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diff --git a/xpcom/ds/ArrayIterator.h b/xpcom/ds/ArrayIterator.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/. */
+
+// Common iterator implementation for array classes e.g. nsTArray.
+
+#ifndef mozilla_ArrayIterator_h
+#define mozilla_ArrayIterator_h
+
+#include <iterator>
+#include <type_traits>
+
+namespace mozilla {
+
+namespace detail {
+template <typename T>
+struct AddInnerConst;
+
+template <typename T>
+struct AddInnerConst<T&> {
+  using Type = const T&;
+};
+
+template <typename T>
+struct AddInnerConst<T*> {
+  using Type = const T*;
+};
+
+template <typename T>
+using AddInnerConstT = typename AddInnerConst<T>::Type;
+}  // namespace detail
+
+// We have implemented a custom iterator class for array rather than using
+// raw pointers into the backing storage to improve the safety of C++11-style
+// range based iteration in the presence of array mutation, or script execution
+// (bug 1299489).
+//
+// Mutating an array which is being iterated is still wrong, and will either
+// cause elements to be missed or firefox to crash, but will not trigger memory
+// safety problems due to the release-mode bounds checking found in ElementAt.
+//
+// Dereferencing this iterator returns type Element. When Element is a reference
+// type, this iterator implements the full standard random access iterator spec,
+// and can be treated in many ways as though it is a pointer. Otherwise, it is
+// just enough to be used in range-based for loop.
+template <class Element, class ArrayType>
+class ArrayIterator {
+ public:
+  typedef ArrayType array_type;
+  typedef ArrayIterator<Element, ArrayType> iterator_type;
+  typedef typename array_type::index_type index_type;
+  typedef std::remove_reference_t<Element> value_type;
+  typedef ptrdiff_t difference_type;
+  typedef value_type* pointer;
+  typedef value_type& reference;
+  typedef std::random_access_iterator_tag iterator_category;
+  typedef ArrayIterator<detail::AddInnerConstT<Element>, ArrayType>
+      const_iterator_type;
+
+ private:
+  const array_type* mArray;
+  index_type mIndex;
+
+ public:
+  ArrayIterator() : mArray(nullptr), mIndex(0) {}
+  ArrayIterator(const iterator_type& aOther)
+      : mArray(aOther.mArray), mIndex(aOther.mIndex) {}
+  ArrayIterator(const array_type& aArray, index_type aIndex)
+      : mArray(&aArray), mIndex(aIndex) {}
+
+  iterator_type& operator=(const iterator_type& aOther) {
+    mArray = aOther.mArray;
+    mIndex = aOther.mIndex;
+    return *this;
+  }
+
+  constexpr operator const_iterator_type() const {
+    return mArray ? const_iterator_type{*mArray, mIndex}
+                  : const_iterator_type{};
+  }
+
+  bool operator==(const iterator_type& aRhs) const {
+    return mIndex == aRhs.mIndex;
+  }
+  bool operator!=(const iterator_type& aRhs) const { return !(*this == aRhs); }
+  bool operator<(const iterator_type& aRhs) const {
+    return mIndex < aRhs.mIndex;
+  }
+  bool operator>(const iterator_type& aRhs) const {
+    return mIndex > aRhs.mIndex;
+  }
+  bool operator<=(const iterator_type& aRhs) const {
+    return mIndex <= aRhs.mIndex;
+  }
+  bool operator>=(const iterator_type& aRhs) const {
+    return mIndex >= aRhs.mIndex;
+  }
+
+  // These operators depend on the release mode bounds checks in
+  // ArrayIterator::ElementAt for safety.
+  value_type* operator->() const {
+    return const_cast<value_type*>(&mArray->ElementAt(mIndex));
+  }
+  Element operator*() const {
+    return const_cast<Element>(mArray->ElementAt(mIndex));
+  }
+
+  iterator_type& operator++() {
+    ++mIndex;
+    return *this;
+  }
+  iterator_type operator++(int) {
+    iterator_type it = *this;
+    ++*this;
+    return it;
+  }
+  iterator_type& operator--() {
+    --mIndex;
+    return *this;
+  }
+  iterator_type operator--(int) {
+    iterator_type it = *this;
+    --*this;
+    return it;
+  }
+
+  iterator_type& operator+=(difference_type aDiff) {
+    mIndex += aDiff;
+    return *this;
+  }
+  iterator_type& operator-=(difference_type aDiff) {
+    mIndex -= aDiff;
+    return *this;
+  }
+
+  iterator_type operator+(difference_type aDiff) const {
+    iterator_type it = *this;
+    it += aDiff;
+    return it;
+  }
+  iterator_type operator-(difference_type aDiff) const {
+    iterator_type it = *this;
+    it -= aDiff;
+    return it;
+  }
+
+  difference_type operator-(const iterator_type& aOther) const {
+    return static_cast<difference_type>(mIndex) -
+           static_cast<difference_type>(aOther.mIndex);
+  }
+
+  Element operator[](difference_type aIndex) const {
+    return *this->operator+(aIndex);
+  }
+
+  constexpr const array_type* GetArray() const { return mArray; }
+
+  constexpr index_type GetIndex() const { return mIndex; }
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ArrayIterator_h