Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 748 insertions, 0 deletions

diff --git a/mfbt/LinkedList.h b/mfbt/LinkedList.h
new file mode 100644
index 0000000000..850b8594c7
--- /dev/null
+++ b/mfbt/LinkedList.h
@@ -0,0 +1,748 @@
+/* -*- 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/. */
+
+/* A type-safe doubly-linked list class. */
+
+/*
+ * The classes LinkedList<T> and LinkedListElement<T> together form a
+ * convenient, type-safe doubly-linked list implementation.
+ *
+ * The class T which will be inserted into the linked list must inherit from
+ * LinkedListElement<T>.  A given object may be in only one linked list at a
+ * time.
+ *
+ * A LinkedListElement automatically removes itself from the list upon
+ * destruction, and a LinkedList will fatally assert in debug builds if it's
+ * non-empty when it's destructed.
+ *
+ * For example, you might use LinkedList in a simple observer list class as
+ * follows.
+ *
+ *   class Observer : public LinkedListElement<Observer>
+ *   {
+ *   public:
+ *     void observe(char* aTopic) { ... }
+ *   };
+ *
+ *   class ObserverContainer
+ *   {
+ *   private:
+ *     LinkedList<Observer> list;
+ *
+ *   public:
+ *     void addObserver(Observer* aObserver)
+ *     {
+ *       // Will assert if |aObserver| is part of another list.
+ *       list.insertBack(aObserver);
+ *     }
+ *
+ *     void removeObserver(Observer* aObserver)
+ *     {
+ *       // Will assert if |aObserver| is not part of some list.
+ *       aObserver.remove();
+ *       // Or, will assert if |aObserver| is not part of |list| specifically.
+ *       // aObserver.removeFrom(list);
+ *     }
+ *
+ *     void notifyObservers(char* aTopic)
+ *     {
+ *       for (Observer* o = list.getFirst(); o != nullptr; o = o->getNext()) {
+ *         o->observe(aTopic);
+ *       }
+ *     }
+ *   };
+ *
+ * Additionally, the class AutoCleanLinkedList<T> is a LinkedList<T> that will
+ * remove and delete each element still within itself upon destruction. Note
+ * that because each element is deleted, elements must have been allocated
+ * using |new|.
+ */
+
+#ifndef mozilla_LinkedList_h
+#define mozilla_LinkedList_h
+
+#include <algorithm>
+#include <utility>
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/RefPtr.h"
+
+#ifdef __cplusplus
+
+namespace mozilla {
+
+template <typename T>
+class LinkedListElement;
+
+namespace detail {
+
+/**
+ * LinkedList supports refcounted elements using this adapter class. Clients
+ * using LinkedList<RefPtr<T>> will get a data structure that holds a strong
+ * reference to T as long as T is in the list.
+ */
+template <typename T>
+struct LinkedListElementTraits {
+  typedef T* RawType;
+  typedef const T* ConstRawType;
+  typedef T* ClientType;
+  typedef const T* ConstClientType;
+
+  // These static methods are called when an element is added to or removed from
+  // a linked list. It can be used to keep track ownership in lists that are
+  // supposed to own their elements. If elements are transferred from one list
+  // to another, no enter or exit calls happen since the elements still belong
+  // to a list.
+  static void enterList(LinkedListElement<T>* elt) {}
+  static void exitList(LinkedListElement<T>* elt) {}
+
+  // This method is called when AutoCleanLinkedList cleans itself
+  // during destruction. It can be used to call delete on elements if
+  // the list is the sole owner.
+  static void cleanElement(LinkedListElement<T>* elt) { delete elt->asT(); }
+};
+
+template <typename T>
+struct LinkedListElementTraits<RefPtr<T>> {
+  typedef T* RawType;
+  typedef const T* ConstRawType;
+  typedef RefPtr<T> ClientType;
+  typedef RefPtr<const T> ConstClientType;
+
+  static void enterList(LinkedListElement<RefPtr<T>>* elt) {
+    elt->asT()->AddRef();
+  }
+  static void exitList(LinkedListElement<RefPtr<T>>* elt) {
+    elt->asT()->Release();
+  }
+  static void cleanElement(LinkedListElement<RefPtr<T>>* elt) {}
+};
+
+} /* namespace detail */
+
+template <typename T>
+class LinkedList;
+
+template <typename T>
+class LinkedListElement {
+  typedef typename detail::LinkedListElementTraits<T> Traits;
+  typedef typename Traits::RawType RawType;
+  typedef typename Traits::ConstRawType ConstRawType;
+  typedef typename Traits::ClientType ClientType;
+  typedef typename Traits::ConstClientType ConstClientType;
+
+  /*
+   * It's convenient that we return nullptr when getNext() or getPrevious()
+   * hits the end of the list, but doing so costs an extra word of storage in
+   * each linked list node (to keep track of whether |this| is the sentinel
+   * node) and a branch on this value in getNext/getPrevious.
