import from UXP: Bug 1328948 - Add is(), as() to Cell. (d646d440)

js/src/gc/Heap.h

@@ -29,6 +29,7 @@
 #include "js/HeapAPI.h"
 #include "js/RootingAPI.h"
 #include "js/TracingAPI.h"
+#include "js/TraceKind.h"
 
 struct JSRuntime;
 
@@ -273,6 +274,23 @@ struct Cell
 
     static MOZ_ALWAYS_INLINE bool needWriteBarrierPre(JS::Zone* zone);
 
+    template <class T>
+    inline bool is() const {
+        return getTraceKind() == JS::MapTypeToTraceKind<T>::kind;
+    }
+
+    template<class T>
+    inline T* as() {
+        MOZ_ASSERT(is<T>());
+        return static_cast<T*>(this);
+    }
+
+    template <class T>
+    inline const T* as() const {
+        MOZ_ASSERT(is<T>());
+        return static_cast<const T*>(this);
+    }
+
 #ifdef DEBUG
     inline bool isAligned() const;
     void dump(FILE* fp) const;