diff -r 000000000000 -r 6474c204b198 xpcom/glue/nsISupportsUtils.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/glue/nsISupportsUtils.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,145 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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/. */
+
+#ifndef nsISupportsUtils_h__
+#define nsISupportsUtils_h__
+
+#include "nscore.h"
+#include "nsISupportsBase.h"
+#include "nsError.h"
+#include "nsDebug.h"
+#include "nsISupportsImpl.h"
+#include "mozilla/TypeTraits.h"
+
+/**
+ * Macro for adding a reference to an interface.
+ * @param _ptr The interface pointer.
+ */
+#define NS_ADDREF(_ptr) \
+  (_ptr)->AddRef()
+
+/**
+ * Macro for adding a reference to this. This macro should be used
+ * because NS_ADDREF (when tracing) may require an ambiguous cast
+ * from the pointers primary type to nsISupports. This macro sidesteps
+ * that entire problem.
+ */
+#define NS_ADDREF_THIS() \
+  AddRef()
+
+
+extern "C++" {
+// ...because some one is accidentally including this file inside
+// an |extern "C"|
+
+
+// Making this a |inline| |template| allows |expr| to be evaluated only once,
+// yet still denies you the ability to |AddRef()| an |nsCOMPtr|.
+template <class T>
+inline
+void
+ns_if_addref( T expr )
+{
+    if (expr) {
+        expr->AddRef();
+    }
+}
+
+} /* extern "C++" */
+
+/**
+ * Macro for adding a reference to an interface that checks for nullptr.
+ * @param _expr The interface pointer.
+ */
+#define NS_IF_ADDREF(_expr) ns_if_addref(_expr)
+
+/*
+ * Given these declarations, it explicitly OK and efficient to end a `getter' with:
+ *
+ *    NS_IF_ADDREF(*result = mThing);
+ *
+ * even if |mThing| is an |nsCOMPtr|.  If |mThing| is an |nsCOMPtr|, however, it is still
+ * _illegal_ to say |NS_IF_ADDREF(mThing)|.
+ */
+
+/**
+ * Macro for releasing a reference to an interface.
+ * @param _ptr The interface pointer.
+ */
+#define NS_RELEASE(_ptr)                                                      \
+  do {                                                                        \
+    (_ptr)->Release();                                                        \
+    (_ptr) = 0;                                                               \
+  } while (0)
+
+/**
+ * Macro for releasing a reference to this interface.
+ */
+#define NS_RELEASE_THIS() \
+    Release()
+
+/**
+ * Macro for releasing a reference to an interface, except that this
+ * macro preserves the return value from the underlying Release call.
+ * The interface pointer argument will only be NULLed if the reference count
+ * goes to zero.
+ *
+ * @param _ptr The interface pointer.
+ * @param _rc  The reference count.
+ */
+#define NS_RELEASE2(_ptr, _rc)                                                \
+  do {                                                                        \
+    _rc = (_ptr)->Release();                                                  \
+    if (0 == (_rc)) (_ptr) = 0;                                               \
+  } while (0)
+
+/**
+ * Macro for releasing a reference to an interface that checks for nullptr;
+ * @param _ptr The interface pointer.
+ */
+#define NS_IF_RELEASE(_ptr)                                                   \
+  do {                                                                        \
+    if (_ptr) {                                                               \
+      (_ptr)->Release();                                                      \
+      (_ptr) = 0;                                                             \
+    }                                                                         \
+  } while (0)
+
+/*
+ * Often you have to cast an implementation pointer, e.g., |this|, to an
+ * |nsISupports*|, but because you have multiple inheritance, a simple cast
+ * is ambiguous.  One could simply say, e.g., (given a base |nsIBase|),
+ * |static_cast<nsIBase*>(this)|; but that disguises the fact that what
+ * you are really doing is disambiguating the |nsISupports|.  You could make
+ * that more obvious with a double cast, e.g., |static_cast<nsISupports*>
+                                                           (* static_cast<nsIBase*>(this))|, but that is bulky and harder to read...
+ *
+ * The following macro is clean, short, and obvious.  In the example above,
+ * you would use it like this: |NS_ISUPPORTS_CAST(nsIBase*, this)|.
+ */
+
+#define NS_ISUPPORTS_CAST(__unambiguousBase, __expr) \
+  static_cast<nsISupports*>(static_cast<__unambiguousBase>(__expr))
+
+// a type-safe shortcut for calling the |QueryInterface()| member function
+template <class T, class DestinationType>
+inline
+nsresult
+CallQueryInterface( T* aSource, DestinationType** aDestination )
+{
+    // We permit nsISupports-to-nsISupports here so that one can still obtain
+    // the canonical nsISupports pointer with CallQueryInterface.
+    static_assert(!mozilla::IsSame<T, DestinationType>::value ||
+                  mozilla::IsSame<DestinationType, nsISupports>::value,
+                  "don't use CallQueryInterface for compile-time-determinable casts");
+
+    NS_PRECONDITION(aSource, "null parameter");
+    NS_PRECONDITION(aDestination, "null parameter");
+
+    return aSource->QueryInterface(NS_GET_TEMPLATE_IID(DestinationType),
+                                   reinterpret_cast<void**>(aDestination));
+}
+
+#endif /* __nsISupportsUtils_h */