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diff --git a/dom/base/ChildIterator.h b/dom/base/ChildIterator.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 ChildIterator_h
+#define ChildIterator_h
+
+#include "nsIContent.h"
+#include "nsIContentInlines.h"
+#include <stdint.h>
+
+class nsIContent;
+
+namespace mozilla::dom {
+
+// Iterates over the flattened children of a node, that is, the regular DOM
+// child nodes of a given DOM node, with assigned nodes as slot children, and
+// shadow host children replaced by their shadow root.
+//
+// The iterator can be initialized to start at the end by providing false for
+// aStartAtBeginning in order to start iterating in reverse from the last child.
+class FlattenedChildIterator {
+ public:
+  explicit FlattenedChildIterator(const nsIContent* aParent,
+                                  bool aStartAtBeginning = true);
+
+  nsIContent* GetNextChild();
+
+  // Looks for aChildToFind respecting insertion points until aChildToFind is
+  // found.  This can be O(1) instead of O(N) in many cases.
+  bool Seek(const nsIContent* aChildToFind);
+
+  // Returns the current target of this iterator (which might be an explicit
+  // child of the node, or a node assigned to a slot.
+  nsIContent* Get() const { return mChild; }
+
+  // Returns the original parent we were initialized with.
+  const nsIContent* Parent() const { return mOriginalParent; }
+
+  // The inverse of GetNextChild. Properly steps in and out of insertion
+  // points.
+  nsIContent* GetPreviousChild();
+
+  bool ShadowDOMInvolved() const { return mShadowDOMInvolved; }
+
+ protected:
+  // The parent of the children being iterated. For shadow hosts this will point
+  // to its shadow root.
+  const nsIContent* mParent;
+
+  // If parent is a slot element with assigned slots, this points to the parent
+  // as HTMLSlotElement, otherwise, it's null.
+  const HTMLSlotElement* mParentAsSlot = nullptr;
+
+  const nsIContent* mOriginalParent = nullptr;
+
+  // The current child.
+  nsIContent* mChild = nullptr;
+
+  // A flag to let us know that we haven't started iterating yet.
+  bool mIsFirst = false;
+
+  // The index of the current element in the slot assigned nodes. One-past the
+  // end to represent the last position.
+  uint32_t mIndexInInserted = 0u;
+
+  // For certain optimizations, nsCSSFrameConstructor needs to know if the child
+  // list of the element that we're iterating matches its .childNodes.
+  bool mShadowDOMInvolved = false;
+};
+
+/**
+ * AllChildrenIterator traverses the children of an element including before /
+ * after content and shadow DOM.  The iterator can be initialized to start at
+ * the end by providing false for aStartAtBeginning in order to start iterating
+ * in reverse from the last child.
+ *
+ * Note: it assumes that no mutation of the DOM or frame tree takes place during
+ * iteration, and will break horribly if that is not true.
+ */
+class AllChildrenIterator : private FlattenedChildIterator {
+ public:
+  AllChildrenIterator(const nsIContent* aNode, uint32_t aFlags,
+                      bool aStartAtBeginning = true)
+      : FlattenedChildIterator(aNode, aStartAtBeginning),
+        mAnonKidsIdx(aStartAtBeginning ? UINT32_MAX : 0),
+        mFlags(aFlags),
+        mPhase(aStartAtBeginning ? eAtBegin : eAtEnd) {}
+
+#ifdef DEBUG
+  AllChildrenIterator(AllChildrenIterator&&) = default;
+
+  AllChildrenIterator& operator=(AllChildrenIterator&& aOther) {
+    // Explicitly call the destructor to ensure the assertion in the destructor
+    // is checked.
+    this->~AllChildrenIterator();
+    new (this) AllChildrenIterator(std::move(aOther));
+    return *this;
+  }
+
+  ~AllChildrenIterator() { MOZ_ASSERT(!mMutationGuard.Mutated(0)); }
+#endif
+
+  // Returns the current target the iterator is at, or null if the iterator
+  // doesn't point to any child node (either eAtBegin or eAtEnd phase).
