ServoStyleSet.h

/* -*- 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 mozilla_ServoStyleSet_h
#define mozilla_ServoStyleSet_h

#include "mozilla/AlreadyAddRefed.h"
#include "mozilla/AnonymousContentKey.h"
#include "mozilla/AtomArray.h"
#include "mozilla/EnumeratedArray.h"
#include "mozilla/Maybe.h"
#include "mozilla/PostTraversalTask.h"
#include "mozilla/ServoBindingTypes.h"
#include "mozilla/ServoUtils.h"
#include "mozilla/dom/RustTypes.h"
#include "mozilla/UniquePtr.h"
#include "MainThreadUtils.h"
#include "nsCSSPseudoElements.h"
#include "nsCSSAnonBoxes.h"
#include "nsChangeHint.h"
#include "nsCoord.h"
#include "nsAtom.h"
#include "nsIMemoryReporter.h"
#include "nsTArray.h"
#include "nsSize.h"

namespace mozilla {
enum class MediaFeatureChangeReason : uint8_t;
enum class StylePageSizeOrientation : uint8_t;
enum class StyleRuleChangeKind : uint32_t;
enum class StyleRelativeSelectorNthEdgeInvalidateFor : uint8_t;
struct StyleRuleChange;

class ErrorResult;

template <typename Integer, typename Number, typename LinearStops>
struct StyleTimingFunction;
struct StylePagePseudoClassFlags;
struct StylePiecewiseLinearFunction;
using StyleComputedTimingFunction =
    StyleTimingFunction<int32_t, float, StylePiecewiseLinearFunction>;

namespace css {
class Rule;
}  // namespace css
namespace dom {
class CSSImportRule;
class Element;
class ShadowRoot;
struct PropertyDefinition;
}  // namespace dom
namespace gfx {
class FontPaletteValueSet;
}  // namespace gfx
class StyleSheet;
struct Keyframe;
class ServoElementSnapshotTable;
class ComputedStyle;
class ServoStyleRuleMap;
class StyleSheet;
}  // namespace mozilla
class gfxFontFeatureValueSet;
class nsIContent;

class nsPresContext;
class nsWindowSizes;
struct TreeMatchContext;

namespace mozilla {

// A few flags used to track which kind of stylist state we may need to
// update.
enum class StylistState : uint8_t {
  // The stylist is not dirty, we should do nothing.
  NotDirty = 0,

  // The style sheets have changed, so we need to update the style data.
  StyleSheetsDirty = 1 << 0,

  // Some of the style sheets of the shadow trees in the document have
  // changed.
  ShadowDOMStyleSheetsDirty = 1 << 1,
};

MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(StylistState)

enum class StyleOrigin : uint8_t;

// Bitfield type to represent Servo stylesheet origins.
enum class OriginFlags : uint8_t {
  UserAgent = 0x01,
  User = 0x02,
  Author = 0x04,
  All = 0x07,
};

MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(OriginFlags)

/**
 * The set of style sheets that apply to a document, backed by a Servo
 * Stylist.  A ServoStyleSet contains StyleSheets.
 */
class ServoStyleSet {
  friend class RestyleManager;
  using SnapshotTable = ServoElementSnapshotTable;
  using Origin = StyleOrigin;

  // We assert that these match the Servo ones in the definition of this array.
  static constexpr Origin kOrigins[] = {
      Origin(static_cast<uint8_t>(OriginFlags::UserAgent)),
      Origin(static_cast<uint8_t>(OriginFlags::User)),
      Origin(static_cast<uint8_t>(OriginFlags::Author)),
  };

 public:
  static bool IsInServoTraversal() { return mozilla::IsInServoTraversal(); }

#ifdef DEBUG
  // Used for debug assertions. We make this debug-only to prevent callers from
  // accidentally using it instead of IsInServoTraversal, which is cheaper. We
  // can change this if a use-case arises.
  static bool IsCurrentThreadInServoTraversal();
#endif

  static ServoStyleSet* Current() { return sInServoTraversal; }

  explicit ServoStyleSet(dom::Document&);
  ~ServoStyleSet();

  void ShellDetachedFromDocument();