+   *
+   * We could get rid of the extra word of storage by shoving the "is
+   * sentinel" bit into one of the pointers, although this would, of course,
+   * have performance implications of its own.
+   *
+   * But the goal here isn't to win an award for the fastest or slimmest
+   * linked list; rather, we want a *convenient* linked list.  So we won't
+   * waste time guessing which micro-optimization strategy is best.
+   *
+   *
+   * Speaking of unnecessary work, it's worth addressing here why we wrote
+   * mozilla::LinkedList in the first place, instead of using stl::list.
+   *
+   * The key difference between mozilla::LinkedList and stl::list is that
+   * mozilla::LinkedList stores the mPrev/mNext pointers in the object itself,
+   * while stl::list stores the mPrev/mNext pointers in a list element which
+   * itself points to the object being stored.
+   *
+   * mozilla::LinkedList's approach makes it harder to store an object in more
+   * than one list.  But the upside is that you can call next() / prev() /
+   * remove() directly on the object.  With stl::list, you'd need to store a
+   * pointer to its iterator in the object in order to accomplish this.  Not
+   * only would this waste space, but you'd have to remember to update that
+   * pointer every time you added or removed the object from a list.
+   *
+   * In-place, constant-time removal is a killer feature of doubly-linked
+   * lists, and supporting this painlessly was a key design criterion.
+   */
+
+ private:
+  LinkedListElement* mNext;
+  LinkedListElement* mPrev;
+  const bool mIsSentinel;
+
+ public:
+  LinkedListElement() : mNext(this), mPrev(this), mIsSentinel(false) {}
+
+  /*
+   * Moves |aOther| into |*this|. If |aOther| is already in a list, then
+   * |aOther| is removed from the list and replaced by |*this|.
+   */
+  LinkedListElement(LinkedListElement<T>&& aOther)
+      : mIsSentinel(aOther.mIsSentinel) {
+    adjustLinkForMove(std::move(aOther));
+  }
+
+  LinkedListElement& operator=(LinkedListElement<T>&& aOther) {
+    MOZ_ASSERT(mIsSentinel == aOther.mIsSentinel, "Mismatch NodeKind!");
+    MOZ_ASSERT(!isInList(),
+               "Assigning to an element in a list messes up that list!");
+    adjustLinkForMove(std::move(aOther));
+    return *this;
+  }
+
+  ~LinkedListElement() {
+    if (!mIsSentinel && isInList()) {
+      remove();
+    }
+  }
+
+  /*
+   * Get the next element in the list, or nullptr if this is the last element
+   * in the list.
+   */
+  RawType getNext() { return mNext->asT(); }
+  ConstRawType getNext() const { return mNext->asT(); }
+
+  /*
+   * Get the previous element in the list, or nullptr if this is the first
+   * element in the list.
+   */
+  RawType getPrevious() { return mPrev->asT(); }
+  ConstRawType getPrevious() const { return mPrev->asT(); }
+
+  /*
+   * Insert aElem after this element in the list.  |this| must be part of a
+   * linked list when you call setNext(); otherwise, this method will assert.
+   */
+  void setNext(RawType aElem) {
+    MOZ_ASSERT(isInList());
+    setNextUnsafe(aElem);
+  }
+
+  /*
+   * Insert aElem before this element in the list.  |this| must be part of a
+   * linked list when you call setPrevious(); otherwise, this method will
+   * assert.
+   */
+  void setPrevious(RawType aElem) {
+    MOZ_ASSERT(isInList());
+    setPreviousUnsafe(aElem);
+  }
+
+  /*
+   * Remove this element from the list which contains it.  If this element is
+   * not currently part of a linked list, this method asserts.
