diff -r 000000000000 -r 6474c204b198 dom/bindings/OwningNonNull.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/dom/bindings/OwningNonNull.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,87 @@
+/* -*- 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/. */
+
+/* A class for non-null strong pointers to reference-counted objects. */
+
+#ifndef mozilla_dom_OwningNonNull_h
+#define mozilla_dom_OwningNonNull_h
+
+#include "nsAutoPtr.h"
+
+namespace mozilla {
+namespace dom {
+
+template<class T>
+class OwningNonNull
+{
+public:
+  OwningNonNull()
+#ifdef DEBUG
+    : mInited(false)
+#endif
+  {}
+
+  operator T&()
+  {
+    MOZ_ASSERT(mInited);
+    MOZ_ASSERT(mPtr, "OwningNonNull<T> was set to null");
+    return *mPtr;
+  }
+
+  operator T*()
+  {
+    MOZ_ASSERT(mInited);
+    MOZ_ASSERT(mPtr, "OwningNonNull<T> was set to null");
+    return mPtr;
+  }
+
+  void operator=(T* aValue)
+  {
+    init(aValue);
+  }
+
+  void operator=(const already_AddRefed<T>& aValue)
+  {
+    init(aValue);
+  }
+
+  already_AddRefed<T> forget()
+  {
+#ifdef DEBUG
+    mInited = false;
+#endif
+    return mPtr.forget();
+  }
+
+  // Make us work with smart pointer helpers that expect a get().
+  T* get() const
+  {
+    MOZ_ASSERT(mInited);
+    MOZ_ASSERT(mPtr);
+    return mPtr;
+  }
+
+protected:
+  template<typename U>
+  void init(U aValue)
+  {
+    mPtr = aValue;
+    MOZ_ASSERT(mPtr);
+#ifdef DEBUG
+    mInited = true;
+#endif
+  }
+
+  nsRefPtr<T> mPtr;
+#ifdef DEBUG
+  bool mInited;
+#endif
+};
+
+} // namespace dom
+} // namespace mozilla
+
+#endif // mozilla_dom_OwningNonNull_h