  // Called when a rules in a stylesheet in this set, or a child sheet of that,
  // are mutated from CSSOM.
  void RuleAdded(StyleSheet&, css::Rule&);
  void RuleRemoved(StyleSheet&, css::Rule&);
  void RuleChanged(StyleSheet&, css::Rule*, const StyleRuleChange&);
  void SheetCloned(StyleSheet&);
  void ImportRuleLoaded(StyleSheet&);

  // Runs style invalidation due to document state changes.
  void InvalidateStyleForDocumentStateChanges(
      dom::DocumentState aStatesChanged);

  void RecordShadowStyleChange(dom::ShadowRoot&);

  bool StyleSheetsHaveChanged() const { return StylistNeedsUpdate(); }

  RestyleHint MediumFeaturesChanged(MediaFeatureChangeReason);

  // Evaluates a given SourceSizeList, returning the optimal viewport width in
  // app units.
  //
  // The SourceSizeList parameter can be null, in which case it will return
  // 100vw.
  inline nscoord EvaluateSourceSizeList(
      const StyleSourceSizeList* aSourceSizeList) const;

  void AddSizeOfIncludingThis(nsWindowSizes& aSizes) const;
  const StylePerDocumentStyleData* RawData() const { return mRawData.get(); }

  bool GetAuthorStyleDisabled() const { return mAuthorStyleDisabled; }

  bool UsesFontMetrics() const;

  void SetAuthorStyleDisabled(bool aStyleDisabled);

  // Get a CopmutedStyle for a text node (which no rules will match).
  //
  // The returned ComputedStyle will have nsCSSAnonBoxes::mozText() as its
  // pseudo.
  //
  // (Perhaps mozText should go away and we shouldn't even create style
  // contexts for such content nodes, when text-combine-upright is not
  // present.  However, not doing any rule matching for them is a first step.)
  already_AddRefed<ComputedStyle> ResolveStyleForText(
      nsIContent* aTextNode, ComputedStyle* aParentStyle);

  // Get a ComputedStyle for a first-letter continuation (which no rules will
  // match).
  //
  // The returned ComputedStyle will have
  // nsCSSAnonBoxes::firstLetterContinuation() as its pseudo.
  //
  // (Perhaps nsCSSAnonBoxes::firstLetterContinuation() should go away and we
  // shouldn't even create ComputedStyles for such frames.  However, not doing
  // any rule matching for them is a first step.  And right now we do use this
  // ComputedStyle for some things)
  already_AddRefed<ComputedStyle> ResolveStyleForFirstLetterContinuation(
      ComputedStyle* aParentStyle);

  // Get a ComputedStyle for a placeholder frame (which no rules will match).
  //
  // The returned ComputedStyle will have nsCSSAnonBoxes::oofPlaceholder() as
  // its pseudo.
  //
  // (Perhaps nsCSSAnonBoxes::oofPaceholder() should go away and we shouldn't
  // even create ComputedStyle for placeholders.  However, not doing any rule
  // matching for them is a first step.)
  already_AddRefed<ComputedStyle> ResolveStyleForPlaceholder();

  // Returns whether a given pseudo-element should exist or not.
  static bool GeneratedContentPseudoExists(const ComputedStyle& aParentStyle,
                                           const ComputedStyle& aPseudoStyle);

  enum class IsProbe {
    No,
    Yes,
  };

  // Get a style for a pseudo-element.
  //
  // If IsProbe is Yes, then no style is returned if there are no rules matching
  // for the pseudo-element, or GeneratedContentPseudoExists returns false.
  //
  // If IsProbe is No, then the style is guaranteed to be non-null.
  already_AddRefed<ComputedStyle> ResolvePseudoElementStyle(
      const dom::Element& aOriginatingElement, PseudoStyleType,
      nsAtom* aFunctionalPseudoParameter, ComputedStyle* aParentStyle,
      IsProbe = IsProbe::No);

  already_AddRefed<ComputedStyle> ProbePseudoElementStyle(
      const dom::Element& aOriginatingElement, PseudoStyleType aType,
      nsAtom* aFunctionalPseudoParameter, ComputedStyle* aParentStyle) {
    return ResolvePseudoElementStyle(aOriginatingElement, aType,
                                     aFunctionalPseudoParameter, aParentStyle,
                                     IsProbe::Yes);
  }

