servo/components/style/matching.rs

/* 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/. */

//! High-level interface to CSS selector matching.

#![allow(unsafe_code)]
#![deny(missing_docs)]

use {Atom, LocalName};
use animation::{self, Animation, PropertyAnimation};
use atomic_refcell::AtomicRefMut;
use cache::LRUCache;
use cascade_info::CascadeInfo;
use context::{SequentialTask, SharedStyleContext, StyleContext};
use data::{ComputedStyle, ElementData, ElementStyles, RestyleData};
use dom::{SendElement, TElement, TNode};
use properties::{CascadeFlags, ComputedValues, SHAREABLE, SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP, cascade};
use properties::longhands::display::computed_value as display;
use restyle_hints::{RESTYLE_STYLE_ATTRIBUTE, RestyleHint};
use rule_tree::{CascadeLevel, StrongRuleNode};
use selector_parser::{PseudoElement, RestyleDamage, SelectorImpl};
use selectors::MatchAttr;
use selectors::bloom::BloomFilter;
use selectors::matching::{ElementSelectorFlags, StyleRelations};
use selectors::matching::AFFECTED_BY_PSEUDO_ELEMENTS;
use servo_config::opts;
use sink::ForgetfulSink;
use std::collections::hash_map::Entry;
use std::slice::IterMut;
use std::sync::Arc;
use stylist::ApplicableDeclarationBlock;

/// Determines the amount of relations where we're going to share style.
#[inline]
fn relations_are_shareable(relations: &StyleRelations) -> bool {
    use selectors::matching::*;
    !relations.intersects(AFFECTED_BY_ID_SELECTOR |
                          AFFECTED_BY_PSEUDO_ELEMENTS | AFFECTED_BY_STATE |
                          AFFECTED_BY_STYLE_ATTRIBUTE |
                          AFFECTED_BY_PRESENTATIONAL_HINTS)
}

fn create_common_style_affecting_attributes_from_element<E: TElement>(element: &E)
                                                         -> CommonStyleAffectingAttributes {
    let mut flags = CommonStyleAffectingAttributes::empty();
    for attribute_info in &common_style_affecting_attributes() {
        match attribute_info.mode {
            CommonStyleAffectingAttributeMode::IsPresent(flag) => {
                if element.has_attr(&ns!(), &attribute_info.attr_name) {
                    flags.insert(flag)
                }
            }
            CommonStyleAffectingAttributeMode::IsEqual(ref target_value, flag) => {
                if element.attr_equals(&ns!(), &attribute_info.attr_name, target_value) {
                    flags.insert(flag)
                }
            }
        }
    }
    flags
}

/// The results returned from running selector matching on an element.
pub struct MatchResults {
    /// A set of style relations (different hints about what rules matched or
    /// could have matched). This is necessary if the style will be shared.
    /// If None, the style will not be shared.
    pub primary_relations: Option<StyleRelations>,
    /// Whether the rule nodes changed during selector matching.
    pub rule_nodes_changed: bool,
}

impl MatchResults {
    /// Returns true if the primary rule node is shareable with other nodes.
    pub fn primary_is_shareable(&self) -> bool {
        self.primary_relations.as_ref()
            .map_or(false, relations_are_shareable)
    }
}

/// Information regarding a style sharing candidate.
///
/// Note that this information is stored in TLS and cleared after the traversal,
/// and once here, the style information of the element is immutable, so it's
/// safe to access.
///
/// TODO: We can stick a lot more info here.
#[derive(Debug)]
struct StyleSharingCandidate<E: TElement> {
    /// The element. We use SendElement here so that the cache may live in
    /// ScopedTLS.
    element: SendElement<E>,
    /// The cached common style affecting attribute info.
    common_style_affecting_attributes: Option<CommonStyleAffectingAttributes>,
    /// The cached class names.
    class_attributes: Option<Vec<Atom>>,
}

impl<E: TElement> PartialEq<StyleSharingCandidate<E>> for StyleSharingCandidate<E> {
    fn eq(&self, other: &Self) -> bool {
        self.element == other.element &&
            self.common_style_affecting_attributes == other.common_style_affecting_attributes
    }
}

/// An LRU cache of the last few nodes seen, so that we can aggressively try to
/// reuse their styles.
///
/// Note that this cache is flushed every time we steal work from the queue, so
/// storing nodes here temporarily is safe.
pub struct StyleSharingCandidateCache<E: TElement> {
    cache: LRUCache<StyleSharingCandidate<E>, ()>,
}

