Adding upstream version 110.0.1.upstream/110.0.1upstream

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

1 files changed, 694 insertions, 0 deletions

diff --git a/servo/components/style/context.rs b/servo/components/style/context.rs
new file mode 100644
index 0000000000..d6b4d2cc10
--- /dev/null
+++ b/servo/components/style/context.rs
@@ -0,0 +1,694 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+//! The context within which style is calculated.
+
+#[cfg(feature = "servo")]
+use crate::animation::DocumentAnimationSet;
+use crate::bloom::StyleBloom;
+use crate::computed_value_flags::ComputedValueFlags;
+use crate::data::{EagerPseudoStyles, ElementData};
+use crate::dom::{SendElement, TElement};
+#[cfg(feature = "gecko")]
+use crate::gecko_bindings::structs;
+use crate::parallel::{STACK_SAFETY_MARGIN_KB, STYLE_THREAD_STACK_SIZE_KB};
+use crate::properties::ComputedValues;
+#[cfg(feature = "servo")]
+use crate::properties::PropertyId;
+use crate::rule_cache::RuleCache;
+use crate::rule_tree::StrongRuleNode;
+use crate::selector_parser::{SnapshotMap, EAGER_PSEUDO_COUNT};
+use crate::shared_lock::StylesheetGuards;
+use crate::sharing::StyleSharingCache;
+use crate::stylist::Stylist;
+use crate::thread_state::{self, ThreadState};
+use crate::traversal::DomTraversal;
+use crate::traversal_flags::TraversalFlags;
+use app_units::Au;
+use euclid::default::Size2D;
+use euclid::Scale;
+#[cfg(feature = "servo")]
+use fxhash::FxHashMap;
+use selectors::NthIndexCache;
+#[cfg(feature = "gecko")]
+use servo_arc::Arc;
+#[cfg(feature = "servo")]
+use servo_atoms::Atom;
+use std::fmt;
+use std::ops;
+use style_traits::CSSPixel;
+use style_traits::DevicePixel;
+#[cfg(feature = "servo")]
+use style_traits::SpeculativePainter;
+use time;
+
+pub use selectors::matching::QuirksMode;
+
+/// A global options structure for the style system. We use this instead of
+/// opts to abstract across Gecko and Servo.
+#[derive(Clone)]
+pub struct StyleSystemOptions {
+    /// Whether the style sharing cache is disabled.
+    pub disable_style_sharing_cache: bool,
+    /// Whether we should dump statistics about the style system.
+    pub dump_style_statistics: bool,
+    /// The minimum number of elements that must be traversed to trigger a dump
+    /// of style statistics.
+    pub style_statistics_threshold: usize,
+}
+
+#[cfg(feature = "gecko")]
+fn get_env_bool(name: &str) -> bool {
+    use std::env;
+    match env::var(name) {
+        Ok(s) => !s.is_empty(),
+        Err(_) => false,
+    }
+}
+
+const DEFAULT_STATISTICS_THRESHOLD: usize = 50;
+
+#[cfg(feature = "gecko")]
+fn get_env_usize(name: &str) -> Option<usize> {
+    use std::env;
+    env::var(name).ok().map(|s| {
+        s.parse::<usize>()
+            .expect("Couldn't parse environmental variable as usize")
+    })
+}
+
+/// A global variable holding the state of
+/// `StyleSystemOptions::default().disable_style_sharing_cache`.
+/// See [#22854](https://github.com/servo/servo/issues/22854).
+#[cfg(feature = "servo")]
+pub static DEFAULT_DISABLE_STYLE_SHARING_CACHE: std::sync::atomic::AtomicBool =
+    std::sync::atomic::AtomicBool::new(false);
+
+/// A global variable holding the state of
+/// `StyleSystemOptions::default().dump_style_statistics`.
+/// See [#22854](https://github.com/servo/servo/issues/22854).