  // Resolves style for a (possibly-pseudo) Element without assuming that the
  // style has been resolved. If the element was unstyled and a new style
  // was resolved, it is not stored in the DOM. (That is, the element remains
  // unstyled.)
  already_AddRefed<ComputedStyle> ResolveStyleLazily(
      const dom::Element&, const PseudoStyleRequest& aPseudoRequest = {},
      StyleRuleInclusion = StyleRuleInclusion::All);

  // Get a ComputedStyle for an anonymous box. The pseudo type must be an
  // inheriting anon box.
  already_AddRefed<ComputedStyle> ResolveInheritingAnonymousBoxStyle(
      PseudoStyleType, ComputedStyle* aParentStyle);

  // Get a ComputedStyle for an anonymous box. The pseudo type must be
  // a non-inheriting anon box, and must not be page-content.
  // See ResolvePageContentStyle for resolving page-content style.
  already_AddRefed<ComputedStyle> ResolveNonInheritingAnonymousBoxStyle(
      PseudoStyleType aType);

  // Get a ComputedStyle for a pageContent box with the specified page-name.
  // A page name that is null or the empty atom and has no pseudo classes gets
  // the global page style.
  already_AddRefed<ComputedStyle> ResolvePageContentStyle(
      const nsAtom* aPageName, const StylePagePseudoClassFlags& aPseudo);

  already_AddRefed<ComputedStyle> ResolveXULTreePseudoStyle(
      dom::Element* aParentElement, nsCSSAnonBoxPseudoStaticAtom* aPseudoTag,
      ComputedStyle* aParentStyle, const AtomArray& aInputWord);

  // Try to resolve the staring style for a given element. Please call this
  // function after checking if it may have rules inside @starting-style.
  already_AddRefed<ComputedStyle> ResolveStartingStyle(dom::Element& aElement);

  size_t SheetCount(Origin) const;
  StyleSheet* SheetAt(Origin, size_t aIndex) const;

  struct PageSizeAndOrientation {
    Maybe<StylePageSizeOrientation> orientation;
    Maybe<nsSize> size;
  };
  // Gets the default page size and orientation (the size/orientation specified
  // by @page rules without a selector list), if any.
  //
  // If the specified size is just an orientation, then the size will be set to
  // nothing and the orientation will be set accordingly.
  // If the specified size is auto or square, then the orientation will be set
  // to nothing.
  // Otherwise, the size will and orientation is determined by the specified
  // page size.
  PageSizeAndOrientation GetDefaultPageSizeAndOrientation();

  void AppendAllNonDocumentAuthorSheets(nsTArray<StyleSheet*>& aArray) const;

  // Manage the set of style sheets in the style set
  void AppendStyleSheet(StyleSheet&);
  void InsertStyleSheetBefore(StyleSheet&, StyleSheet& aReferenceSheet);
  void RemoveStyleSheet(StyleSheet&);
  void AddDocStyleSheet(StyleSheet&);

  /**
   * Performs a Servo traversal to compute style for all dirty nodes in the
   * document.
   *
   * This will traverse all of the document's style roots (that is, its document
   * element, and the roots of the document-level native anonymous content).
   *
   * We specify |ForCSSRuleChanges| to try to update all CSS animations
   * when we call this function due to CSS rule changes since @keyframes rules
   * may have changed.
   *
   * Returns true if a post-traversal is required.
   */
  bool StyleDocument(ServoTraversalFlags aFlags);

  /**
   * Eagerly styles a subtree of unstyled nodes that was just appended to the
   * tree. This is used in situations where we need the style immediately and
   * cannot wait for a future batch restyle.
   */
  void StyleNewSubtree(dom::Element* aRoot);

