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diff --git a/js/src/vm/ReceiverGuard.h b/js/src/vm/ReceiverGuard.h
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+/* -*- 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/. */
+
+#ifndef vm_ReceiverGuard_h
+#define vm_ReceiverGuard_h
+
+#include "vm/Shape.h"
+
+namespace js {
+
+// [SMDOC] Receiver Guard
+//
+// A ReceiverGuard encapsulates the information about an object that needs to
+// be tested to determine if it has the same 'structure' as another object.
+// The guard includes the shape and/or group of the object, and which of these
+// is tested, as well as the meaning here of 'structure', depends on the kind
+// of object being tested:
+//
+// NativeObject: The structure of a native object is determined by its shape.
+//   Two objects with the same shape have the same class, prototype, flags,
+//   and all properties except those stored in dense elements.
+//
+// ProxyObject: The structure of a proxy object is determined by its shape.
+//   Proxies with the same shape have the same class and prototype, but no
+//   other commonality is guaranteed.
+//
+// TypedObject: The structure of a typed object is determined by its group.
+//   All typed objects with the same group have the same class, prototype, and
+//   own properties.
+//
+// In all cases, a ReceiverGuard has *either* a shape or a group active, and
+// never both.
+
+class ReceiverGuard {
+  ObjectGroup* group_;
+  Shape* shape_;
+
+ public:
+  ReceiverGuard() : group_(nullptr), shape_(nullptr) {}
+
+  explicit MOZ_ALWAYS_INLINE ReceiverGuard(JSObject* obj);
+
+  bool operator==(const ReceiverGuard& other) const {
+    return group_ == other.group_ && shape_ == other.shape_;
+  }
+
+  bool operator!=(const ReceiverGuard& other) const {
+    return !(*this == other);
+  }
+
+  uintptr_t hash() const {
+    return (uintptr_t(group_) >> 3) ^ (uintptr_t(shape_) >> 3);
+  }
+
+  Shape* getShape() const { return shape_; }
+  ObjectGroup* getGroup() const { return group_; }
+};
+
+// Heap storage for ReceiverGuards.
+//
+// This is a storage only class -- all computation is actually
+// done by converting this back to a RecieverGuard, hence why
+// there are no accessors.
+class HeapReceiverGuard {
+  friend class ReceiverGuard;
+
+  const GCPtrObjectGroup group_;
+  const GCPtrShape shape_;
+
+ public:
+  explicit HeapReceiverGuard(const ReceiverGuard& guard)
+      : group_(guard.getGroup()), shape_(guard.getShape()) {}
+
+  void trace(JSTracer* trc);
+};
+
+}  // namespace js
+
+#endif /* vm_ReceiverGuard_h */