+   */
+  void remove() {
+    MOZ_ASSERT(isInList());
+
+    mPrev->mNext = mNext;
+    mNext->mPrev = mPrev;
+    mNext = this;
+    mPrev = this;
+
+    Traits::exitList(this);
+  }
+
+  /*
+   * Remove this element from the list containing it.  Returns a pointer to the
+   * element that follows this element (before it was removed).  This method
+   * asserts if the element does not belong to a list. Note: In a refcounted
+   * list, |this| may be destroyed.
+   */
+  RawType removeAndGetNext() {
+    RawType r = getNext();
+    remove();
+    return r;
+  }
+
+  /*
+   * Remove this element from the list containing it.  Returns a pointer to the
+   * previous element in the containing list (before the removal).  This method
+   * asserts if the element does not belong to a list. Note: In a refcounted
+   * list, |this| may be destroyed.
+   */
+  RawType removeAndGetPrevious() {
+    RawType r = getPrevious();
+    remove();
+    return r;
+  }
+
+  /*
+   * Identical to remove(), but also asserts in debug builds that this element
+   * is in aList.
+   */
+  void removeFrom(const LinkedList<T>& aList) {
+    aList.assertContains(asT());
+    remove();
+  }
+
+  /*
+   * Return true if |this| part is of a linked list, and false otherwise.
+   */
+  bool isInList() const {
+    MOZ_ASSERT((mNext == this) == (mPrev == this));
+    return mNext != this;
+  }
+
+ private:
+  friend class LinkedList<T>;
+  friend struct detail::LinkedListElementTraits<T>;
+
+  enum class NodeKind { Normal, Sentinel };
+
+  explicit LinkedListElement(NodeKind nodeKind)
+      : mNext(this), mPrev(this), mIsSentinel(nodeKind == NodeKind::Sentinel) {}
+
+  /*
+   * Return |this| cast to T* if we're a normal node, or return nullptr if
+   * we're a sentinel node.
+   */
+  RawType asT() { return mIsSentinel ? nullptr : static_cast<RawType>(this); }
+  ConstRawType asT() const {
+    return mIsSentinel ? nullptr : static_cast<ConstRawType>(this);
+  }
+
+  /*
+   * Insert aElem after this element, but don't check that this element is in
+   * the list.  This is called by LinkedList::insertFront().
+   */
+  void setNextUnsafe(RawType aElem) {
+    LinkedListElement* listElem = static_cast<LinkedListElement*>(aElem);
+    MOZ_RELEASE_ASSERT(!listElem->isInList());
+
+    listElem->mNext = this->mNext;
+    listElem->mPrev = this;
+    this->mNext->mPrev = listElem;
+    this->mNext = listElem;
+
+    Traits::enterList(aElem);
+  }
+
+  /*
+   * Insert aElem before this element, but don't check that this element is in
+   * the list.  This is called by LinkedList::insertBack().
+   */
+  void setPreviousUnsafe(RawType aElem) {
+    LinkedListElement<T>* listElem = static_cast<LinkedListElement<T>*>(aElem);
+    MOZ_RELEASE_ASSERT(!listElem->isInList());
+
+    listElem->mNext = this;
+    listElem->mPrev = this->mPrev;
+    this->mPrev->mNext = listElem;
+    this->mPrev = listElem;
+
+    Traits::enterList(aElem);
+  }
+
+  /*
+   * Transfers the elements [aBegin, aEnd) before the "this" list element.
+   */
+  void transferBeforeUnsafe(LinkedListElement<T>& aBegin,
+                            LinkedListElement<T>& aEnd) {
+    MOZ_RELEASE_ASSERT(!aBegin.mIsSentinel);
+    if (!aBegin.isInList() || !aEnd.isInList()) {
+      return;
+    }
+
+    auto otherPrev = aBegin.mPrev;
+
+    aBegin.mPrev = this->mPrev;
+    this->mPrev->mNext = &aBegin;
+    this->mPrev = aEnd.mPrev;
+    aEnd.mPrev->mNext = this;
+
+    // Patch the gap in the source list
+    otherPrev->mNext = &aEnd;
+    aEnd.mPrev = otherPrev;
+  }
+
+  /*
+   * Adjust mNext and mPrev for implementing move constructor and move
+   * assignment.
