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diff --git a/js/public/GCPolicyAPI.h b/js/public/GCPolicyAPI.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/. */
+
+// GC Policy Mechanism
+
+// A GCPolicy controls how the GC interacts with both direct pointers to GC
+// things (e.g. JSObject* or JSString*), tagged and/or optional pointers to GC
+// things (e.g.  Value or jsid), and C++ container types (e.g.
+// JSPropertyDescriptor or GCHashMap).
+//
+// The GCPolicy provides at a minimum:
+//
+//   static void trace(JSTracer, T* tp, const char* name)
+//       - Trace the edge |*tp|, calling the edge |name|. Containers like
+//         GCHashMap and GCHashSet use this method to trace their children.
+//
+//   static bool needsSweep(T* tp)
+//       - [DEPRECATED], use traceWeak instead.
+//         Return true if |*tp| is about to be finalized. Otherwise, update the
+//         edge for moving GC, and return false. Containers like GCHashMap and
+//         GCHashSet use this method to decide when to remove an entry: if this
+//         function returns true on a key/value/member/etc, its entry is dropped
+//         from the container. Specializing this method is the standard way to
+//         get custom weak behavior from a container type.
+//
+//   static bool traceWeak(T* tp)
+//       - Return false if |*tp| has been set to nullptr. Otherwise, update the
+//         edge for moving GC, and return true. Containers like GCHashMap and
+//         GCHashSet use this method to decide when to remove an entry: if this
+//         function returns false on a key/value/member/etc, its entry is
+//         dropped from the container. Specializing this method is the standard
+//         way to get custom weak behavior from a container type.
+//
+//   static bool isValid(const T& t)
+//       - Return false only if |t| is corrupt in some way. The built-in GC
+//         types do some memory layout checks. For debugging only; it is ok
+//         to always return true or even to omit this member entirely.
+//
+// The default GCPolicy<T> assumes that T has a default constructor and |trace|
+// and |traceWeak| methods, and forwards to them. GCPolicy has appropriate
+// specializations for pointers to GC things and pointer-like types like
+// JS::Heap<T> and mozilla::UniquePtr<T>.
+//
+// There are some stock structs your specializations can inherit from.
+// IgnoreGCPolicy<T> does nothing. StructGCPolicy<T> forwards the methods to the
+// referent type T.
+
+#ifndef GCPolicyAPI_h
+#define GCPolicyAPI_h
+
+#include "mozilla/Maybe.h"
+#include "mozilla/UniquePtr.h"
+
+#include <type_traits>
+
+#include "js/GCTypeMacros.h"  // JS_FOR_EACH_PUBLIC_GC_POINTER_TYPE
+#include "js/TraceKind.h"
+#include "js/TracingAPI.h"
+#include "js/TypeDecls.h"
+
+namespace JS {
+
+// Defines a policy for container types with non-GC, i.e. C storage. This
+// policy dispatches to the underlying struct for GC interactions.
+template <typename T>
+struct StructGCPolicy {
+  static_assert(!std::is_pointer_v<T>,
+                "Pointer type not allowed for StructGCPolicy");
+
+  static void trace(JSTracer* trc, T* tp, const char* name) { tp->trace(trc); }
+
+  static void sweep(T* tp) { return tp->sweep(); }
+
+  static bool needsSweep(T* tp) { return tp->needsSweep(); }
+
+  static bool traceWeak(JSTracer* trc, T* tp) { return tp->traceWeak(trc); }
+
+  static bool isValid(const T& tp) { return true; }
+};
+
+// The default GC policy attempts to defer to methods on the underlying type.
+// Most C++ structures that contain a default constructor, a trace function and
+// a sweep function will work out of the box with Rooted, Handle, GCVector,
+// and GCHash{Set,Map}.
+template <typename T>
+struct GCPolicy : public StructGCPolicy<T> {};
+
+// This policy ignores any GC interaction, e.g. for non-GC types.
+template <typename T>
+struct IgnoreGCPolicy {
+  static void trace(JSTracer* trc, T* t, const char* name) {}
+  static bool needsSweep(T* v) { return false; }
+  static bool traceWeak(JSTracer*, T* v) { return true; }
+  static bool isValid(const T& v) { return true; }
+};
+template <>
+struct GCPolicy<uint32_t> : public IgnoreGCPolicy<uint32_t> {};
+template <>
+struct GCPolicy<uint64_t> : public IgnoreGCPolicy<uint64_t> {};
+
+template <typename T>
+struct GCPointerPolicy {
+  static_assert(std::is_pointer_v<T>,
+                "Non-pointer type not allowed for GCPointerPolicy");
+
+  static void trace(JSTracer* trc, T* vp, const char* name) {
+    // It's not safe to trace unbarriered pointers except as part of root
+    // marking.