  /**
   * Helper for correctly calling UpdateStylist without paying the cost of an
   * extra function call in the common no-rebuild-needed case.
   */
  void UpdateStylistIfNeeded() {
    if (StylistNeedsUpdate()) {
      UpdateStylist();
    }
  }

  /**
   * Checks whether the rule tree has crossed its threshold for unused nodes,
   * and if so, frees them.
   */
  void MaybeGCRuleTree();

  /**
   * Returns true if the given element may be used as the root of a style
   * traversal. Reasons for false include having an unstyled parent, or having
   * a parent that is display:none.
   *
   * Most traversal callsites don't need to check this, but some do.
   */
  static bool MayTraverseFrom(const dom::Element* aElement);

#ifdef DEBUG
  void AssertTreeIsClean();
#else
  void AssertTreeIsClean() {}
#endif

  /**
   * Clears any cached style data that may depend on all sorts of computed
   * values.
   *
   * Right now this clears the non-inheriting ComputedStyle cache, resets the
   * default computed values, and clears cached anonymous content style.
   *
   * This does _not_, however, clear the stylist.
   */
  void ClearCachedStyleData();

  /**
   * Notifies the Servo stylesheet that the document's compatibility mode has
   * changed.
   */
  void CompatibilityModeChanged();

  template <typename T>
  void EnumerateStyleSheets(T aCb) {
    for (auto origin : kOrigins) {
      for (size_t i = 0, count = SheetCount(origin); i < count; ++i) {
        aCb(*SheetAt(origin, i));
      }
    }
  }

  /**
   * Resolve style for the given element, and return it as a
   * ComputedStyle.
   *
   * FIXME(emilio): Is there a point in this after bug 1367904?
   */
  static inline already_AddRefed<ComputedStyle> ResolveServoStyle(
      const dom::Element&);

  bool GetKeyframesForName(const dom::Element&, const ComputedStyle&,
                           nsAtom* aName,
                           const StyleComputedTimingFunction& aTimingFunction,
                           nsTArray<Keyframe>& aKeyframes);

  nsTArray<ComputedKeyframeValues> GetComputedKeyframeValuesFor(
      const nsTArray<Keyframe>& aKeyframes, dom::Element* aElement,
      const PseudoStyleRequest& aPseudoRequest, const ComputedStyle* aStyle);

  void GetAnimationValues(
      StyleLockedDeclarationBlock* aDeclarations, dom::Element* aElement,
      const mozilla::ComputedStyle* aStyle,
      nsTArray<RefPtr<StyleAnimationValue>>& aAnimationValues);

  void AppendFontFaceRules(nsTArray<nsFontFaceRuleContainer>& aArray);

  const StyleLockedCounterStyleRule* CounterStyleRuleForName(nsAtom* aName);

  // Get all the currently-active font feature values set.
  already_AddRefed<gfxFontFeatureValueSet> BuildFontFeatureValueSet();

  // Get the set of all currently-active font-palette-values.
  already_AddRefed<gfx::FontPaletteValueSet> BuildFontPaletteValueSet();

  already_AddRefed<ComputedStyle> GetBaseContextForElement(
      dom::Element* aElement, const ComputedStyle* aStyle);

  /**
   * Resolve style for a given declaration block with/without the parent style.
   * If the parent style is not specified, the document default computed values
   * is used.
   */
  already_AddRefed<ComputedStyle> ResolveForDeclarations(
      const ComputedStyle* aParentOrNull,
      const StyleLockedDeclarationBlock* aDeclarations);

  already_AddRefed<StyleAnimationValue> ComputeAnimationValue(
      dom::Element* aElement, StyleLockedDeclarationBlock* aDeclaration,
      const mozilla::ComputedStyle* aStyle);

  void AppendTask(PostTraversalTask aTask) {
    MOZ_ASSERT(IsInServoTraversal());

    // We currently only use PostTraversalTasks while the Servo font metrics
    // mutex is locked.  If we need to use them in other situations during
    // a traversal, we should assert that we've taken appropriate
    // synchronization measures.
    AssertIsMainThreadOrServoFontMetricsLocked();

    mPostTraversalTasks.AppendElement(aTask);
  }

  // Returns true if a restyle of the document is needed due to cloning
  // sheet inners.
  bool EnsureUniqueInnerOnCSSSheets();