/// A cache miss result.
#[derive(Clone, Debug)]
pub enum CacheMiss {
    /// The parents don't match.
    Parent,
    /// The local name of the element and the candidate don't match.
    LocalName,
    /// The namespace of the element and the candidate don't match.
    Namespace,
    /// One of the element or the candidate was a link, but the other one
    /// wasn't.
    Link,
    /// The element and the candidate match different kind of rules. This can
    /// only happen in Gecko.
    UserAndAuthorRules,
    /// The element and the candidate are in a different state.
    State,
    /// The element had an id attribute, which qualifies for a unique style.
    IdAttr,
    /// The element had a style attribute, which qualifies for a unique style.
    StyleAttr,
    /// The element and the candidate class names didn't match.
    Class,
    /// The element and the candidate common style affecting attributes didn't
    /// match.
    CommonStyleAffectingAttributes,
    /// The presentation hints didn't match.
    PresHints,
    /// The element and the candidate didn't match the same set of
    /// sibling-affecting rules.
    SiblingRules,
    /// The element and the candidate didn't match the same set of non-common
    /// style affecting attribute selectors.
    NonCommonAttrRules,
}

fn element_matches_candidate<E: TElement>(element: &E,
                                          candidate: &mut StyleSharingCandidate<E>,
                                          candidate_element: &E,
                                          shared_context: &SharedStyleContext)
                                          -> Result<ComputedStyle, CacheMiss> {
    macro_rules! miss {
        ($miss: ident) => {
            return Err(CacheMiss::$miss);
        }
    }

    if element.parent_element() != candidate_element.parent_element() {
        miss!(Parent)
    }

    if *element.get_local_name() != *candidate_element.get_local_name() {
        miss!(LocalName)
    }

    if *element.get_namespace() != *candidate_element.get_namespace() {
        miss!(Namespace)
    }

    if element.is_link() != candidate_element.is_link() {
        miss!(Link)
    }

    if element.matches_user_and_author_rules() != candidate_element.matches_user_and_author_rules() {
        miss!(UserAndAuthorRules)
    }

    if element.get_state() != candidate_element.get_state() {
        miss!(State)
    }

    if element.get_id().is_some() {
        miss!(IdAttr)
    }

    if element.style_attribute().is_some() {
        miss!(StyleAttr)
    }

    if !have_same_class(element, candidate, candidate_element) {
        miss!(Class)
    }

    if !have_same_common_style_affecting_attributes(element,
                                                    candidate,
                                                    candidate_element) {
        miss!(CommonStyleAffectingAttributes)
    }

    if !have_same_presentational_hints(element, candidate_element) {
        miss!(PresHints)
    }

    if !match_same_sibling_affecting_rules(element,
                                           candidate_element,
                                           shared_context) {
        miss!(SiblingRules)
    }

    if !match_same_not_common_style_affecting_attributes_rules(element,
                                                               candidate_element,
                                                               shared_context) {
        miss!(NonCommonAttrRules)
    }

    let data = candidate_element.borrow_data().unwrap();
    debug_assert!(data.has_current_styles());
    let current_styles = data.styles();

    Ok(current_styles.primary.clone())
}

fn have_same_common_style_affecting_attributes<E: TElement>(element: &E,
                                                            candidate: &mut StyleSharingCandidate<E>,
                                                            candidate_element: &E) -> bool {
    if candidate.common_style_affecting_attributes.is_none() {
        candidate.common_style_affecting_attributes =
            Some(create_common_style_affecting_attributes_from_element(candidate_element))
    }
    create_common_style_affecting_attributes_from_element(element) ==
        candidate.common_style_affecting_attributes.unwrap()
}

fn have_same_presentational_hints<E: TElement>(element: &E, candidate: &E) -> bool {
    let mut first = ForgetfulSink::new();
    element.synthesize_presentational_hints_for_legacy_attributes(&mut first);
    if cfg!(debug_assertions) {
        let mut second = vec![];
        candidate.synthesize_presentational_hints_for_legacy_attributes(&mut second);
        debug_assert!(second.is_empty(),
                      "Should never have inserted an element with preshints in the cache!");
    }

    first.is_empty()
}

bitflags! {
    /// A set of common style-affecting attributes we check separately to
    /// optimize the style sharing cache.
    pub flags CommonStyleAffectingAttributes: u8 {
        /// The `hidden` attribute.
        const HIDDEN_ATTRIBUTE = 0x01,
        /// The `nowrap` attribute.
        const NO_WRAP_ATTRIBUTE = 0x02,
        /// The `align="left"` attribute.
        const ALIGN_LEFT_ATTRIBUTE = 0x04,
        /// The `align="center"` attribute.
        const ALIGN_CENTER_ATTRIBUTE = 0x08,
        /// The `align="right"` attribute.
        const ALIGN_RIGHT_ATTRIBUTE = 0x10,
    }
}

/// The information of how to match a given common-style affecting attribute.
pub struct CommonStyleAffectingAttributeInfo {
    /// The attribute name.
    pub attr_name: LocalName,
    /// The matching mode for the attribute.
    pub mode: CommonStyleAffectingAttributeMode,
}