+    UnsafeTraceRoot(trc, vp, name);
+  }
+  static bool needsSweep(T* vp) {
+    if (*vp) {
+      return js::gc::IsAboutToBeFinalizedUnbarriered(vp);
+    }
+    return false;
+  }
+  static bool traceWeak(JSTracer* trc, T* vp) {
+    if (*vp) {
+      return js::TraceManuallyBarrieredWeakEdge(trc, vp, "traceWeak");
+    }
+    return true;
+  }
+  static bool isTenured(T v) { return !js::gc::IsInsideNursery(v); }
+  static bool isValid(T v) { return js::gc::IsCellPointerValidOrNull(v); }
+};
+#define EXPAND_SPECIALIZE_GCPOLICY(Type)                   \
+  template <>                                              \
+  struct GCPolicy<Type> : public GCPointerPolicy<Type> {}; \
+  template <>                                              \
+  struct GCPolicy<Type const> : public GCPointerPolicy<Type const> {};
+JS_FOR_EACH_PUBLIC_GC_POINTER_TYPE(EXPAND_SPECIALIZE_GCPOLICY)
+#undef EXPAND_SPECIALIZE_GCPOLICY
+
+template <typename T>
+struct NonGCPointerPolicy {
+  static void trace(JSTracer* trc, T* vp, const char* name) {
+    if (*vp) {
+      (*vp)->trace(trc);
+    }
+  }
+  static bool needsSweep(T* vp) {
+    if (*vp) {
+      return (*vp)->needsSweep();
+    }
+    return false;
+  }
+  static bool traceWeak(JSTracer* trc, T* vp) {
+    if (*vp) {
+      return (*vp)->traceWeak(trc);
+    }
+    return true;
+  }
+
+  static bool isValid(T v) { return true; }
+};
+
+template <typename T>
+struct GCPolicy<JS::Heap<T>> {
+  static void trace(JSTracer* trc, JS::Heap<T>* thingp, const char* name) {
+    TraceEdge(trc, thingp, name);
+  }
+  static bool needsSweep(JS::Heap<T>* thingp) {
+    return *thingp && js::gc::EdgeNeedsSweep(thingp);
+  }
+  static bool traceWeak(JSTracer* trc, JS::Heap<T>* thingp) {
+    if (*thingp) {
+      return js::TraceWeakEdge(trc, thingp, "traceWeak");
+    }
+    return true;
+  }
+};
+
+// GCPolicy<UniquePtr<T>> forwards the contained pointer to GCPolicy<T>.
+template <typename T, typename D>
+struct GCPolicy<mozilla::UniquePtr<T, D>> {
+  static void trace(JSTracer* trc, mozilla::UniquePtr<T, D>* tp,
+                    const char* name) {
+    if (tp->get()) {
+      GCPolicy<T>::trace(trc, tp->get(), name);
+    }
+  }
+  static bool needsSweep(mozilla::UniquePtr<T, D>* tp) {
+    if (tp->get()) {
+      return GCPolicy<T>::needsSweep(tp->get());
+    }
+    return false;
+  }
+  static bool traceWeak(JSTracer* trc, mozilla::UniquePtr<T, D>* tp) {
+    if (tp->get()) {
+      return GCPolicy<T>::traceWeak(trc, tp->get());
+    }
+    return true;
+  }
+  static bool isValid(const mozilla::UniquePtr<T, D>& t) {
+    if (t.get()) {
+      return GCPolicy<T>::isValid(*t.get());
+    }
+    return true;
+  }
+};
+
+template <>
+struct GCPolicy<mozilla::Nothing> : public IgnoreGCPolicy<mozilla::Nothing> {};
+
+// GCPolicy<Maybe<T>> forwards tracing/sweeping to GCPolicy<T*> if
+// when the Maybe<T> is full.
+template <typename T>
+struct GCPolicy<mozilla::Maybe<T>> {
+  static void trace(JSTracer* trc, mozilla::Maybe<T>* tp, const char* name) {
+    if (tp->isSome()) {
+      GCPolicy<T>::trace(trc, tp->ptr(), name);
+    }
+  }
+  static bool needsSweep(mozilla::Maybe<T>* tp) {
+    if (tp->isSome()) {
+      return GCPolicy<T>::needsSweep(tp->ptr());
+    }
+    return false;
+  }
+  static bool traceWeak(JSTracer* trc, mozilla::Maybe<T>* tp) {
+    if (tp->isSome()) {
+      return GCPolicy<T>::traceWeak(trc, tp->ptr());
+    }
+    return true;
+  }
+  static bool isValid(const mozilla::Maybe<T>& t) {
+    if (t.isSome()) {
+      return GCPolicy<T>::isValid(t.ref());
+    }
+    return true;
+  }
+};
+
+template <>
+struct GCPolicy<JS::Realm*>;  // see Realm.h
+
+}  // namespace JS
+
+#endif  // GCPolicyAPI_h