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diff --git a/xpcom/ds/nsTHashSet.h b/xpcom/ds/nsTHashSet.h
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+++ b/xpcom/ds/nsTHashSet.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 XPCOM_DS_NSTHASHSET_H_
+#define XPCOM_DS_NSTHASHSET_H_
+
+#include "nsHashtablesFwd.h"
+#include "nsTHashMap.h"  // for nsKeyClass
+
+/**
+ * Templated hash set. Don't use this directly, but use nsTHashSet instead
+ * (defined as a type alias in nsHashtablesFwd.h).
+ *
+ * @param KeyClass a wrapper-class for the hashtable key, see nsHashKeys.h
+ *   for a complete specification.
+ */
+template <class KeyClass>
+class nsTBaseHashSet : protected nsTHashtable<KeyClass> {
+  using Base = nsTHashtable<KeyClass>;
+  typedef mozilla::fallible_t fallible_t;
+
+ public:
+  // XXX We have a similar situation here like with DataType/UserDataType in
+  // nsBaseHashtable. It's less problematic here due to the more constrained
+  // interface, but it may still be confusing. KeyType is not the stored key
+  // type, but the one exposed to the user, i.e. as a parameter type, and as the
+  // value type when iterating. It is currently impossible to move-insert a
+  // RefPtr<T>, e.g., since the KeyType is T* in that case.
+  using ValueType = typename KeyClass::KeyType;
+
+  // KeyType is defined just for compatibility with nsTHashMap. For a set, the
+  // key type is conceptually equivalent to the value type.
+  using KeyType = typename KeyClass::KeyType;
+
+  using Base::Base;
+
+  // Query operations.
+
+  using Base::Contains;
+  using Base::GetGeneration;
+  using Base::ShallowSizeOfExcludingThis;
+  using Base::ShallowSizeOfIncludingThis;
+  using Base::SizeOfExcludingThis;
+  using Base::SizeOfIncludingThis;
+
+  /**
+   * Return the number of entries in the table.
+   * @return    number of entries
+   */
+  [[nodiscard]] uint32_t Count() const { return Base::Count(); }
+
+  /**
+   * Return whether the table is empty.
+   * @return    whether empty
+   */
+  [[nodiscard]] bool IsEmpty() const { return Base::IsEmpty(); }
+
+  using iterator = ::detail::nsTHashtableKeyIterator<KeyClass>;
+  using const_iterator = iterator;
+
+  [[nodiscard]] auto begin() const { return Base::Keys().begin(); }
+
+  [[nodiscard]] auto end() const { return Base::Keys().end(); }
+
+  [[nodiscard]] auto cbegin() const { return Base::Keys().cbegin(); }
+
+  [[nodiscard]] auto cend() const { return Base::Keys().cend(); }
+
+  // Mutating operations.
+
+  using Base::Clear;
+  using Base::MarkImmutable;
+
+  /**
+   * Inserts a value into the set. Has no effect if the value is already in the
+   * set. This overload is infallible and crashes if memory is exhausted.
+   *
+   * \note For strict consistency with nsTHashtable::EntryHandle method naming,
+   * this should rather be called OrInsert, as it is legal to call it when the
+   * value is already in the set. For simplicity, as we don't have two methods,
+   * we still use "Insert" instead.
+   */
+  void Insert(ValueType aValue) { Base::PutEntry(aValue); }
+
+  /**
+   * Inserts a value into the set. Has no effect if the value is already in the
+   * set. This overload is fallible and returns false if memory is exhausted.
+   *
+   * \note See note on infallible overload.
+   */
+  [[nodiscard]] bool Insert(ValueType aValue, const mozilla::fallible_t&) {
+    return Base::PutEntry(aValue, mozilla::fallible);
+  }
+
+  /**
+   * Inserts a value into the set. Has no effect if the value is already in the
+   * set. This member function is infallible and crashes if memory is exhausted.
+   *
+   * \return true if the value was actually inserted, false if it was already in
+   * the set.
+   */
+  bool EnsureInserted(ValueType aValue) { return Base::EnsureInserted(aValue); }
+
+  using Base::Remove;
+
+  /**
+   * Removes a value from the set. Has no effect if the value is not in the set.
+   *
+   * \note For strict consistency with nsTHashtable::EntryHandle method naming,
+   * this should rather be called OrRemove, as it is legal to call it when the
+   * value is not in the set. For simplicity, as we don't have two methods,
+   * we still use "Remove" instead.
+   */
+  void Remove(ValueType aValue) { Base::RemoveEntry(aValue); }
+
+  using Base::EnsureRemoved;
+
+  /**
+   * Removes all elements matching a predicate.
+   *
+   * The predicate must be compatible with signature bool (ValueType).
+   */
+  template <typename Pred>
+  void RemoveIf(Pred&& aPred) {
+    for (auto it = cbegin(), end = cend(); it != end; ++it) {
+      if (aPred(static_cast<ValueType>(*it))) {
+        Remove(it);
+      }
+    }
+  }
+};
+
+template <typename KeyClass>
+auto RangeSize(const nsTBaseHashSet<KeyClass>& aRange) {
+  return aRange.Count();
+}
+
+class nsCycleCollectionTraversalCallback;
+
+template <class KeyClass>
+inline void ImplCycleCollectionUnlink(nsTBaseHashSet<KeyClass>& aField) {
+  aField.Clear();
+}
+
+template <class KeyClass>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    const nsTBaseHashSet<KeyClass>& aField, const char* aName,
+    uint32_t aFlags = 0) {
+  for (const auto& entry : aField) {
+    CycleCollectionNoteChild(aCallback, mozilla::detail::PtrGetWeak(entry),
+                             aName, aFlags);
+  }
+}
+
+namespace mozilla {
+template <typename SetT>
+class nsTSetInserter {
+  SetT* mSet;
+
+  class Proxy {
+    SetT& mSet;
+
+   public:
+    explicit Proxy(SetT& aSet) : mSet{aSet} {}
+
+    template <typename E2>
+    void operator=(E2&& aValue) {
+      mSet.Insert(std::forward<E2>(aValue));
+    }
+  };
+
+ public:
+  using iterator_category = std::output_iterator_tag;
+
+  explicit nsTSetInserter(SetT& aSet) : mSet{&aSet} {}
+
+  Proxy operator*() { return Proxy(*mSet); }
+
+  nsTSetInserter& operator++() { return *this; }
+  nsTSetInserter& operator++(int) { return *this; }
+};
+}  // namespace mozilla
+
+template <typename E>
+auto MakeInserter(nsTBaseHashSet<E>& aSet) {
+  return mozilla::nsTSetInserter<nsTBaseHashSet<E>>{aSet};
+}
+
+#endif  // XPCOM_DS_NSTHASHSET_H_