/// How should we match a given common style-affecting attribute?
#[derive(Clone)]
pub enum CommonStyleAffectingAttributeMode {
    /// Just for presence?
    IsPresent(CommonStyleAffectingAttributes),
    /// For presence and equality with a given value.
    IsEqual(Atom, CommonStyleAffectingAttributes),
}

/// The common style affecting attribute array.
///
/// TODO: This should be a `const static` or similar, but couldn't be because
/// `Atom`s have destructors.
#[inline]
pub fn common_style_affecting_attributes() -> [CommonStyleAffectingAttributeInfo; 5] {
    [
        CommonStyleAffectingAttributeInfo {
            attr_name: local_name!("hidden"),
            mode: CommonStyleAffectingAttributeMode::IsPresent(HIDDEN_ATTRIBUTE),
        },
        CommonStyleAffectingAttributeInfo {
            attr_name: local_name!("nowrap"),
            mode: CommonStyleAffectingAttributeMode::IsPresent(NO_WRAP_ATTRIBUTE),
        },
        CommonStyleAffectingAttributeInfo {
            attr_name: local_name!("align"),
            mode: CommonStyleAffectingAttributeMode::IsEqual(atom!("left"), ALIGN_LEFT_ATTRIBUTE),
        },
        CommonStyleAffectingAttributeInfo {
            attr_name: local_name!("align"),
            mode: CommonStyleAffectingAttributeMode::IsEqual(atom!("center"), ALIGN_CENTER_ATTRIBUTE),
        },
        CommonStyleAffectingAttributeInfo {
            attr_name: local_name!("align"),
            mode: CommonStyleAffectingAttributeMode::IsEqual(atom!("right"), ALIGN_RIGHT_ATTRIBUTE),
        }
    ]
}

/// Attributes that, if present, disable style sharing. All legacy HTML
/// attributes must be in either this list or
/// `common_style_affecting_attributes`. See the comment in
/// `synthesize_presentational_hints_for_legacy_attributes`.
///
/// TODO(emilio): This is not accurate now, we don't disable style sharing for
/// this now since we check for attribute selectors in the stylesheet. Consider
/// removing this.
pub fn rare_style_affecting_attributes() -> [LocalName; 4] {
    [local_name!("bgcolor"), local_name!("border"), local_name!("colspan"), local_name!("rowspan")]
}

fn have_same_class<E: TElement>(element: &E,
                                candidate: &mut StyleSharingCandidate<E>,
                                candidate_element: &E) -> bool {
    // XXX Efficiency here, I'm only validating ideas.
    let mut element_class_attributes = vec![];
    element.each_class(|c| element_class_attributes.push(c.clone()));

    if candidate.class_attributes.is_none() {
        let mut attrs = vec![];
        candidate_element.each_class(|c| attrs.push(c.clone()));
        candidate.class_attributes = Some(attrs)
    }

    element_class_attributes == *candidate.class_attributes.as_ref().unwrap()
}

// TODO: These re-match the candidate every time, which is suboptimal.
#[inline]
fn match_same_not_common_style_affecting_attributes_rules<E: TElement>(element: &E,
                                                                       candidate: &E,
                                                                       ctx: &SharedStyleContext) -> bool {
    ctx.stylist.match_same_not_common_style_affecting_attributes_rules(element, candidate)
}

#[inline]
fn match_same_sibling_affecting_rules<E: TElement>(element: &E,
                                                   candidate: &E,
                                                   ctx: &SharedStyleContext) -> bool {
    ctx.stylist.match_same_sibling_affecting_rules(element, candidate)
}

static STYLE_SHARING_CANDIDATE_CACHE_SIZE: usize = 8;

impl<E: TElement> StyleSharingCandidateCache<E> {
    /// Create a new style sharing candidate cache.
    pub fn new() -> Self {
        StyleSharingCandidateCache {
            cache: LRUCache::new(STYLE_SHARING_CANDIDATE_CACHE_SIZE),
        }
    }

    fn iter_mut(&mut self) -> IterMut<(StyleSharingCandidate<E>, ())> {
        self.cache.iter_mut()
    }

    /// Tries to insert an element in the style sharing cache.
    ///
    /// Fails if we know it should never be in the cache.
    pub fn insert_if_possible(&mut self,
                              element: &E,
                              style: &Arc<ComputedValues>,
                              relations: StyleRelations) {
        let parent = match element.parent_element() {
            Some(element) => element,
            None => {
                debug!("Failing to insert to the cache: no parent element");
                return;
            }
        };

