diff -r 000000000000 -r 6474c204b198 mfbt/DebugOnly.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mfbt/DebugOnly.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,82 @@
+/* -*- 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/. */
+
+/*
+ * Provides DebugOnly, a type for variables used only in debug builds (i.e. by
+ * assertions).
+ */
+
+#ifndef mozilla_DebugOnly_h
+#define mozilla_DebugOnly_h
+
+namespace mozilla {
+
+/**
+ * DebugOnly contains a value of type T, but only in debug builds.  In release
+ * builds, it does not contain a value.  This helper is intended to be used with
+ * MOZ_ASSERT()-style macros, allowing one to write:
+ *
+ *   DebugOnly<bool> check = func();
+ *   MOZ_ASSERT(check);
+ *
+ * more concisely than declaring |check| conditional on #ifdef DEBUG, but also
+ * without allocating storage space for |check| in release builds.
+ *
+ * DebugOnly instances can only be coerced to T in debug builds.  In release
+ * builds they don't have a value, so type coercion is not well defined.
+ *
+ * Note that DebugOnly instances still take up one byte of space, plus padding,
+ * when used as members of structs.
+ */
+template<typename T>
+class DebugOnly
+{
+  public:
+#ifdef DEBUG
+    T value;
+
+    DebugOnly() { }
+    DebugOnly(const T& other) : value(other) { }
+    DebugOnly(const DebugOnly& other) : value(other.value) { }
+    DebugOnly& operator=(const T& rhs) {
+      value = rhs;
+      return *this;
+    }
+    void operator++(int) {
+      value++;
+    }
+    void operator--(int) {
+      value--;
+    }
+
+    T* operator&() { return &value; }
+
+    operator T&() { return value; }
+    operator const T&() const { return value; }
+
+    T& operator->() { return value; }
+    const T& operator->() const { return value; }
+
+#else
+    DebugOnly() { }
+    DebugOnly(const T&) { }
+    DebugOnly(const DebugOnly&) { }
+    DebugOnly& operator=(const T&) { return *this; }
+    void operator++(int) { }
+    void operator--(int) { }
+#endif
+
+    /*
+     * DebugOnly must always have a destructor or else it will
+     * generate "unused variable" warnings, exactly what it's intended
+     * to avoid!
+     */
+    ~DebugOnly() {}
+};
+
+}
+
+#endif /* mozilla_DebugOnly_h */