+#[cfg(feature = "servo")]
+pub static DEFAULT_DUMP_STYLE_STATISTICS: std::sync::atomic::AtomicBool =
+    std::sync::atomic::AtomicBool::new(false);
+
+impl Default for StyleSystemOptions {
+    #[cfg(feature = "servo")]
+    fn default() -> Self {
+        use std::sync::atomic::Ordering;
+
+        StyleSystemOptions {
+            disable_style_sharing_cache: DEFAULT_DISABLE_STYLE_SHARING_CACHE
+                .load(Ordering::Relaxed),
+            dump_style_statistics: DEFAULT_DUMP_STYLE_STATISTICS.load(Ordering::Relaxed),
+            style_statistics_threshold: DEFAULT_STATISTICS_THRESHOLD,
+        }
+    }
+
+    #[cfg(feature = "gecko")]
+    fn default() -> Self {
+        StyleSystemOptions {
+            disable_style_sharing_cache: get_env_bool("DISABLE_STYLE_SHARING_CACHE"),
+            dump_style_statistics: get_env_bool("DUMP_STYLE_STATISTICS"),
+            style_statistics_threshold: get_env_usize("STYLE_STATISTICS_THRESHOLD")
+                .unwrap_or(DEFAULT_STATISTICS_THRESHOLD),
+        }
+    }
+}
+
+/// A shared style context.
+///
+/// There's exactly one of these during a given restyle traversal, and it's
+/// shared among the worker threads.
+pub struct SharedStyleContext<'a> {
+    /// The CSS selector stylist.
+    pub stylist: &'a Stylist,
+
+    /// Whether visited styles are enabled.
+    ///
+    /// They may be disabled when Gecko's pref layout.css.visited_links_enabled
+    /// is false, or when in private browsing mode.
+    pub visited_styles_enabled: bool,
+
+    /// Configuration options.
+    pub options: StyleSystemOptions,
+
+    /// Guards for pre-acquired locks
+    pub guards: StylesheetGuards<'a>,
+
+    /// The current time for transitions and animations. This is needed to ensure
+    /// a consistent sampling time and also to adjust the time for testing.
+    pub current_time_for_animations: f64,
+
+    /// Flags controlling how we traverse the tree.
+    pub traversal_flags: TraversalFlags,
+
+    /// A map with our snapshots in order to handle restyle hints.
+    pub snapshot_map: &'a SnapshotMap,
+
+    /// The state of all animations for our styled elements.
+    #[cfg(feature = "servo")]
+    pub animations: DocumentAnimationSet,
+
+    /// Paint worklets
+    #[cfg(feature = "servo")]
+    pub registered_speculative_painters: &'a dyn RegisteredSpeculativePainters,
+}
+
+impl<'a> SharedStyleContext<'a> {
+    /// Return a suitable viewport size in order to be used for viewport units.
+    pub fn viewport_size(&self) -> Size2D<Au> {
+        self.stylist.device().au_viewport_size()
+    }
+
+    /// The device pixel ratio
+    pub fn device_pixel_ratio(&self) -> Scale<f32, CSSPixel, DevicePixel> {
+        self.stylist.device().device_pixel_ratio()
+    }
+
+    /// The quirks mode of the document.
+    pub fn quirks_mode(&self) -> QuirksMode {
+        self.stylist.quirks_mode()
+    }
+}
+
+/// The structure holds various intermediate inputs that are eventually used by
+/// by the cascade.
+///
+/// The matching and cascading process stores them in this format temporarily
+/// within the `CurrentElementInfo`. At the end of the cascade, they are folded
+/// down into the main `ComputedValues` to reduce memory usage per element while
+/// still remaining accessible.
+#[derive(Clone, Debug, Default)]
+pub struct CascadeInputs {
+    /// The rule node representing the ordered list of rules matched for this
+    /// node.
+    pub rules: Option<StrongRuleNode>,
+
+    /// The rule node representing the ordered list of rules matched for this
+    /// node if visited, only computed if there's a relevant link for this
+    /// element. A element's "relevant link" is the element being matched if it
+    /// is a link or the nearest ancestor link.
+    pub visited_rules: Option<StrongRuleNode>,
+
+    /// The set of flags from container queries that we need for invalidation.
+    pub flags: ComputedValueFlags,
+}
+
+impl CascadeInputs {
+    /// Construct inputs from previous cascade results, if any.
+    pub fn new_from_style(style: &ComputedValues) -> Self {
+        Self {
+            rules: style.rules.clone(),
+            visited_rules: style.visited_style().and_then(|v| v.rules.clone()),
+            flags: style.flags.for_cascade_inputs(),
+        }
+    }
+}
+
+/// A list of cascade inputs for eagerly-cascaded pseudo-elements.
+/// The list is stored inline.