        // These are things we don't check in the candidate match because they
        // are either uncommon or expensive.
        if !relations_are_shareable(&relations) {
            debug!("Failing to insert to the cache: {:?}", relations);
            return;
        }

        let box_style = style.get_box();
        if box_style.transition_property_count() > 0 {
            debug!("Failing to insert to the cache: transitions");
            return;
        }

        if box_style.specifies_animations() {
            debug!("Failing to insert to the cache: animations");
            return;
        }

        debug!("Inserting into cache: {:?} with parent {:?}",
               element, parent);

        self.cache.insert(StyleSharingCandidate {
            element: unsafe { SendElement::new(*element) },
            common_style_affecting_attributes: None,
            class_attributes: None,
        }, ());
    }

    /// Touch a given index in the style sharing candidate cache.
    pub fn touch(&mut self, index: usize) {
        self.cache.touch(index);
    }

    /// Clear the style sharing candidate cache.
    pub fn clear(&mut self) {
        self.cache.evict_all()
    }
}

/// The results of attempting to share a style.
pub enum StyleSharingResult {
    /// We didn't find anybody to share the style with.
    CannotShare,
    /// The node's style can be shared. The integer specifies the index in the
    /// LRU cache that was hit and the damage that was done.
    StyleWasShared(usize),
}

/// Callers need to pass several boolean flags to cascade_primary_or_pseudo.
/// We encapsulate them in this struct to avoid mixing them up.
///
/// FIXME(pcwalton): Unify with `CascadeFlags`, perhaps?
struct CascadeBooleans {
    shareable: bool,
    animate: bool,
}

trait PrivateMatchMethods: TElement {
    /// Returns the closest parent element that doesn't have a display: contents
    /// style (and thus generates a box).
    ///
    /// This is needed to correctly handle blockification of flex and grid
    /// items.
    ///
    /// Returns itself if the element has no parent. In practice this doesn't
    /// happen because the root element is blockified per spec, but it could
    /// happen if we decide to not blockify for roots of disconnected subtrees,
    /// which is a kind of dubious beahavior.
    fn layout_parent(&self) -> Self {
        let mut current = self.clone();
        loop {
            current = match current.parent_element() {
                Some(el) => el,
                None => return current,
            };

            let is_display_contents =
                current.borrow_data().unwrap().styles().primary.values().is_display_contents();

            if !is_display_contents {
                return current;
            }
        }
    }

    fn cascade_internal(&self,
                        context: &StyleContext<Self>,
                        primary_style: &ComputedStyle,
                        pseudo_style: &mut Option<(&PseudoElement, &mut ComputedStyle)>,
                        booleans: &CascadeBooleans)
                        -> Arc<ComputedValues> {
        let shared_context = context.shared;
        let mut cascade_info = CascadeInfo::new();
        let mut cascade_flags = CascadeFlags::empty();
        if booleans.shareable {
            cascade_flags.insert(SHAREABLE)
        }
        if self.skip_root_and_item_based_display_fixup() {
            cascade_flags.insert(SKIP_ROOT_AND_ITEM_BASED_DISPLAY_FIXUP)
        }

        // Grab the rule node.
        let rule_node = &pseudo_style.as_ref().map_or(primary_style, |p| &*p.1).rules;

        // Grab the inherited values.
        let parent_el;
        let parent_data;
        let inherited_values_ = if pseudo_style.is_none() {
            parent_el = self.parent_element();
            parent_data = parent_el.as_ref().and_then(|e| e.borrow_data());
            let parent_values = parent_data.as_ref().map(|d| {
                // Sometimes Gecko eagerly styles things without processing
                // pending restyles first. In general we'd like to avoid this,
                // but there can be good reasons (for example, needing to
                // construct a frame for some small piece of newly-added
                // content in order to do something specific with that frame,
                // but not wanting to flush all of layout).
                debug_assert!(cfg!(feature = "gecko") || d.has_current_styles());
                d.styles().primary.values()
            });

            parent_values
        } else {
            parent_el = Some(self.clone());
            Some(primary_style.values())
        };

        let mut layout_parent_el = parent_el.clone();
        let layout_parent_data;
        let mut layout_parent_style = inherited_values_;
        if inherited_values_.map_or(false, |s| s.is_display_contents()) {
            layout_parent_el = Some(layout_parent_el.unwrap().layout_parent());
            layout_parent_data = layout_parent_el.as_ref().unwrap().borrow_data().unwrap();
            layout_parent_style = Some(layout_parent_data.styles().primary.values())
        }

        let inherited_values = inherited_values_.map(|x| &**x);
        let layout_parent_style = layout_parent_style.map(|x| &**x);

        // Propagate the "can be fragmented" bit. It would be nice to
        // encapsulate this better.
        //
        // Note that this is not needed for pseudos since we already do that
        // when we resolve the non-pseudo style.
        if pseudo_style.is_none() {
            if let Some(ref p) = layout_parent_style {
                let can_be_fragmented =
                    p.is_multicol() ||
                    layout_parent_el.as_ref().unwrap().as_node().can_be_fragmented();
                unsafe { self.as_node().set_can_be_fragmented(can_be_fragmented); }
            }
        }

