diff -r 000000000000 -r 6474c204b198 layout/generic/nsQueryFrame.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/layout/generic/nsQueryFrame.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,96 @@
+/* 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 nsQueryFrame_h
+#define nsQueryFrame_h
+
+#include "nscore.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/TypeTraits.h"
+
+// NOTE: the long lines in this file are intentional to make compiler error
+// messages more readable.
+
+#define NS_DECL_QUERYFRAME_TARGET(classname)                    \
+  static const nsQueryFrame::FrameIID kFrameIID = nsQueryFrame::classname##_id; \
+  typedef classname Has_NS_DECL_QUERYFRAME_TARGET;
+
+#define NS_DECL_QUERYFRAME                                      \
+  virtual void* QueryFrame(FrameIID id);
+
+#define NS_QUERYFRAME_HEAD(class)                               \
+  void* class::QueryFrame(FrameIID id) { switch (id) {
+
+#define NS_QUERYFRAME_ENTRY(class)                              \
+  case class::kFrameIID: {                                      \
+    static_assert(mozilla::IsSame<class, class::Has_NS_DECL_QUERYFRAME_TARGET>::value, \
+                  #class " must declare itself as a queryframe target"); \
+    return static_cast<class*>(this);                           \
+  }
+
+#define NS_QUERYFRAME_ENTRY_CONDITIONAL(class, condition)       \
+  case class::kFrameIID:                                        \
+  if (condition) {                                              \
+    static_assert(mozilla::IsSame<class, class::Has_NS_DECL_QUERYFRAME_TARGET>::value, \
+                  #class " must declare itself as a queryframe target"); \
+    return static_cast<class*>(this);                           \
+  }                                                             \
+  break;
+
+#define NS_QUERYFRAME_TAIL_INHERITING(class)                    \
+  default: break;                                               \
+  }                                                             \
+  return class::QueryFrame(id);                                 \
+}
+
+#define NS_QUERYFRAME_TAIL_INHERITANCE_ROOT                     \
+  default: break;                                               \
+  }                                                             \
+  MOZ_ASSERT(id != GetFrameId(),                                \
+    "A frame failed to QueryFrame to its *own type*. "          \
+    "It may be missing NS_DECL_QUERYFRAME, or a "               \
+    "NS_QUERYFRAME_ENTRY() line with its own type name");       \
+  return nullptr;                                               \
+}
+
+class nsQueryFrame
+{
+public:
+  enum FrameIID {
+#define FRAME_ID(classname) classname##_id,
+#include "nsFrameIdList.h"
+#undef FRAME_ID
+
+    // The PresArena implementation uses this bit to distinguish objects
+    // allocated by size from objects allocated by type ID (that is, frames
+    // using AllocateByFrameID, and other objects using AllocateByObjectID).
+    // It should not collide with any frame ID (above) or Object ID (in
+    // nsPresArena.h).  It is not 0x80000000 to avoid the question of
+    // whether enumeration constants are signed.
+    NON_FRAME_MARKER = 0x20000000
+  };
+
+  virtual void* QueryFrame(FrameIID id) = 0;
+};
+
+class do_QueryFrame
+{
+public:
+  do_QueryFrame(nsQueryFrame *s) : mRawPtr(s) { }
+
+  template<class Dest>
+  operator Dest*() {
+    static_assert(mozilla::IsSame<Dest, typename Dest::Has_NS_DECL_QUERYFRAME_TARGET>::value,
+                  "Dest must declare itself as a queryframe target");
+    if (!mRawPtr)
+      return nullptr;
+
+    return reinterpret_cast<Dest*>(mRawPtr->QueryFrame(Dest::kFrameIID));
+  }
+
+private:
+  nsQueryFrame *mRawPtr;
+};
+
+#endif // nsQueryFrame_h