+#[derive(Debug)]
+pub struct EagerPseudoCascadeInputs(Option<[Option<CascadeInputs>; EAGER_PSEUDO_COUNT]>);
+
+// Manually implement `Clone` here because the derived impl of `Clone` for
+// array types assumes the value inside is `Copy`.
+impl Clone for EagerPseudoCascadeInputs {
+    fn clone(&self) -> Self {
+        if self.0.is_none() {
+            return EagerPseudoCascadeInputs(None);
+        }
+        let self_inputs = self.0.as_ref().unwrap();
+        let mut inputs: [Option<CascadeInputs>; EAGER_PSEUDO_COUNT] = Default::default();
+        for i in 0..EAGER_PSEUDO_COUNT {
+            inputs[i] = self_inputs[i].clone();
+        }
+        EagerPseudoCascadeInputs(Some(inputs))
+    }
+}
+
+impl EagerPseudoCascadeInputs {
+    /// Construct inputs from previous cascade results, if any.
+    fn new_from_style(styles: &EagerPseudoStyles) -> Self {
+        EagerPseudoCascadeInputs(styles.as_optional_array().map(|styles| {
+            let mut inputs: [Option<CascadeInputs>; EAGER_PSEUDO_COUNT] = Default::default();
+            for i in 0..EAGER_PSEUDO_COUNT {
+                inputs[i] = styles[i].as_ref().map(|s| CascadeInputs::new_from_style(s));
+            }
+            inputs
+        }))
+    }
+
+    /// Returns the list of rules, if they exist.
+    pub fn into_array(self) -> Option<[Option<CascadeInputs>; EAGER_PSEUDO_COUNT]> {
+        self.0
+    }
+}
+
+/// The cascade inputs associated with a node, including those for any
+/// pseudo-elements.
+///
+/// The matching and cascading process stores them in this format temporarily
+/// within the `CurrentElementInfo`. At the end of the cascade, they are folded
+/// down into the main `ComputedValues` to reduce memory usage per element while
+/// still remaining accessible.
+#[derive(Clone, Debug)]
+pub struct ElementCascadeInputs {
+    /// The element's cascade inputs.
+    pub primary: CascadeInputs,
+    /// A list of the inputs for the element's eagerly-cascaded pseudo-elements.
+    pub pseudos: EagerPseudoCascadeInputs,
+}
+
+impl ElementCascadeInputs {
+    /// Construct inputs from previous cascade results, if any.
+    #[inline]
+    pub fn new_from_element_data(data: &ElementData) -> Self {
+        debug_assert!(data.has_styles());
+        ElementCascadeInputs {
+            primary: CascadeInputs::new_from_style(data.styles.primary()),
+            pseudos: EagerPseudoCascadeInputs::new_from_style(&data.styles.pseudos),
+        }
+    }
+}
+
+/// Statistics gathered during the traversal. We gather statistics on each
+/// thread and then combine them after the threads join via the Add
+/// implementation below.
+#[derive(AddAssign, Clone, Default)]
+pub struct PerThreadTraversalStatistics {
+    /// The total number of elements traversed.
+    pub elements_traversed: u32,
+    /// The number of elements where has_styles() went from false to true.
+    pub elements_styled: u32,
+    /// The number of elements for which we performed selector matching.
+    pub elements_matched: u32,
+    /// The number of cache hits from the StyleSharingCache.
+    pub styles_shared: u32,
+    /// The number of styles reused via rule node comparison from the
+    /// StyleSharingCache.
+    pub styles_reused: u32,
+}
+
+/// Statistics gathered during the traversal plus some information from
+/// other sources including stylist.
+#[derive(Default)]
+pub struct TraversalStatistics {
+    /// Aggregated statistics gathered during the traversal.
+    pub aggregated: PerThreadTraversalStatistics,
+    /// The number of selectors in the stylist.
+    pub selectors: u32,
+    /// The number of revalidation selectors.
+    pub revalidation_selectors: u32,
+    /// The number of state/attr dependencies in the dependency set.
+    pub dependency_selectors: u32,
+    /// The number of declarations in the stylist.
+    pub declarations: u32,
+    /// The number of times the stylist was rebuilt.
+    pub stylist_rebuilds: u32,
+    /// Time spent in the traversal, in milliseconds.
