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diff --git a/js/public/Object.h b/js/public/Object.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 js_public_Object_h
+#define js_public_Object_h
+
+#include "js/shadow/Object.h"  // JS::shadow::Object
+
+#include "mozilla/Assertions.h"  // MOZ_ASSERT
+
+#include <stddef.h>  // size_t
+#include <stdint.h>  // uint32_t
+
+#include "jstypes.h"  // JS_PUBLIC_API
+
+#include "js/Class.h"       // js::ESClass, JSCLASS_RESERVED_SLOTS
+#include "js/Realm.h"       // JS::GetCompartmentForRealm
+#include "js/RootingAPI.h"  // JS::{,Mutable}Handle
+#include "js/Value.h"       // JS::Value
+
+struct JS_PUBLIC_API JSContext;
+class JS_PUBLIC_API JSObject;
+
+namespace JS {
+
+class JS_PUBLIC_API Compartment;
+
+/**
+ * Determine the ECMAScript "class" -- Date, String, RegExp, and all the other
+ * builtin object types (described in ECMAScript in terms of an objecting having
+ * "an [[ArrayBufferData]] internal slot" or similar language for other kinds of
+ * object -- of the provided object.
+ *
+ * If this function is passed a wrapper that can be unwrapped, the determination
+ * is performed on that object.  If the wrapper can't be unwrapped, and it's not
+ * a wrapper that prefers to treat this operation as a failure, this function
+ * will indicate that the object is |js::ESClass::Other|.
+ */
+extern JS_PUBLIC_API bool GetBuiltinClass(JSContext* cx, Handle<JSObject*> obj,
+                                          js::ESClass* cls);
+
+/** Get the |JSClass| of an object. */
+inline const JSClass* GetClass(const JSObject* obj) {
+  return reinterpret_cast<const shadow::Object*>(obj)->shape->base->clasp;
+}
+
+/**
+ * Get the |JS::Compartment*| of an object.
+ *
+ * Note that the compartment of an object in this realm, that is a
+ * cross-compartment wrapper around an object from another realm, is the
+ * compartment of this realm.
+ */
+static MOZ_ALWAYS_INLINE Compartment* GetCompartment(JSObject* obj) {
+  Realm* realm = reinterpret_cast<shadow::Object*>(obj)->shape->base->realm;
+  return GetCompartmentForRealm(realm);
+}
+
+/**
+ * Get the value stored in a reserved slot in an object.
+ *
+ * If |obj| is known to be a proxy and you're willing to use friend APIs,
+ * |js::GetProxyReservedSlot| in "js/Proxy.h" is very slightly more efficient.
+ */
+inline const Value& GetReservedSlot(JSObject* obj, size_t slot) {
+  MOZ_ASSERT(slot < JSCLASS_RESERVED_SLOTS(GetClass(obj)));
+  return reinterpret_cast<const shadow::Object*>(obj)->slotRef(slot);
+}
+
+namespace detail {
+
+extern JS_PUBLIC_API void SetReservedSlotWithBarrier(JSObject* obj, size_t slot,
+                                                     const Value& value);
+
+}  // namespace detail
+
+/**
+ * Store a value in an object's reserved slot.
+ *
+ * This can be used with both native objects and proxies.  However, if |obj| is
+ * known to be a proxy, |js::SetProxyReservedSlot| in "js/Proxy.h" is very
+ * slightly more efficient.
+ */
+inline void SetReservedSlot(JSObject* obj, size_t slot, const Value& value) {
+  MOZ_ASSERT(slot < JSCLASS_RESERVED_SLOTS(GetClass(obj)));
+  auto* sobj = reinterpret_cast<shadow::Object*>(obj);
+  if (sobj->slotRef(slot).isGCThing() || value.isGCThing()) {
+    detail::SetReservedSlotWithBarrier(obj, slot, value);
+  } else {
+    sobj->slotRef(slot) = value;
+  }
+}
+
+/**
+ * Helper function to get the pointer value (or nullptr if not set) from an
+ * object's reserved slot. The slot must contain either a PrivateValue(T*) or
+ * UndefinedValue.
+ */
+template <typename T>
+inline T* GetMaybePtrFromReservedSlot(JSObject* obj, size_t slot) {
+  Value v = GetReservedSlot(obj, slot);
+  return v.isUndefined() ? nullptr : static_cast<T*>(v.toPrivate());
+}
+
+/**
+ * Helper function to get the pointer value (or nullptr if not set) from the
+ * object's first reserved slot. Must only be used for objects with a JSClass
+ * that has the JSCLASS_SLOT0_IS_NSISUPPORTS flag.
+ */
+template <typename T>
+inline T* GetObjectISupports(JSObject* obj) {
+  MOZ_ASSERT(GetClass(obj)->slot0IsISupports());
+  return GetMaybePtrFromReservedSlot<T>(obj, 0);
+}
+
+/**
+ * Helper function to store |PrivateValue(nsISupportsValue)| in the object's
+ * first reserved slot. Must only be used for objects with a JSClass that has
+ * the JSCLASS_SLOT0_IS_NSISUPPORTS flag.
+ *
+ * Note: the pointer is opaque to the JS engine (including the GC) so it's the
+ * embedding's responsibility to trace or free this value.
+ */
+inline void SetObjectISupports(JSObject* obj, void* nsISupportsValue) {
+  MOZ_ASSERT(GetClass(obj)->slot0IsISupports());
+  SetReservedSlot(obj, 0, PrivateValue(nsISupportsValue));
+}
+
+}  // namespace JS
+
+// JSObject* is an aligned pointer, but this information isn't available in the
+// public header. We specialize HasFreeLSB here so that JS::Result<JSObject*>
+// compiles.
+
+namespace mozilla {
+namespace detail {
+template <>
+struct HasFreeLSB<JSObject*> {
+  static constexpr bool value = true;
+};
+}  // namespace detail
+}  // namespace mozilla
+
+#endif  // js_public_Object_h