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diff --git a/js/public/friend/DOMProxy.h b/js/public/friend/DOMProxy.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/. */
+
+/*
+ * Specify information about DOMProxy proxies in the DOM, for use by ICs.
+ *
+ * Embedders who don't need to define particularly high-performance proxies that
+ * can have random properties added to them can ignore this header.
+ */
+
+#ifndef js_friend_DOMProxy_h
+#define js_friend_DOMProxy_h
+
+#include <stddef.h>  // size_t
+#include <stdint.h>  // uint64_t
+
+#include "jstypes.h"  // JS_FRIEND_API
+
+#include "js/Id.h"          // JS::PropertyKey
+#include "js/RootingAPI.h"  // JS::Handle, JS::Heap
+#include "js/Value.h"       // JS::UndefinedValue, JS::Value
+
+struct JS_PUBLIC_API JSContext;
+class JS_PUBLIC_API JSObject;
+
+namespace JS {
+
+/*
+ * The DOMProxyShadowsCheck function will be called to check if the property for
+ * id should be gotten from the prototype, or if there is an own property that
+ * shadows it.
+ * * If ShadowsViaDirectExpando is returned, then the slot at
+ *   listBaseExpandoSlot contains an expando object which has the property in
+ *   question.
+ * * If ShadowsViaIndirectExpando is returned, then the slot at
+ *   listBaseExpandoSlot contains a private pointer to an ExpandoAndGeneration
+ *   and the expando object in the ExpandoAndGeneration has the property in
+ *   question.
+ * * If DoesntShadow is returned then the slot at listBaseExpandoSlot should
+ *   either be undefined or point to an expando object that would contain the
+ *   own property.
+ * * If DoesntShadowUnique is returned then the slot at listBaseExpandoSlot
+ *   should contain a private pointer to a ExpandoAndGeneration, which contains
+ *   a JS::Value that should either be undefined or point to an expando object,
+ *   and a uint64 value. If that value changes then the IC for getting a
+ *   property will be invalidated.
+ * * If Shadows is returned, that means the property is an own property of the
+ *   proxy but doesn't live on the expando object.
+ */
+
+struct ExpandoAndGeneration {
+  ExpandoAndGeneration() : expando(JS::UndefinedValue()), generation(0) {}
+
+  void OwnerUnlinked() { ++generation; }
+
+  static constexpr size_t offsetOfExpando() {
+    return offsetof(ExpandoAndGeneration, expando);
+  }
+
+  static constexpr size_t offsetOfGeneration() {
+    return offsetof(ExpandoAndGeneration, generation);
+  }
+
+  Heap<Value> expando;
+  uint64_t generation;
+};
+
+enum class DOMProxyShadowsResult {
+  ShadowCheckFailed,
+  Shadows,
+  DoesntShadow,
+  DoesntShadowUnique,
+  ShadowsViaDirectExpando,
+  ShadowsViaIndirectExpando
+};
+
+using DOMProxyShadowsCheck = DOMProxyShadowsResult (*)(JSContext*,
+                                                       Handle<JSObject*>,
+                                                       Handle<JS::PropertyKey>);
+
+extern JS_FRIEND_API void SetDOMProxyInformation(
+    const void* domProxyHandlerFamily,
+    DOMProxyShadowsCheck domProxyShadowsCheck,
+    const void* domRemoteProxyHandlerFamily);
+
+}  // namespace JS
+
+#endif  // js_friend_DOMProxy_h