        // Invoke the cascade algorithm.
        let values =
            Arc::new(cascade(shared_context.viewport_size,
                             rule_node,
                             inherited_values,
                             layout_parent_style,
                             &shared_context.default_computed_values,
                             Some(&mut cascade_info),
                             shared_context.error_reporter.clone(),
                             cascade_flags));

        cascade_info.finish(&self.as_node());
        values
    }

    /// Computes values and damage for the primary or pseudo style of an element,
    /// setting them on the ElementData.
    fn cascade_primary_or_pseudo<'a>(&self,
                                     context: &StyleContext<Self>,
                                     data: &mut ElementData,
                                     pseudo: Option<&PseudoElement>,
                                     possibly_expired_animations: &mut Vec<PropertyAnimation>,
                                     booleans: CascadeBooleans) {
        // Collect some values.
        let shared_context = context.shared;
        let (mut styles, restyle) = data.styles_and_restyle_mut();
        let mut primary_style = &mut styles.primary;
        let pseudos = &mut styles.pseudos;
        let mut pseudo_style = pseudo.map(|p| (p, pseudos.get_mut(p).unwrap()));
        let mut old_values =
            pseudo_style.as_mut().map_or_else(|| primary_style.values.take(), |p| p.1.values.take());

        // Compute the new values.
        let mut new_values = self.cascade_internal(context, primary_style,
                                                   &mut pseudo_style, &booleans);

        // Handle animations.
        if booleans.animate && cfg!(feature = "servo") {
            if let Some(ref mut old) = old_values {
                self.update_animations_for_cascade(shared_context, old,
                                                   possibly_expired_animations);
            }

            let new_animations_sender = &context.thread_local.new_animations_sender;
            let this_opaque = self.as_node().opaque();
            // Trigger any present animations if necessary.
            animation::maybe_start_animations(&shared_context,
                                              new_animations_sender,
                                              this_opaque, &new_values);

            // Trigger transitions if necessary. This will reset `new_values` back
            // to its old value if it did trigger a transition.
            if let Some(ref values) = old_values {
                animation::start_transitions_if_applicable(
                    new_animations_sender,
                    this_opaque,
                    self.as_node().to_unsafe(),
                    &**values,
                    &mut new_values,
                    &shared_context.timer,
                    &possibly_expired_animations);
            }
        }

        // Accumulate restyle damage.
        if let Some(old) = old_values {
            self.accumulate_damage(restyle.unwrap(), &old, &new_values, pseudo);
        }

        // Set the new computed values.
        if let Some((_, ref mut style)) = pseudo_style {
            style.values = Some(new_values);
        } else {
            primary_style.values = Some(new_values);
        }
    }

    /// Computes and applies non-redundant damage.
    #[cfg(feature = "gecko")]
    fn accumulate_damage(&self,
                         restyle: &mut RestyleData,
                         old_values: &Arc<ComputedValues>,
                         new_values: &Arc<ComputedValues>,
                         pseudo: Option<&PseudoElement>) {
        // If an ancestor is already getting reconstructed by Gecko's top-down
        // frame constructor, no need to apply damage.
        if restyle.damage_handled.contains(RestyleDamage::reconstruct()) {
            restyle.damage = RestyleDamage::empty();
            return;
        }

        // Add restyle damage, but only the bits that aren't redundant with respect
        // to damage applied on our ancestors.
        //
        // See https://bugzilla.mozilla.org/show_bug.cgi?id=1301258#c12
        // for followup work to make the optimization here more optimal by considering
        // each bit individually.
        if !restyle.damage.contains(RestyleDamage::reconstruct()) {
            let new_damage = self.compute_restyle_damage(&old_values, &new_values, pseudo);
            if !restyle.damage_handled.contains(new_damage) {
                restyle.damage |= new_damage;
            }
        }
    }

    /// Computes and applies restyle damage unless we've already maxed it out.
    #[cfg(feature = "servo")]
    fn accumulate_damage(&self,
                         restyle: &mut RestyleData,
                         old_values: &Arc<ComputedValues>,
                         new_values: &Arc<ComputedValues>,
                         pseudo: Option<&PseudoElement>) {
        if restyle.damage != RestyleDamage::rebuild_and_reflow() {
            let d = self.compute_restyle_damage(&old_values, &new_values, pseudo);
            restyle.damage |= d;
        }
    }

    fn update_animations_for_cascade(&self,
                                     context: &SharedStyleContext,
                                     style: &mut Arc<ComputedValues>,
                                     possibly_expired_animations: &mut Vec<PropertyAnimation>) {
        // Finish any expired transitions.
        let this_opaque = self.as_node().opaque();
        animation::complete_expired_transitions(this_opaque, style, context);

