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diff --git a/mfbt/MacroArgs.h b/mfbt/MacroArgs.h
new file mode 100644
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+++ b/mfbt/MacroArgs.h
@@ -0,0 +1,109 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 various macros meant to ease the use of variadic macros.
+ */
+
+#ifndef mozilla_MacroArgs_h
+#define mozilla_MacroArgs_h
+
+// Concatenates pre-processor tokens in a way that can be used with __LINE__.
+#define MOZ_CONCAT2(x, y) x ## y
+#define MOZ_CONCAT(x, y) MOZ_CONCAT2(x, y)
+
+/*
+ * MOZ_PASTE_PREFIX_AND_ARG_COUNT(aPrefix, ...) counts the number of variadic
+ * arguments and prefixes it with |aPrefix|. For example:
+ *
+ *   MOZ_PASTE_PREFIX_AND_ARG_COUNT(, foo, 42) expands to 2
+ *   MOZ_PASTE_PREFIX_AND_ARG_COUNT(A, foo, 42, bar) expands to A3
+ *
+ * You must pass in between 1 and 50 (inclusive) variadic arguments, past
+ * |aPrefix|. It is not legal to do
+ *
+ *   MOZ_PASTE_PREFIX_AND_ARG_COUNT(prefix)
+ *
+ * (that is, pass in 0 variadic arguments). To ensure that a compile-time
+ * error occurs when these constraints are violated, use the
+ * MOZ_STATIC_ASSERT_VALID_ARG_COUNT macro with the same variaidc arguments
+ * wherever this macro is used.
+ *
+ * Passing (__VA_ARGS__, <rest of arguments>) rather than simply calling
+ * MOZ_MACROARGS_ARG_COUNT_HELPER2(__VA_ARGS__, <rest of arguments>) very
+ * carefully tiptoes around a MSVC bug where it improperly expands __VA_ARGS__
+ * as a single token in argument lists. For details, see:
+ *
+ *   http://connect.microsoft.com/VisualStudio/feedback/details/380090/variadic-macro-replacement
+ *   http://cplusplus.co.il/2010/07/17/variadic-macro-to-count-number-of-arguments/#comment-644
+ */
+#define MOZ_PASTE_PREFIX_AND_ARG_COUNT(aPrefix, ...) \
+  MOZ_MACROARGS_ARG_COUNT_HELPER((__VA_ARGS__, \
+    aPrefix##50, aPrefix##49, aPrefix##48, aPrefix##47, aPrefix##46, \
+    aPrefix##45, aPrefix##44, aPrefix##43, aPrefix##42, aPrefix##41, \
+    aPrefix##40, aPrefix##39, aPrefix##38, aPrefix##37, aPrefix##36, \
+    aPrefix##35, aPrefix##34, aPrefix##33, aPrefix##32, aPrefix##31, \
+    aPrefix##30, aPrefix##29, aPrefix##28, aPrefix##27, aPrefix##26, \
+    aPrefix##25, aPrefix##24, aPrefix##23, aPrefix##22, aPrefix##21, \
+    aPrefix##20, aPrefix##19, aPrefix##18, aPrefix##17, aPrefix##16, \
+    aPrefix##15, aPrefix##14, aPrefix##13, aPrefix##12, aPrefix##11, \
+    aPrefix##10, aPrefix##9,  aPrefix##8,  aPrefix##7,  aPrefix##6,  \
+    aPrefix##5,  aPrefix##4,  aPrefix##3,  aPrefix##2,  aPrefix##1, aPrefix##0))
+
+#define MOZ_MACROARGS_ARG_COUNT_HELPER(aArgs) \
+  MOZ_MACROARGS_ARG_COUNT_HELPER2 aArgs
+
+#define MOZ_MACROARGS_ARG_COUNT_HELPER2( \
+   a1,  a2,  a3,  a4,  a5,  a6,  a7,  a8,  a9, a10, \
+  a11, a12, a13, a14, a15, a16, a17, a18, a19, a20, \
+  a21, a22, a23, a24, a25, a26, a27, a28, a29, a30, \
+  a31, a32, a33, a34, a35, a36, a37, a38, a39, a40, \
+  a41, a42, a43, a44, a45, a46, a47, a48, a49, a50, \
+  a51, ...) a51
+
+/*
+ * MOZ_STATIC_ASSERT_VALID_ARG_COUNT ensures that a compile-time error occurs
+ * when the argument count constraints of MOZ_PASTE_PREFIX_AND_ARG_COUNT are
+ * violated. Use this macro wherever MOZ_PASTE_PREFIX_AND_ARG_COUNT is used
+ * and pass it the same variadic arguments.
+ *
+ * This macro employs a few dirty tricks to function. To detect the zero
+ * argument case, |(__VA_ARGS__)| is stringified, sizeof-ed, and compared to
+ * what it should be in the absence of arguments.
+ *
+ * Detecting too many arguments is a little trickier. With a valid argument
+ * count and a prefix of 1, MOZ_PASTE_PREFIX_AND_ARG_COUNT expands to e.g. 14.
+ * With a prefix of 0.0, it expands to e.g. 0.04. If there are too many
+ * arguments, it expands to the first argument over the limit. If this
+ * exceeding argument is a number, the assertion will fail as there is no
+ * number than can simultaneously be both > 10 and == 0. If the exceeding
+ * argument is not a number, a compile-time error should still occur due to
+ * the operations performed on it.
+ */
+#define MOZ_MACROARGS_STRINGIFY_HELPER(x) #x
+#define MOZ_STATIC_ASSERT_VALID_ARG_COUNT(...) \
+  static_assert( \
+    sizeof(MOZ_MACROARGS_STRINGIFY_HELPER((__VA_ARGS__))) != sizeof("()") && \
+      (MOZ_PASTE_PREFIX_AND_ARG_COUNT(1, __VA_ARGS__)) > 10 && \
+      (int)(MOZ_PASTE_PREFIX_AND_ARG_COUNT(0.0, __VA_ARGS__)) == 0, \
+    "MOZ_STATIC_ASSERT_VALID_ARG_COUNT requires 1 to 50 arguments") /* ; */
+
+/*
+ * MOZ_ARGS_AFTER_N expands to its arguments excluding the first |N|
+ * arguments. For example:
+ *
+ *   MOZ_ARGS_AFTER_2(a, b, c, d) expands to: c, d
+ */
+#define MOZ_ARGS_AFTER_1(a1, ...) __VA_ARGS__
+#define MOZ_ARGS_AFTER_2(a1, a2, ...) __VA_ARGS__
+
+/*
+ * MOZ_ARG_N expands to its |N|th argument.
+ */
+#define MOZ_ARG_1(a1, ...) a1
+#define MOZ_ARG_2(a1, a2, ...) a2
+
+#endif /* mozilla_MacroArgs_h */