount);
+  if (!lowSurrogate8)
+  {
+    fail("out of memory creating lowSurrogate8");
+    return NS_ERROR_OUT_OF_MEMORY;
+  }
+
+  if (byteCount != 7)
+  {
+    fail("wrong number of bytes; expected 7, got %lu", byteCount);
+    return NS_ERROR_FAILURE;
+  }
+
+  const unsigned char expected8[] =
+    { 0xEF, 0xBF, 0xBD, 0x74, 0x65, 0x78, 0x74, 0x0 };
+  if (0 != memcmp(expected8, lowSurrogate8, sizeof(expected8)))
+  {
+    fail("wrong translation to UTF8");
+    return NS_ERROR_FAILURE;
+  }
+
+  // This takes a different code path from the above, so test it to make sure
+  // the UTF-16 enumeration remains in sync with the UTF-8 enumeration.
+  nsDependentCString expected((const char*)expected8);
+  if (0 != CompareUTF8toUTF16(expected, lowSurrogate16))
+  {
+    fail("bad comparison between UTF-8 and malformed but equivalent UTF-16");
+    return NS_ERROR_FAILURE;
+  }
+
+  NS_Free(lowSurrogate8);
+
+  passed("TestMalformedUTF16OrphanLowSurrogate");
+  return NS_OK;
+}
+
+
+int main(int argc, char** argv)
+{
+  ScopedXPCOM xpcom("TestEncoding");
+  if (xpcom.failed())
+    return 1;
+
+  int rv = 0;
+
+  if (NS_FAILED(TestGoodSurrogatePair()))
+    rv = 1;
+  if (NS_FAILED(TestBackwardsSurrogatePair()))
+    rv = 1;
+  if (NS_FAILED(TestMalformedUTF16OrphanHighSurrogate()))
+    rv = 1;
+  if (NS_FAILED(TestMalformedUTF16OrphanLowSurrogate()))
+    rv = 1;
+
+  return rv;
+}