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diff --git a/js/public/GCHashTable.h b/js/public/GCHashTable.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 GCHashTable_h
+#define GCHashTable_h
+
+#include "mozilla/Maybe.h"
+
+#include "js/GCPolicyAPI.h"
+#include "js/HashTable.h"
+#include "js/RootingAPI.h"
+#include "js/TypeDecls.h"
+
+class JSTracer;
+
+namespace JS {
+
+// Define a reasonable default GC policy for GC-aware Maps.
+template <typename Key, typename Value>
+struct DefaultMapEntryGCPolicy {
+  static bool traceWeak(JSTracer* trc, Key* key, Value* value) {
+    return GCPolicy<Key>::traceWeak(trc, key) &&
+           GCPolicy<Value>::traceWeak(trc, value);
+  }
+};
+
+// A GCHashMap is a GC-aware HashMap, meaning that it has additional trace
+// methods that know how to visit all keys and values in the table. HashMaps
+// that contain GC pointers will generally want to use this GCHashMap
+// specialization instead of HashMap, because this conveniently supports tracing
+// keys and values, and cleaning up weak entries.
+//
+// GCHashMap::trace applies GCPolicy<T>::trace to each entry's key and value.
+// Most types of GC pointers already have appropriate specializations of
+// GCPolicy, so they should just work as keys and values. Any struct type with a
+// default constructor and trace function should work as well. If you need to
+// define your own GCPolicy specialization, generic helpers can be found in
+// js/public/TracingAPI.h.
+//
+// The MapEntryGCPolicy template parameter controls how the table drops entries
+// when edges are weakly held. GCHashMap::traceWeak applies the
+// MapEntryGCPolicy's traceWeak method to each table entry; if it returns true,
+// the entry is dropped. The default MapEntryGCPolicy drops the entry if either
+// the key or value is about to be finalized, according to its
+// GCPolicy<T>::traceWeak method. (This default is almost always fine: it's hard
+// to imagine keeping such an entry around anyway.)
+//
+// Note that this HashMap only knows *how* to trace, but it does not itself
+// cause tracing to be invoked. For tracing, it must be used as
+// Rooted<GCHashMap> or PersistentRooted<GCHashMap>, or barriered and traced
+// manually.
+template <typename Key, typename Value,
+          typename HashPolicy = js::DefaultHasher<Key>,
+          typename AllocPolicy = js::TempAllocPolicy,
+          typename MapEntryGCPolicy = DefaultMapEntryGCPolicy<Key, Value>>
+class GCHashMap : public js::HashMap<Key, Value, HashPolicy, AllocPolicy> {
+  using Base = js::HashMap<Key, Value, HashPolicy, AllocPolicy>;
+
+ public:
+  using EntryGCPolicy = MapEntryGCPolicy;
+
+  explicit GCHashMap(AllocPolicy a = AllocPolicy()) : Base(std::move(a)) {}
+  explicit GCHashMap(size_t length) : Base(length) {}
+  GCHashMap(AllocPolicy a, size_t length) : Base(std::move(a), length) {}
+
+  void trace(JSTracer* trc) {
+    for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
+      GCPolicy<Value>::trace(trc, &e.front().value(), "hashmap value");
+      GCPolicy<Key>::trace(trc, &e.front().mutableKey(), "hashmap key");
+    }
+  }
+
+  bool traceWeak(JSTracer* trc) {
+    typename Base::Enum e(*this);
+    traceWeakEntries(trc, e);
+    return !this->empty();
+  }
+
+  void traceWeakEntries(JSTracer* trc, typename Base::Enum& e) {
+    for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
+      if (!MapEntryGCPolicy::traceWeak(trc, &e.front().mutableKey(),
+                                       &e.front().value())) {
+        e.removeFront();
+      }
+    }
+  }
+
+  // GCHashMap is movable
+  GCHashMap(GCHashMap&& rhs) : Base(std::move(rhs)) {}
+  void operator=(GCHashMap&& rhs) {
+    MOZ_ASSERT(this != &rhs, "self-move assignment is prohibited");
+    Base::operator=(std::move(rhs));
+  }
+
+ private:
+  // GCHashMap is not copyable or assignable
+  GCHashMap(const GCHashMap& hm) = delete;
+  GCHashMap& operator=(const GCHashMap& hm) = delete;
+} MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS;
+
+}  // namespace JS
+
+namespace js {
+
+// HashMap that supports rekeying.
+//
+// If your keys are pointers to something like JSObject that can be tenured or
+// compacted, prefer to use GCHashMap with StableCellHasher, which takes
+// advantage of the Zone's stable id table to make rekeying unnecessary.