+  nsIContent* Get() const;
+
+  // Seeks the given node in children of a parent element, starting from
+  // the current iterator's position, and sets the iterator at the given child
+  // node if it was found.
+  bool Seek(const nsIContent* aChildToFind);
+
+  nsIContent* GetNextChild();
+  nsIContent* GetPreviousChild();
+
+  enum IteratorPhase {
+    eAtBegin,
+    eAtMarkerKid,
+    eAtBeforeKid,
+    eAtFlatTreeKids,
+    eAtAnonKids,
+    eAtAfterKid,
+    eAtEnd
+  };
+  IteratorPhase Phase() const { return mPhase; }
+
+ private:
+  // Helpers.
+  void AppendNativeAnonymousChildren();
+
+  // mAnonKids is an array of native anonymous children, mAnonKidsIdx is index
+  // in the array. If mAnonKidsIdx < mAnonKids.Length() and mPhase is
+  // eAtAnonKids then the iterator points at a child at mAnonKidsIdx index. If
+  // mAnonKidsIdx == mAnonKids.Length() then the iterator is somewhere after
+  // the last native anon child. If mAnonKidsIdx == UINT32_MAX then the iterator
+  // is somewhere before the first native anon child.
+  nsTArray<nsIContent*> mAnonKids;
+  uint32_t mAnonKidsIdx;
+
+  uint32_t mFlags;
+  IteratorPhase mPhase;
+#ifdef DEBUG
+  // XXX we should really assert there are no frame tree changes as well, but
+  // there's no easy way to do that.
+  nsMutationGuard mMutationGuard;
+#endif
+};
+
+/**
+ * StyleChildrenIterator traverses the children of the element from the
+ * perspective of the style system, particularly the children we need to
+ * traverse during restyle.
+ *
+ * At present, this is identical to AllChildrenIterator with
+ * (eAllChildren | eSkipDocumentLevelNativeAnonymousContent). We used to have
+ * detect and skip any native anonymous children that are used to implement some
+ * special magic in here that went away, but we keep the separate class so
+ * we can reintroduce special magic back if needed.
+ *
+ * Note: it assumes that no mutation of the DOM or frame tree takes place during
+ * iteration, and will break horribly if that is not true.
+ *
+ * We require this to be memmovable since Rust code can create and move
+ * StyleChildrenIterators.
+ */
+class MOZ_NEEDS_MEMMOVABLE_MEMBERS StyleChildrenIterator
+    : private AllChildrenIterator {
+ public:
+  static nsIContent* GetParent(const nsIContent& aContent) {
+    nsINode* node = aContent.GetFlattenedTreeParentNodeForStyle();
+    return node && node->IsContent() ? node->AsContent() : nullptr;
+  }
+
+  explicit StyleChildrenIterator(const nsIContent* aContent,
+                                 bool aStartAtBeginning = true)
+      : AllChildrenIterator(
+            aContent,
+            nsIContent::eAllChildren |
+                nsIContent::eSkipDocumentLevelNativeAnonymousContent,
+            aStartAtBeginning) {
+    MOZ_COUNT_CTOR(StyleChildrenIterator);
+  }
+
+  StyleChildrenIterator(StyleChildrenIterator&& aOther)
+      : AllChildrenIterator(std::move(aOther)) {
+    MOZ_COUNT_CTOR(StyleChildrenIterator);
+  }
+
+  StyleChildrenIterator& operator=(StyleChildrenIterator&& aOther) = default;
+
+  MOZ_COUNTED_DTOR(StyleChildrenIterator)
+
+  using AllChildrenIterator::GetNextChild;
+  using AllChildrenIterator::GetPreviousChild;
+  using AllChildrenIterator::Seek;
+};
+
+}  // namespace mozilla::dom
+
+#endif