+   */
+  void adjustLinkForMove(LinkedListElement<T>&& aOther) {
+    if (!aOther.isInList()) {
+      mNext = this;
+      mPrev = this;
+      return;
+    }
+
+    if (!mIsSentinel) {
+      Traits::enterList(this);
+    }
+
+    MOZ_ASSERT(aOther.mNext->mPrev == &aOther);
+    MOZ_ASSERT(aOther.mPrev->mNext == &aOther);
+
+    /*
+     * Initialize |this| with |aOther|'s mPrev/mNext pointers, and adjust those
+     * element to point to this one.
+     */
+    mNext = aOther.mNext;
+    mPrev = aOther.mPrev;
+
+    mNext->mPrev = this;
+    mPrev->mNext = this;
+
+    /*
+     * Adjust |aOther| so it doesn't think it's in a list.  This makes it
+     * safely destructable.
+     */
+    aOther.mNext = &aOther;
+    aOther.mPrev = &aOther;
+
+    if (!mIsSentinel) {
+      Traits::exitList(&aOther);
+    }
+  }
+
+  LinkedListElement& operator=(const LinkedListElement<T>& aOther) = delete;
+  LinkedListElement(const LinkedListElement<T>& aOther) = delete;
+};
+
+template <typename T>
+class LinkedList {
+ private:
+  typedef typename detail::LinkedListElementTraits<T> Traits;
+  typedef typename Traits::RawType RawType;
+  typedef typename Traits::ConstRawType ConstRawType;
+  typedef typename Traits::ClientType ClientType;
+  typedef typename Traits::ConstClientType ConstClientType;
+  typedef LinkedListElement<T>* ElementType;
+  typedef const LinkedListElement<T>* ConstElementType;
+
+  LinkedListElement<T> sentinel;
+
+ public:
+  template <typename Type, typename Element>
+  class Iterator {
+    Type mCurrent;
+
+   public:
+    using iterator_category = std::forward_iterator_tag;
+    using value_type = T;
+    using difference_type = std::ptrdiff_t;
+    using pointer = T*;
+    using reference = T&;
+
+    explicit Iterator(Type aCurrent) : mCurrent(aCurrent) {}
+
+    Type operator*() const { return mCurrent; }
+
+    const Iterator& operator++() {
+      mCurrent = static_cast<Element>(mCurrent)->getNext();
+      return *this;
+    }
+
+    bool operator!=(const Iterator& aOther) const {
+      return mCurrent != aOther.mCurrent;
+    }
+  };
+
+  LinkedList() : sentinel(LinkedListElement<T>::NodeKind::Sentinel) {}
+
+  LinkedList(LinkedList<T>&& aOther) : sentinel(std::move(aOther.sentinel)) {}
+
+  LinkedList& operator=(LinkedList<T>&& aOther) {
+    MOZ_ASSERT(isEmpty(),
+               "Assigning to a non-empty list leaks elements in that list!");
+    sentinel = std::move(aOther.sentinel);
+    return *this;
+  }
+
+  ~LinkedList() {
+#  ifdef DEBUG
+    if (!isEmpty()) {
+      MOZ_CRASH_UNSAFE_PRINTF(
+          "%s has a buggy user: "
+          "it should have removed all this list's elements before "
+          "the list's destruction",
+          __PRETTY_FUNCTION__);
+    }
+#  endif
+  }
+
+  /*
+   * Add aElem to the front of the list.
+   */
+  void insertFront(RawType aElem) {
+    /* Bypass setNext()'s this->isInList() assertion. */
+    sentinel.setNextUnsafe(aElem);
+  }
+
+  /*
+   * Add aElem to the back of the list.