        // Merge any running transitions into the current style, and cancel them.
        let had_running_animations = context.running_animations
                                            .read()
                                            .get(&this_opaque)
                                            .is_some();
        if had_running_animations {
            let mut all_running_animations = context.running_animations.write();
            for running_animation in all_running_animations.get_mut(&this_opaque).unwrap() {
                // This shouldn't happen frequently, but under some
                // circumstances mainly huge load or debug builds, the
                // constellation might be delayed in sending the
                // `TickAllAnimations` message to layout.
                //
                // Thus, we can't assume all the animations have been already
                // updated by layout, because other restyle due to script might
                // be triggered by layout before the animation tick.
                //
                // See #12171 and the associated PR for an example where this
                // happened while debugging other release panic.
                if !running_animation.is_expired() {
                    animation::update_style_for_animation(context,
                                                          running_animation,
                                                          style);
                    if let Animation::Transition(_, _, _, ref frame, _) = *running_animation {
                        possibly_expired_animations.push(frame.property_animation.clone())
                    }
                }
            }
        }
    }

    fn share_style_with_candidate_if_possible(&self,
                                              shared_context: &SharedStyleContext,
                                              candidate: &mut StyleSharingCandidate<Self>)
                                              -> Result<ComputedStyle, CacheMiss> {
        let candidate_element = *candidate.element;
        element_matches_candidate(self, candidate, &candidate_element, shared_context)
    }
}

fn compute_rule_node<E: TElement>(context: &StyleContext<E>,
                                  applicable_declarations: &mut Vec<ApplicableDeclarationBlock>)
                                  -> StrongRuleNode
{
    let rules = applicable_declarations.drain(..).map(|d| (d.source, d.level));
    let rule_node = context.shared.stylist.rule_tree.insert_ordered_rules(rules);
    rule_node
}

impl<E: TElement> PrivateMatchMethods for E {}

/// The public API that elements expose for selector matching.
pub trait MatchMethods : TElement {
    /// Runs selector matching to (re)compute rule nodes for this element.
    fn match_element(&self,
                     context: &mut StyleContext<Self>,
                     data: &mut ElementData)
                     -> MatchResults
    {
        let mut applicable_declarations =
            Vec::<ApplicableDeclarationBlock>::with_capacity(16);

        let stylist = &context.shared.stylist;
        let style_attribute = self.style_attribute();
        let animation_rules = self.get_animation_rules(None);
        let mut flags = ElementSelectorFlags::empty();
        let mut rule_nodes_changed = false;

        // Compute the primary rule node.
        let mut primary_relations =
            stylist.push_applicable_declarations(self,
                                                 Some(context.thread_local.bloom_filter.filter()),
                                                 style_attribute,
                                                 animation_rules,
                                                 None,
                                                 &mut applicable_declarations,
                                                 &mut flags);
        let primary_rule_node = compute_rule_node(context, &mut applicable_declarations);
        if !data.has_styles() {
            data.set_styles(ElementStyles::new(ComputedStyle::new_partial(primary_rule_node)));
            rule_nodes_changed = true;
        } else if data.styles().primary.rules != primary_rule_node {
            data.styles_mut().primary.rules = primary_rule_node;
            rule_nodes_changed = true;
        }

        // Compute rule nodes for eagerly-cascaded pseudo-elements.
        let mut matches_different_pseudos = false;
        SelectorImpl::each_eagerly_cascaded_pseudo_element(|pseudo| {
            let mut per_pseudo = &mut data.styles_mut().pseudos;
            debug_assert!(applicable_declarations.is_empty());
            let pseudo_animation_rules = self.get_animation_rules(Some(&pseudo));
            stylist.push_applicable_declarations(self,
                                                 Some(context.thread_local.bloom_filter.filter()),
                                                 None, pseudo_animation_rules,
                                                 Some(&pseudo),
                                                 &mut applicable_declarations,
                                                 &mut flags);

            if !applicable_declarations.is_empty() {
                let new_rules = compute_rule_node(context, &mut applicable_declarations);
                match per_pseudo.entry(pseudo) {
                    Entry::Occupied(mut e) => {
                        if e.get().rules != new_rules {
                            e.get_mut().rules = new_rules;
                            rule_nodes_changed = true;
                        }
                    },
                    Entry::Vacant(e) => {
                        e.insert(ComputedStyle::new_partial(new_rules));
                        matches_different_pseudos = true;
                    }
                }
            } else if per_pseudo.remove(&pseudo).is_some() {
                matches_different_pseudos = true;
            }
        });

        if matches_different_pseudos {
            rule_nodes_changed = true;
            if let Some(r) = data.get_restyle_mut() {
                // Any changes to the matched pseudo-elements trigger
                // reconstruction.
                r.damage |= RestyleDamage::reconstruct();
            }
        }