  // Returns the style rule map.
  ServoStyleRuleMap* StyleRuleMap();

  /**
   * Returns true if a modification to an an attribute with the specified
   * local name might require us to restyle the element.
   *
   * This function allows us to skip taking a an attribute snapshot when
   * the modified attribute doesn't appear in an attribute selector in
   * a style sheet.
   */
  bool MightHaveAttributeDependency(const dom::Element&,
                                    nsAtom* aAttribute) const;

  /**
   * Returns true if a modification to an attribute with the specified local
   * name might require us to restyle the element's siblings.
   */
  bool MightHaveNthOfAttributeDependency(const dom::Element&,
                                         nsAtom* aAttribute) const;

  /**
   * Returns true if a modification to a class might require us to restyle the
   * element's siblings.
   */
  bool MightHaveNthOfClassDependency(const dom::Element&);

  /**
   * Returns true if a modification to an ID might require us to restyle the
   * element's siblings.
   */
  bool MightHaveNthOfIDDependency(const dom::Element&, nsAtom* aOldID,
                                  nsAtom* aNewID) const;

  /**
   * Maybe invalidate if a modification to an ID might require us to restyle
   * the relative selector it refers to.
   */
  void MaybeInvalidateRelativeSelectorIDDependency(
      const dom::Element&, nsAtom* aOldID, nsAtom* aNewID,
      const ServoElementSnapshotTable& aSnapshots);

  /**
   * Maybe invalidate if a modification to an attribute with the specified local
   * name might require us to restyle the relative selector it refers to.
   */
  void MaybeInvalidateRelativeSelectorClassDependency(
      const dom::Element&, const ServoElementSnapshotTable& aSnapshots);

  /**
   * Maybe invalidate if a modification to a Custom State might require us to
   * restyle the relative selector it refers to.
   */
  void MaybeInvalidateRelativeSelectorCustomStateDependency(
      const dom::Element&, nsAtom* state,
      const ServoElementSnapshotTable& aSnapshots);

  /**
   * Maybe invalidate if a modification to an ID might require us to restyle
   * the relative selector it refers to.
   */
  void MaybeInvalidateRelativeSelectorAttributeDependency(
      const dom::Element&, nsAtom* aAttribute,
      const ServoElementSnapshotTable& aSnapshots);

  /**
   * Maybe invalidate if a change in event state on an element might require us
   * to restyle the relative selector it refers to.
   */
  void MaybeInvalidateRelativeSelectorStateDependency(
      const dom::Element&, dom::ElementState,
      const ServoElementSnapshotTable& aSnapshots);

  /**
   * Maybe invalidate if a change on an element that might be selected by :empty
   * that might require us to restyle the relative selector it refers to.
   */
  void MaybeInvalidateRelativeSelectorForEmptyDependency(const dom::Element&);

  /**
   * Maybe invalidate if a state change on an element that might be selected
   * by a selector that can only selector first/last child, that
   * might require us to restyle the relative selector it refers to.
   */
  void MaybeInvalidateRelativeSelectorForNthEdgeDependency(
      const dom::Element&, StyleRelativeSelectorNthEdgeInvalidateFor);

  /**
   * Maybe invalidate if a state change on an element that might be selected by
   * :nth-* (Or :nth-like) selectors that might require us to restyle the
   * relative selector it refers to.
   */
  void MaybeInvalidateRelativeSelectorForNthDependencyFromSibling(
      const dom::Element*, bool aForceRestyleSiblings);

  /**
   * Maybe invalidate if a DOM element insertion might require us to restyle
   * the relative selector to ancestors/previous siblings.
   */
  void MaybeInvalidateForElementInsertion(const dom::Element&);

  /**
   * Maybe invalidate if a series of nodes is appended, among which may
   * be element(s) that might require us to restyle the relative selector
   * to ancestors/previous siblings.
   */
  void MaybeInvalidateForElementAppend(const nsIContent&);