+   */
+  void insertBack(RawType aElem) { sentinel.setPreviousUnsafe(aElem); }
+
+  /*
+   * Move all elements from another list to the back
+   */
+  void extendBack(LinkedList<T>&& aOther) {
+    MOZ_RELEASE_ASSERT(this != &aOther);
+    if (aOther.isEmpty()) {
+      return;
+    }
+    sentinel.transferBeforeUnsafe(**aOther.begin(), aOther.sentinel);
+  }
+
+  /*
+   * Move elements from another list to the specified position
+   */
+  void splice(size_t aDestinationPos, LinkedList<T>& aListFrom,
+              size_t aSourceStart, size_t aSourceLen) {
+    MOZ_RELEASE_ASSERT(this != &aListFrom);
+    if (aListFrom.isEmpty() || !aSourceLen) {
+      return;
+    }
+
+    const auto safeForward = [](LinkedList<T>& aList,
+                                LinkedListElement<T>& aBegin,
+                                size_t aPos) -> LinkedListElement<T>& {
+      auto* iter = &aBegin;
+      for (size_t i = 0; i < aPos; ++i, (iter = iter->mNext)) {
+        if (iter->mIsSentinel) {
+          break;
+        }
+      }
+      return *iter;
+    };
+
+    auto& sourceBegin =
+        safeForward(aListFrom, *aListFrom.sentinel.mNext, aSourceStart);
+    if (sourceBegin.mIsSentinel) {
+      return;
+    }
+    auto& sourceEnd = safeForward(aListFrom, sourceBegin, aSourceLen);
+    auto& destination = safeForward(*this, *sentinel.mNext, aDestinationPos);
+
+    destination.transferBeforeUnsafe(sourceBegin, sourceEnd);
+  }
+
+  /*
+   * Get the first element of the list, or nullptr if the list is empty.
+   */
+  RawType getFirst() { return sentinel.getNext(); }
+  ConstRawType getFirst() const { return sentinel.getNext(); }
+
+  /*
+   * Get the last element of the list, or nullptr if the list is empty.
+   */
+  RawType getLast() { return sentinel.getPrevious(); }
+  ConstRawType getLast() const { return sentinel.getPrevious(); }
+
+  /*
+   * Get and remove the first element of the list.  If the list is empty,
+   * return nullptr.
+   */
+  ClientType popFirst() {
+    ClientType ret = sentinel.getNext();
+    if (ret) {
+      static_cast<LinkedListElement<T>*>(RawType(ret))->remove();
+    }
+    return ret;
+  }
+
+  /*
+   * Get and remove the last element of the list.  If the list is empty,
+   * return nullptr.
+   */
+  ClientType popLast() {
+    ClientType ret = sentinel.getPrevious();
+    if (ret) {
+      static_cast<LinkedListElement<T>*>(RawType(ret))->remove();
+    }
+    return ret;
+  }
+
+  /*
+   * Return true if the list is empty, or false otherwise.
+   */
+  bool isEmpty() const { return !sentinel.isInList(); }
+
+  /**
+   * Returns whether the given element is in the list.
+   */
+  bool contains(ConstRawType aElm) const {
+    return std::find(begin(), end(), aElm) != end();
+  }
+
+  /*
+   * Remove all the elements from the list.
+   *
+   * This runs in time linear to the list's length, because we have to mark
+   * each element as not in the list.
+   */
+  void clear() {
+    while (popFirst()) {
+    }
+  }
+
+  /**
+   * Return the length of elements in the list.
+   */
+  size_t length() const { return std::distance(begin(), end()); }
+
+  /*
+   * Allow range-based iteration:
+   *
+   *     for (MyElementType* elt : myList) { ... }
+   */
+  Iterator<RawType, ElementType> begin() {
+    return Iterator<RawType, ElementType>(getFirst());
+  }
+  Iterator<ConstRawType, ConstElementType> begin() const {
+    return Iterator<ConstRawType, ConstElementType>(getFirst());
+  }
+  Iterator<RawType, ElementType> end() {
+    return Iterator<RawType, ElementType>(nullptr);
+  }
+  Iterator<ConstRawType, ConstElementType> end() const {
+    return Iterator<ConstRawType, ConstElementType>(nullptr);
+  }
+
+  /*
+   * Measures the memory consumption of the list excluding |this|.  Note that
+   * it only measures the list elements themselves.  If the list elements
+   * contain pointers to other memory blocks, those blocks must be measured
+   * separately during a subsequent iteration over the list.
+   */
+  size_t sizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const {
+    size_t n = 0;
+    ConstRawType t = getFirst();
+    while (t) {
+      n += aMallocSizeOf(t);
+      t = static_cast<const LinkedListElement<T>*>(t)->getNext();
+    }
+    return n;
+  }
+
+  /*
+   * Like sizeOfExcludingThis(), but measures |this| as well.
+   */
+  size_t sizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const {
+    return aMallocSizeOf(this) + sizeOfExcludingThis(aMallocSizeOf);
+  }
+
+  /*
+   * In a debug build, make sure that the list is sane (no cycles, consistent
+   * mNext/mPrev pointers, only one sentinel).  Has no effect in release builds.