        // If we have any pseudo elements, indicate so in the primary StyleRelations.
        if !data.styles().pseudos.is_empty() {
            primary_relations |= AFFECTED_BY_PSEUDO_ELEMENTS;
        }

        // Apply the selector flags.
        let self_flags = flags.for_self();
        if !self_flags.is_empty() {
            unsafe { self.set_selector_flags(self_flags); }
        }
        let parent_flags = flags.for_parent();
        if !parent_flags.is_empty() {
            if let Some(p) = self.parent_element() {
                // Avoid the overhead of the SequentialTask if the flags are already set.
                if !p.has_selector_flags(parent_flags) {
                    let task = SequentialTask::set_selector_flags(p, parent_flags);
                    context.thread_local.tasks.push(task);
                }
            }
        }

        MatchResults {
            primary_relations: Some(primary_relations),
            rule_nodes_changed: rule_nodes_changed,
        }
    }

    /// Updates the rule nodes without re-running selector matching, using just
    /// the rule tree. Returns true if the rule nodes changed.
    fn cascade_with_replacements(&self,
                                 hint: RestyleHint,
                                 context: &StyleContext<Self>,
                                 data: &mut AtomicRefMut<ElementData>)
                                 -> bool {
        let primary_rules = &mut data.styles_mut().primary.rules;
        let mut rule_node_changed = false;

        if hint.contains(RESTYLE_STYLE_ATTRIBUTE) {
            let style_attribute = self.style_attribute();

            let new_node = context.shared.stylist.rule_tree
                .update_rule_at_level(CascadeLevel::StyleAttributeNormal,
                                      style_attribute,
                                      primary_rules);
            if let Some(n) = new_node {
                *primary_rules = n;
                rule_node_changed = true;
            }

            let new_node = context.shared.stylist.rule_tree
                .update_rule_at_level(CascadeLevel::StyleAttributeImportant,
                                      style_attribute,
                                      primary_rules);
            if let Some(n) = new_node {
                *primary_rules = n;
                rule_node_changed = true;
            }
        }

        // The per-pseudo rule nodes never change in this path.
        rule_node_changed
    }

    /// Attempts to share a style with another node. This method is unsafe
    /// because it depends on the `style_sharing_candidate_cache` having only
    /// live nodes in it, and we have no way to guarantee that at the type
    /// system level yet.
    unsafe fn share_style_if_possible(&self,
                                      style_sharing_candidate_cache:
                                        &mut StyleSharingCandidateCache<Self>,
                                      shared_context: &SharedStyleContext,
                                      data: &mut AtomicRefMut<ElementData>)
                                      -> StyleSharingResult {
        if opts::get().disable_share_style_cache {
            return StyleSharingResult::CannotShare
        }

        if self.parent_element().is_none() {
            return StyleSharingResult::CannotShare
        }

        if self.style_attribute().is_some() {
            return StyleSharingResult::CannotShare
        }

        if self.has_attr(&ns!(), &local_name!("id")) {
            return StyleSharingResult::CannotShare
        }

        let mut should_clear_cache = false;
        for (i, &mut (ref mut candidate, ())) in style_sharing_candidate_cache.iter_mut().enumerate() {
            let sharing_result = self.share_style_with_candidate_if_possible(shared_context, candidate);
            match sharing_result {
                Ok(shared_style) => {
                    // Yay, cache hit. Share the style.

                    // Accumulate restyle damage.
                    debug_assert_eq!(data.has_styles(), data.has_restyle());
                    let old_values = data.get_styles_mut()
                                         .and_then(|s| s.primary.values.take());
                    if let Some(old) = old_values {
                        self.accumulate_damage(data.restyle_mut(), &old,
                                               shared_style.values(), None);
                    }

                    // We never put elements with pseudo style into the style
                    // sharing cache, so we can just mint an ElementStyles
                    // directly here.
                    //
                    // See https://bugzilla.mozilla.org/show_bug.cgi?id=1329361
                    let styles = ElementStyles::new(shared_style);
                    data.set_styles(styles);

                    return StyleSharingResult::StyleWasShared(i)
                }
                Err(miss) => {
                    debug!("Cache miss: {:?}", miss);

                    // Cache miss, let's see what kind of failure to decide
                    // whether we keep trying or not.
                    match miss {
                        // Cache miss because of parent, clear the candidate cache.
                        CacheMiss::Parent => {
                            should_clear_cache = true;
                            break;
                        },
                        // Too expensive failure, give up, we don't want another
                        // one of these.
                        CacheMiss::CommonStyleAffectingAttributes |
                        CacheMiss::PresHints |
                        CacheMiss::SiblingRules |
                        CacheMiss::NonCommonAttrRules => break,
                        _ => {}
                    }
                }
            }
        }
        if should_clear_cache {
            style_sharing_candidate_cache.clear();
        }