  /**
   * Maybe invalidate if a DOM element removal might require us to restyle
   * the relative selector to ancestors/previous siblings.
   */
  void MaybeInvalidateForElementRemove(const dom::Element& aElement);

  /**
   * Returns true if a change in event state on an element might require
   * us to restyle the element.
   *
   * This function allows us to skip taking a state snapshot when
   * the changed state isn't depended upon by any pseudo-class selectors
   * in a style sheet.
   */
  bool HasStateDependency(const dom::Element&, dom::ElementState) const;

  /**
   * Returns true if a change in event state on an element might require
   * us to restyle the element's siblings.
   */
  bool HasNthOfStateDependency(const dom::Element&, dom::ElementState) const;

  /**
   * Returns true if a change in Custom State on an element might require
   * us to restyle the element's siblings.
   */
  bool HasNthOfCustomStateDependency(const dom::Element&, nsAtom*) const;

  /**
   * Restyle this element's siblings in order to propagate any potential change
   * in :nth-child(of) styling.
   */
  void RestyleSiblingsForNthOf(const dom::Element&, uint32_t) const;

  /**
   * Returns true if a change in document state might require us to restyle the
   * document.
   */
  bool HasDocumentStateDependency(dom::DocumentState) const;

  /**
   * Get a new ComputedStyle that uses the same rules as the given ComputedStyle
   * but has a different parent.
   *
   * aElement is non-null if this is a ComputedStyle for a frame whose mContent
   * is an element and which has no pseudo on its ComputedStyle (so it's the
   * actual style for the element being passed).
   */
  already_AddRefed<ComputedStyle> ReparentComputedStyle(
      ComputedStyle* aComputedStyle, ComputedStyle* aNewParent,
      ComputedStyle* aNewLayoutParent, dom::Element* aElement);

  /**
   * Invalidate styles where there's any viewport units dependent style.
   */
  enum class OnlyDynamic : bool { No, Yes };
  void InvalidateForViewportUnits(OnlyDynamic);

 private:
  friend class AutoSetInServoTraversal;
  friend class AutoPrepareTraversal;
  friend class PostTraversalTask;

  bool ShouldTraverseInParallel() const;

  void RuleChangedInternal(StyleSheet&, css::Rule&, const StyleRuleChange&);

  /**
   * Forces all the ShadowRoot styles to be dirty.
   *
   * Only to be used for:
   *
   *  * Devtools (dealing with sheet cloning).
   *  * Compatibility-mode changes.
   *
   * Try to do something more incremental for other callers that are exposed to
   * the web.
   */
  void ForceDirtyAllShadowStyles();

  /**
   * Gets the pending snapshots to handle from the restyle manager.
   */
  const SnapshotTable& Snapshots();

  /**
   * Clear our cached mNonInheritingComputedStyles.
   *
   * We do this when we want to make sure those ComputedStyles won't live too
   * long (e.g. when rebuilding all style data or when shutting down the style
   * set).
   */
  void ClearNonInheritingComputedStyles();

  /**
   * Perform processes that we should do before traversing.
   *
   * When aRoot is null, the entire document is pre-traversed.  Otherwise,
   * only the subtree rooted at aRoot is pre-traversed.
   */
  void PreTraverse(ServoTraversalFlags aFlags, dom::Element* aRoot = nullptr);

  // Subset of the pre-traverse steps that involve syncing up data
  void PreTraverseSync();

  /**
   * Records that the contents of style sheets at the specified origin have
   * changed since the last.  Calling this will ensure that the Stylist
   * rebuilds its selector maps.
   */
  void MarkOriginsDirty(OriginFlags aChangedOrigins);

  /**
   * Note that the stylist needs a style flush due to style sheet changes.
   */
  void SetStylistStyleSheetsDirty();

  void SetStylistShadowDOMStyleSheetsDirty();

  bool StylistNeedsUpdate() const {
    return mStylistState != StylistState::NotDirty;
  }