+template <typename Key, typename Value,
+          typename HashPolicy = DefaultHasher<Key>,
+          typename AllocPolicy = TempAllocPolicy,
+          typename MapEntryGCPolicy = JS::DefaultMapEntryGCPolicy<Key, Value>>
+class GCRekeyableHashMap : public JS::GCHashMap<Key, Value, HashPolicy,
+                                                AllocPolicy, MapEntryGCPolicy> {
+  using Base = JS::GCHashMap<Key, Value, HashPolicy, AllocPolicy>;
+
+ public:
+  explicit GCRekeyableHashMap(AllocPolicy a = AllocPolicy())
+      : Base(std::move(a)) {}
+  explicit GCRekeyableHashMap(size_t length) : Base(length) {}
+  GCRekeyableHashMap(AllocPolicy a, size_t length)
+      : Base(std::move(a), length) {}
+
+  bool traceWeak(JSTracer* trc) {
+    for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
+      Key key(e.front().key());
+      if (!MapEntryGCPolicy::traceWeak(trc, &key, &e.front().value())) {
+        e.removeFront();
+      } else if (!HashPolicy::match(key, e.front().key())) {
+        e.rekeyFront(key);
+      }
+    }
+    return !this->empty();
+  }
+
+  // GCRekeyableHashMap is movable
+  GCRekeyableHashMap(GCRekeyableHashMap&& rhs) : Base(std::move(rhs)) {}
+  void operator=(GCRekeyableHashMap&& rhs) {
+    MOZ_ASSERT(this != &rhs, "self-move assignment is prohibited");
+    Base::operator=(std::move(rhs));
+  }
+} MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS;
+
+template <typename Wrapper, typename... Args>
+class WrappedPtrOperations<JS::GCHashMap<Args...>, Wrapper> {
+  using Map = JS::GCHashMap<Args...>;
+  using Lookup = typename Map::Lookup;
+
+  const Map& map() const { return static_cast<const Wrapper*>(this)->get(); }
+
+ public:
+  using AddPtr = typename Map::AddPtr;
+  using Ptr = typename Map::Ptr;
+  using Range = typename Map::Range;
+
+  Ptr lookup(const Lookup& l) const { return map().lookup(l); }
+  Range all() const { return map().all(); }
+  bool empty() const { return map().empty(); }
+  uint32_t count() const { return map().count(); }
+  size_t capacity() const { return map().capacity(); }
+  bool has(const Lookup& l) const { return map().lookup(l).found(); }
+  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return map().sizeOfExcludingThis(mallocSizeOf);
+  }
+  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return mallocSizeOf(this) + map().sizeOfExcludingThis(mallocSizeOf);
+  }
+};
+
+template <typename Wrapper, typename... Args>
+class MutableWrappedPtrOperations<JS::GCHashMap<Args...>, Wrapper>
+    : public WrappedPtrOperations<JS::GCHashMap<Args...>, Wrapper> {
+  using Map = JS::GCHashMap<Args...>;
+  using Lookup = typename Map::Lookup;
+
+  Map& map() { return static_cast<Wrapper*>(this)->get(); }
+
+ public:
+  using AddPtr = typename Map::AddPtr;
+  struct Enum : public Map::Enum {
+    explicit Enum(Wrapper& o) : Map::Enum(o.map()) {}
+  };
+  using Ptr = typename Map::Ptr;
+  using Range = typename Map::Range;
+
+  void clear() { map().clear(); }
+  void clearAndCompact() { map().clearAndCompact(); }
+  void remove(Ptr p) { map().remove(p); }
+  AddPtr lookupForAdd(const Lookup& l) { return map().lookupForAdd(l); }
+
+  template <typename KeyInput, typename ValueInput>
+  bool add(AddPtr& p, KeyInput&& k, ValueInput&& v) {
+    return map().add(p, std::forward<KeyInput>(k), std::forward<ValueInput>(v));
+  }
+
+  template <typename KeyInput>
+  bool add(AddPtr& p, KeyInput&& k) {
+    return map().add(p, std::forward<KeyInput>(k), Map::Value());
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  bool relookupOrAdd(AddPtr& p, KeyInput&& k, ValueInput&& v) {
+    return map().relookupOrAdd(p, k, std::forward<KeyInput>(k),
+                               std::forward<ValueInput>(v));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  bool put(KeyInput&& k, ValueInput&& v) {
+    return map().put(std::forward<KeyInput>(k), std::forward<ValueInput>(v));
+  }
+
+  template <typename KeyInput, typename ValueInput>
+  bool putNew(KeyInput&& k, ValueInput&& v) {
+    return map().putNew(std::forward<KeyInput>(k), std::forward<ValueInput>(v));
+  }
+};
+
+}  // namespace js
+
+namespace JS {
+
+// A GCHashSet is a HashSet with an additional trace method that knows
+// be traced to be kept alive will generally want to use this GCHashSet
+// specialization in lieu of HashSet.
+//
+// Most types of GC pointers can be traced with no extra infrastructure. For
+// structs and non-gc-pointer members, ensure that there is a specialization of
+// GCPolicy<T> with an appropriate trace method available to handle the custom
+// type. Generic helpers can be found in js/public/TracingAPI.h.
+//
+// Note that although this HashSet's trace will deal correctly with moved
+// elements, it does not itself know when to barrier or trace elements. To
+// function properly it must either be used with Rooted or barriered and traced
+// manually.