        StyleSharingResult::CannotShare
    }

    // The below two functions are copy+paste because I can't figure out how to
    // write a function which takes a generic function. I don't think it can
    // be done.
    //
    // Ideally, I'd want something like:
    //
    //   > fn with_really_simple_selectors(&self, f: <H: Hash>|&H|);


    // In terms of `SimpleSelector`s, these two functions will insert and remove:
    //   - `SimpleSelector::LocalName`
    //   - `SimpleSelector::Namepace`
    //   - `SimpleSelector::ID`
    //   - `SimpleSelector::Class`

    /// Inserts and removes the matching `Descendant` selectors from a bloom
    /// filter. This is used to speed up CSS selector matching to remove
    /// unnecessary tree climbs for `Descendant` queries.
    ///
    /// A bloom filter of the local names, namespaces, IDs, and classes is kept.
    /// Therefore, each node must have its matching selectors inserted _after_
    /// its own selector matching and _before_ its children start.
    fn insert_into_bloom_filter(&self, bf: &mut BloomFilter) {
        bf.insert(&*self.get_local_name());
        bf.insert(&*self.get_namespace());
        self.get_id().map(|id| bf.insert(&id));

        // TODO: case-sensitivity depends on the document type and quirks mode
        self.each_class(|class| bf.insert(class));
    }

    /// After all the children are done css selector matching, this must be
    /// called to reset the bloom filter after an `insert`.
    fn remove_from_bloom_filter(&self, bf: &mut BloomFilter) {
        bf.remove(&*self.get_local_name());
        bf.remove(&*self.get_namespace());
        self.get_id().map(|id| bf.remove(&id));

        // TODO: case-sensitivity depends on the document type and quirks mode
        self.each_class(|class| bf.remove(class));
    }

    /// Given the old and new style of this element, and whether it's a
    /// pseudo-element, compute the restyle damage used to determine which
    /// kind of layout or painting operations we'll need.
    fn compute_restyle_damage(&self,
                              old_values: &Arc<ComputedValues>,
                              new_values: &Arc<ComputedValues>,
                              pseudo: Option<&PseudoElement>)
                              -> RestyleDamage
    {
        match self.existing_style_for_restyle_damage(old_values, pseudo) {
            Some(ref source) => RestyleDamage::compute(source, new_values),
            None => {
                // If there's no style source, that likely means that Gecko
                // couldn't find a style context. This happens with display:none
                // elements, and probably a number of other edge cases that
                // we don't handle well yet (like display:contents).
                if new_values.get_box().clone_display() == display::T::none &&
                    old_values.get_box().clone_display() == display::T::none {
                    // The style remains display:none. No need for damage.
                    RestyleDamage::empty()
                } else {
                    // Something else. Be conservative for now.
                    RestyleDamage::reconstruct()
                }
            }
        }
    }

    /// Run the CSS cascade and compute values for the element, potentially
    /// starting any new transitions or animations.
    fn cascade_element(&self,
                       context: &StyleContext<Self>,
                       mut data: &mut AtomicRefMut<ElementData>,
                       primary_is_shareable: bool)
    {
        let mut possibly_expired_animations = vec![];

        // Cascade the primary style.
        self.cascade_primary_or_pseudo(context, data, None,
                                       &mut possibly_expired_animations,
                                       CascadeBooleans {
                                           shareable: primary_is_shareable,
                                           animate: true,
                                       });

        // Check whether the primary style is display:none.
        let display_none = data.styles().primary.values().get_box().clone_display() ==
                           display::T::none;

        // Cascade each pseudo-element.
        //
        // Note that we've already set up the map of matching pseudo-elements
        // in match_element (and handled the damage implications of changing
        // which pseudos match), so now we can just iterate the map. This does
        // mean collecting the keys, so that the borrow checker will let us pass
        // the mutable |data| to the inner cascade function.
        let matched_pseudos: Vec<PseudoElement> =
            data.styles().pseudos.keys().cloned().collect();
        for pseudo in matched_pseudos {
            // If the new primary style is display:none, we don't need pseudo
            // styles, but we still need to clear any stale values.
            if display_none {
                data.styles_mut().pseudos.get_mut(&pseudo).unwrap().values = None;
                continue;
            }

            // Only ::before and ::after are animatable.
            let animate = <Self as MatchAttr>::Impl::pseudo_is_before_or_after(&pseudo);
            self.cascade_primary_or_pseudo(context, data, Some(&pseudo),
                                           &mut possibly_expired_animations,
                                           CascadeBooleans {
                                               shareable: false,
                                               animate: animate,
                                           });
        }
    }
}

impl<E: TElement> MatchMethods for E {}