+   */
+  void debugAssertIsSane() const {
+#  ifdef DEBUG
+    const LinkedListElement<T>* slow;
+    const LinkedListElement<T>* fast1;
+    const LinkedListElement<T>* fast2;
+
+    /*
+     * Check for cycles in the forward singly-linked list using the
+     * tortoise/hare algorithm.
+     */
+    for (slow = sentinel.mNext, fast1 = sentinel.mNext->mNext,
+        fast2 = sentinel.mNext->mNext->mNext;
+         slow != &sentinel && fast1 != &sentinel && fast2 != &sentinel;
+         slow = slow->mNext, fast1 = fast2->mNext, fast2 = fast1->mNext) {
+      MOZ_ASSERT(slow != fast1);
+      MOZ_ASSERT(slow != fast2);
+    }
+
+    /* Check for cycles in the backward singly-linked list. */
+    for (slow = sentinel.mPrev, fast1 = sentinel.mPrev->mPrev,
+        fast2 = sentinel.mPrev->mPrev->mPrev;
+         slow != &sentinel && fast1 != &sentinel && fast2 != &sentinel;
+         slow = slow->mPrev, fast1 = fast2->mPrev, fast2 = fast1->mPrev) {
+      MOZ_ASSERT(slow != fast1);
+      MOZ_ASSERT(slow != fast2);
+    }
+
+    /*
+     * Check that |sentinel| is the only node in the list with
+     * mIsSentinel == true.
+     */
+    for (const LinkedListElement<T>* elem = sentinel.mNext; elem != &sentinel;
+         elem = elem->mNext) {
+      MOZ_ASSERT(!elem->mIsSentinel);
+    }
+
+    /* Check that the mNext/mPrev pointers match up. */
+    const LinkedListElement<T>* prev = &sentinel;
+    const LinkedListElement<T>* cur = sentinel.mNext;
+    do {
+      MOZ_ASSERT(cur->mPrev == prev);
+      MOZ_ASSERT(prev->mNext == cur);
+
+      prev = cur;
+      cur = cur->mNext;
+    } while (cur != &sentinel);
+#  endif /* ifdef DEBUG */
+  }
+
+ private:
+  friend class LinkedListElement<T>;
+
+  void assertContains(const RawType aValue) const {
+#  ifdef DEBUG
+    for (ConstRawType elem = getFirst(); elem; elem = elem->getNext()) {
+      if (elem == aValue) {
+        return;
+      }
+    }
+    MOZ_CRASH("element wasn't found in this list!");
+#  endif
+  }
+
+  LinkedList& operator=(const LinkedList<T>& aOther) = delete;
+  LinkedList(const LinkedList<T>& aOther) = delete;
+};
+
+template <typename T>
+inline void ImplCycleCollectionUnlink(LinkedList<RefPtr<T>>& aField) {
+  aField.clear();
+}
+
+template <typename T>
+inline void ImplCycleCollectionTraverse(
+    nsCycleCollectionTraversalCallback& aCallback,
+    LinkedList<RefPtr<T>>& aField, const char* aName, uint32_t aFlags = 0) {
+  typedef typename detail::LinkedListElementTraits<T> Traits;
+  typedef typename Traits::RawType RawType;
+  for (RawType element : aField) {
+    // RefPtr is stored as a raw pointer in LinkedList.
+    // So instead of creating a new RefPtr from the raw
+    // pointer (which is not allowed), we simply call
+    // CycleCollectionNoteChild against the raw pointer
+    CycleCollectionNoteChild(aCallback, element, aName, aFlags);
+  }
+}
+
+template <typename T>
+class AutoCleanLinkedList : public LinkedList<T> {
+ private:
+  using Traits = detail::LinkedListElementTraits<T>;
+  using ClientType = typename detail::LinkedListElementTraits<T>::ClientType;
+
+ public:
+  AutoCleanLinkedList() = default;
+  AutoCleanLinkedList(AutoCleanLinkedList&&) = default;
+  ~AutoCleanLinkedList() { clear(); }
+
+  AutoCleanLinkedList& operator=(AutoCleanLinkedList&& aOther) = default;
+
+  void clear() {
+    while (ClientType element = this->popFirst()) {
+      Traits::cleanElement(element);
+    }
+  }
+};
+
+} /* namespace mozilla */
+
+#endif /* __cplusplus */
+
+#endif /* mozilla_LinkedList_h */