+template <typename T, typename HashPolicy = js::DefaultHasher<T>,
+          typename AllocPolicy = js::TempAllocPolicy>
+class GCHashSet : public js::HashSet<T, HashPolicy, AllocPolicy> {
+  using Base = js::HashSet<T, HashPolicy, AllocPolicy>;
+
+ public:
+  explicit GCHashSet(AllocPolicy a = AllocPolicy()) : Base(std::move(a)) {}
+  explicit GCHashSet(size_t length) : Base(length) {}
+  GCHashSet(AllocPolicy a, size_t length) : Base(std::move(a), length) {}
+
+  void trace(JSTracer* trc) {
+    for (typename Base::Enum e(*this); !e.empty(); e.popFront()) {
+      GCPolicy<T>::trace(trc, &e.mutableFront(), "hashset element");
+    }
+  }
+
+  bool traceWeak(JSTracer* trc) {
+    typename Base::Enum e(*this);
+    traceWeakEntries(trc, e);
+    return !this->empty();
+  }
+
+  void traceWeakEntries(JSTracer* trc, typename Base::Enum& e) {
+    for (; !e.empty(); e.popFront()) {
+      if (!GCPolicy<T>::traceWeak(trc, &e.mutableFront())) {
+        e.removeFront();
+      }
+    }
+  }
+
+  // GCHashSet is movable
+  GCHashSet(GCHashSet&& rhs) : Base(std::move(rhs)) {}
+  void operator=(GCHashSet&& rhs) {
+    MOZ_ASSERT(this != &rhs, "self-move assignment is prohibited");
+    Base::operator=(std::move(rhs));
+  }
+
+ private:
+  // GCHashSet is not copyable or assignable
+  GCHashSet(const GCHashSet& hs) = delete;
+  GCHashSet& operator=(const GCHashSet& hs) = delete;
+} MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS;
+
+}  // namespace JS
+
+namespace js {
+
+template <typename Wrapper, typename... Args>
+class WrappedPtrOperations<JS::GCHashSet<Args...>, Wrapper> {
+  using Set = JS::GCHashSet<Args...>;
+
+  const Set& set() const { return static_cast<const Wrapper*>(this)->get(); }
+
+ public:
+  using Lookup = typename Set::Lookup;
+  using AddPtr = typename Set::AddPtr;
+  using Entry = typename Set::Entry;
+  using Ptr = typename Set::Ptr;
+  using Range = typename Set::Range;
+
+  Ptr lookup(const Lookup& l) const { return set().lookup(l); }
+  Range all() const { return set().all(); }
+  bool empty() const { return set().empty(); }
+  uint32_t count() const { return set().count(); }
+  size_t capacity() const { return set().capacity(); }
+  bool has(const Lookup& l) const { return set().lookup(l).found(); }
+  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return set().sizeOfExcludingThis(mallocSizeOf);
+  }
+  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return mallocSizeOf(this) + set().sizeOfExcludingThis(mallocSizeOf);
+  }
+};
+
+template <typename Wrapper, typename... Args>
+class MutableWrappedPtrOperations<JS::GCHashSet<Args...>, Wrapper>
+    : public WrappedPtrOperations<JS::GCHashSet<Args...>, Wrapper> {
+  using Set = JS::GCHashSet<Args...>;
+  using Lookup = typename Set::Lookup;
+
+  Set& set() { return static_cast<Wrapper*>(this)->get(); }
+
+ public:
+  using AddPtr = typename Set::AddPtr;
+  using Entry = typename Set::Entry;
+  struct Enum : public Set::Enum {
+    explicit Enum(Wrapper& o) : Set::Enum(o.set()) {}
+  };
+  using Ptr = typename Set::Ptr;
+  using Range = typename Set::Range;
+
+  void clear() { set().clear(); }
+  void clearAndCompact() { set().clearAndCompact(); }
+  [[nodiscard]] bool reserve(uint32_t len) { return set().reserve(len); }
+  void remove(Ptr p) { set().remove(p); }
+  void remove(const Lookup& l) { set().remove(l); }
+  AddPtr lookupForAdd(const Lookup& l) { return set().lookupForAdd(l); }
+
+  template <typename TInput>
+  void replaceKey(Ptr p, const Lookup& l, TInput&& newValue) {
+    set().replaceKey(p, l, std::forward<TInput>(newValue));
+  }
+
+  template <typename TInput>
+  bool add(AddPtr& p, TInput&& t) {
+    return set().add(p, std::forward<TInput>(t));
+  }
+
+  template <typename TInput>
+  bool relookupOrAdd(AddPtr& p, const Lookup& l, TInput&& t) {
+    return set().relookupOrAdd(p, l, std::forward<TInput>(t));
+  }
+
+  template <typename TInput>
+  bool put(TInput&& t) {
+    return set().put(std::forward<TInput>(t));
+  }
+
+  template <typename TInput>
+  bool putNew(TInput&& t) {
+    return set().putNew(std::forward<TInput>(t));
+  }
+
+  template <typename TInput>
+  bool putNew(const Lookup& l, TInput&& t) {
+    return set().putNew(l, std::forward<TInput>(t));
+  }
+};
+
+} /* namespace js */
+
+#endif /* GCHashTable_h */