  /**
   * Update the stylist as needed to ensure style data is up-to-date.
   *
   * This should only be called if StylistNeedsUpdate returns true.
   */
  void UpdateStylist();

  void RunPostTraversalTasks();

  void PrependSheetOfType(Origin, StyleSheet*);
  void AppendSheetOfType(Origin, StyleSheet*);
  void InsertSheetOfType(Origin, StyleSheet*, StyleSheet* aBeforeSheet);
  void RemoveSheetOfType(Origin, StyleSheet*);

  const nsPresContext* GetPresContext() const {
    return const_cast<ServoStyleSet*>(this)->GetPresContext();
  }

  /**
   * Return the associated pres context if we're the master style set and we
   * have an associated pres shell.
   */
  nsPresContext* GetPresContext();

  // The owner document of this style set. Never null, and always outlives the
  // StyleSet.
  dom::Document* mDocument;
  UniquePtr<StylePerDocumentStyleData> mRawData;

  // Map from raw Servo style rule to Gecko's wrapper object.
  // Constructed lazily when requested by devtools.
  UniquePtr<ServoStyleRuleMap> mStyleRuleMap;
  uint64_t mUserFontSetUpdateGeneration = 0;

  // Tasks to perform after a traversal, back on the main thread.
  //
  // These are similar to Servo's SequentialTasks, except that they are
  // posted by C++ code running on style worker threads.
  nsTArray<PostTraversalTask> mPostTraversalTasks;

  // Stores pointers to our cached ComputedStyles for non-inheriting anonymous
  // boxes.
  EnumeratedArray<nsCSSAnonBoxes::NonInheriting, RefPtr<ComputedStyle>,
                  size_t(nsCSSAnonBoxes::NonInheriting::_Count)>
      mNonInheritingComputedStyles;

 public:
  void PutCachedAnonymousContentStyles(
      AnonymousContentKey aKey, nsTArray<RefPtr<ComputedStyle>>&& aStyles) {
    auto index = static_cast<size_t>(aKey);

    MOZ_ASSERT(mCachedAnonymousContentStyles.Length() + aStyles.Length() < 256,
               "(index, length) pairs must be bigger");
    MOZ_ASSERT(mCachedAnonymousContentStyleIndexes[index].second == 0,
               "shouldn't need to overwrite existing cached styles");
    MOZ_ASSERT(!aStyles.IsEmpty(), "should have some styles to cache");

    mCachedAnonymousContentStyleIndexes[index] = std::make_pair(
        mCachedAnonymousContentStyles.Length(), aStyles.Length());
    mCachedAnonymousContentStyles.AppendElements(std::move(aStyles));
  }

  void GetCachedAnonymousContentStyles(
      AnonymousContentKey aKey, nsTArray<RefPtr<ComputedStyle>>& aStyles) {
    auto index = static_cast<size_t>(aKey);
    auto loc = mCachedAnonymousContentStyleIndexes[index];
    aStyles.AppendElements(mCachedAnonymousContentStyles.Elements() + loc.first,
                           loc.second);
  }

  void RegisterProperty(const dom::PropertyDefinition&, ErrorResult&);

 private:
  // Map of AnonymousContentKey values to an (index, length) pair pointing into
  // mCachedAnonymousContentStyles.
  //
  // We assert that the index and length values fit into uint8_ts.
  Array<std::pair<uint8_t, uint8_t>, 1 << sizeof(AnonymousContentKey) * 8>
      mCachedAnonymousContentStyleIndexes;

  // Stores cached ComputedStyles for certain native anonymous content.
  nsTArray<RefPtr<ComputedStyle>> mCachedAnonymousContentStyles;

  StylistState mStylistState = StylistState::NotDirty;
  bool mAuthorStyleDisabled = false;
  bool mNeedsRestyleAfterEnsureUniqueInner = false;
};

class UACacheReporter final : public nsIMemoryReporter {
  NS_DECL_ISUPPORTS
  NS_DECL_NSIMEMORYREPORTER

 private:
  ~UACacheReporter() = default;
};

}  // namespace mozilla

#endif  // mozilla_ServoStyleSet_h