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diff --git a/js/public/Zone.h b/js/public/Zone.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/. */
+
+/* JavaScript API. */
+
+#ifndef js_Zone_h
+#define js_Zone_h
+
+#include "mozilla/MemoryReporting.h"  // mozilla::MallocSizeOf
+
+#include <stddef.h>  // size_t
+
+#include "jstypes.h"        // JS_PUBLIC_API
+#include "js/RootingAPI.h"  // JS::Handle
+#include "js/TypeDecls.h"  // JSContext, JSObject, jsid, JS::Compartment, JS::GCContext, JS::Value, JS::Zone
+
+// [SMDOC] Nested GC Data Structures (Compartments and Zones)
+//
+// The GC has two nested data structures, Zones and Compartents. Each has
+// distint responsibilities. Zones contain compartments, along with other GC
+// resources used by a tab. Compartments contain realms, which are specification
+// defined.
+//
+// See also the SMDoc on Realms.
+//
+// Compartment
+// -----------
+// Security membrane; when an object from compartment A is used in compartment
+// B, a cross-compartment wrapper (a kind of proxy) is used. In the browser,
+// same-origin realms can share a compartment.
+//
+// Zone
+// ----
+// A Zone is a group of compartments that share GC resources (arenas, strings,
+// etc) for memory usage and performance reasons. Zone is the GC unit: the GC
+// can operate on one or more zones at a time. The browser uses roughly one zone
+// per tab.
+
+using JSDestroyZoneCallback = void (*)(JS::GCContext*, JS::Zone*);
+
+using JSDestroyCompartmentCallback = void (*)(JS::GCContext*, JS::Compartment*);
+
+using JSSizeOfIncludingThisCompartmentCallback =
+    size_t (*)(mozilla::MallocSizeOf, JS::Compartment*);
+
+extern JS_PUBLIC_API void JS_SetDestroyZoneCallback(
+    JSContext* cx, JSDestroyZoneCallback callback);
+
+extern JS_PUBLIC_API void JS_SetDestroyCompartmentCallback(
+    JSContext* cx, JSDestroyCompartmentCallback callback);
+
+extern JS_PUBLIC_API void JS_SetSizeOfIncludingThisCompartmentCallback(
+    JSContext* cx, JSSizeOfIncludingThisCompartmentCallback callback);
+
+extern JS_PUBLIC_API void JS_SetCompartmentPrivate(JS::Compartment* compartment,
+                                                   void* data);
+
+extern JS_PUBLIC_API void* JS_GetCompartmentPrivate(
+    JS::Compartment* compartment);
+
+extern JS_PUBLIC_API void JS_SetZoneUserData(JS::Zone* zone, void* data);
+
+extern JS_PUBLIC_API void* JS_GetZoneUserData(JS::Zone* zone);
+
+extern JS_PUBLIC_API bool JS_RefreshCrossCompartmentWrappers(
+    JSContext* cx, JS::Handle<JSObject*> obj);
+
+/**
+ * Mark a jsid after entering a new compartment. Different zones separately
+ * mark the ids in a runtime, and this must be used any time an id is obtained
+ * from one compartment and then used in another compartment, unless the two
+ * compartments are guaranteed to be in the same zone.
+ */
+extern JS_PUBLIC_API void JS_MarkCrossZoneId(JSContext* cx, jsid id);
+
+/**
+ * If value stores a jsid (an atomized string or symbol), mark that id as for
+ * JS_MarkCrossZoneId.
+ */
+extern JS_PUBLIC_API void JS_MarkCrossZoneIdValue(JSContext* cx,
+                                                  const JS::Value& value);
+
+#endif  // js_Zone_h