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diff --git a/js/src/vm/ShapedObject.h b/js/src/vm/ShapedObject.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * 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 vm_ShapedObject_h
+#define vm_ShapedObject_h
+
+#include "jsobj.h"
+
+namespace js {
+
+/*
+ * Shaped objects extend the base implementation of an object with a shape
+ * field.  Subclasses of ShapedObject ascribe meaning to this field.
+ *
+ * ShapedObject is only created as the base class of some other class.  It's
+ * never created as a most-derived class.
+ */
+class ShapedObject : public JSObject
+{
+  protected:
+    // Property layout description and other state.
+    GCPtrShape shape_;
+
+  public:
+    // Set the shape of an object. This pointer is valid for native objects and
+    // some non-native objects. After creating an object, the objects for which
+    // the shape pointer is invalid need to overwrite this pointer before a GC
+    // can occur.
+    void initShape(Shape* shape) {
+        this->shape_.init(shape);
+    }
+
+    void setShape(Shape* shape) {
+        this->shape_ = shape;
+    }
+
+    Shape* shape() const { return this->shape_; }
+
+    void traceShape(JSTracer* trc) {
+        TraceEdge(trc, &shape_, "shape");
+    }
+
+    static size_t offsetOfShape() { return offsetof(ShapedObject, shape_); }
+
+  private:
+    static void staticAsserts() {
+        static_assert(offsetof(ShapedObject, shape_) == offsetof(shadow::Object, shape),
+                      "shadow shape must match actual shape");
+    }
+};
+
+} // namespace js
+
+#endif /* vm_ShapedObject_h */