+    pub traversal_time_ms: f64,
+    /// Whether this was a parallel traversal.
+    pub is_parallel: bool,
+    /// Whether this is a "large" traversal.
+    pub is_large: bool,
+}
+
+/// Format the statistics in a way that the performance test harness understands.
+/// See https://bugzilla.mozilla.org/show_bug.cgi?id=1331856#c2
+impl fmt::Display for TraversalStatistics {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        debug_assert!(
+            self.traversal_time_ms != 0.0,
+            "should have set traversal time"
+        );
+        writeln!(f, "[PERF] perf block start")?;
+        writeln!(
+            f,
+            "[PERF],traversal,{}",
+            if self.is_parallel {
+                "parallel"
+            } else {
+                "sequential"
+            }
+        )?;
+        writeln!(
+            f,
+            "[PERF],elements_traversed,{}",
+            self.aggregated.elements_traversed
+        )?;
+        writeln!(
+            f,
+            "[PERF],elements_styled,{}",
+            self.aggregated.elements_styled
+        )?;
+        writeln!(
+            f,
+            "[PERF],elements_matched,{}",
+            self.aggregated.elements_matched
+        )?;
+        writeln!(f, "[PERF],styles_shared,{}", self.aggregated.styles_shared)?;
+        writeln!(f, "[PERF],styles_reused,{}", self.aggregated.styles_reused)?;
+        writeln!(f, "[PERF],selectors,{}", self.selectors)?;
+        writeln!(
+            f,
+            "[PERF],revalidation_selectors,{}",
+            self.revalidation_selectors
+        )?;
+        writeln!(
+            f,
+            "[PERF],dependency_selectors,{}",
+            self.dependency_selectors
+        )?;
+        writeln!(f, "[PERF],declarations,{}", self.declarations)?;
+        writeln!(f, "[PERF],stylist_rebuilds,{}", self.stylist_rebuilds)?;
+        writeln!(f, "[PERF],traversal_time_ms,{}", self.traversal_time_ms)?;
+        writeln!(f, "[PERF] perf block end")
+    }
+}
+
+impl TraversalStatistics {
+    /// Generate complete traversal statistics.
+    ///
+    /// The traversal time is computed given the start time in seconds.
+    pub fn new<E, D>(
+        aggregated: PerThreadTraversalStatistics,
+        traversal: &D,
+        parallel: bool,
+        start: f64,
+    ) -> TraversalStatistics
+    where
+        E: TElement,
+        D: DomTraversal<E>,
+    {
+        let threshold = traversal
+            .shared_context()
+            .options
+            .style_statistics_threshold;
+        let stylist = traversal.shared_context().stylist;
+        let is_large = aggregated.elements_traversed as usize >= threshold;
+        TraversalStatistics {
+            aggregated,
+            selectors: stylist.num_selectors() as u32,
+            revalidation_selectors: stylist.num_revalidation_selectors() as u32,
+            dependency_selectors: stylist.num_invalidations() as u32,
+            declarations: stylist.num_declarations() as u32,
+            stylist_rebuilds: stylist.num_rebuilds() as u32,
+            traversal_time_ms: (time::precise_time_s() - start) * 1000.0,
+            is_parallel: parallel,
+            is_large,
+        }
+    }
+}
+
+#[cfg(feature = "gecko")]
+bitflags! {
+    /// Represents which tasks are performed in a SequentialTask of
+    /// UpdateAnimations which is a result of normal restyle.
+    pub struct UpdateAnimationsTasks: u8 {
+        /// Update CSS Animations.
+        const CSS_ANIMATIONS = structs::UpdateAnimationsTasks_CSSAnimations;
+        /// Update CSS Transitions.
+        const CSS_TRANSITIONS = structs::UpdateAnimationsTasks_CSSTransitions;
+        /// Update effect properties.
+        const EFFECT_PROPERTIES = structs::UpdateAnimationsTasks_EffectProperties;
+        /// Update animation cacade results for animations running on the compositor.
+        const CASCADE_RESULTS = structs::UpdateAnimationsTasks_CascadeResults;
+        /// Display property was changed from none.
+        /// Script animations keep alive on display:none elements, so we need to trigger
+        /// the second animation restyles for the script animations in the case where
+        /// the display property was changed from 'none' to others.
+        const DISPLAY_CHANGED_FROM_NONE = structs::UpdateAnimationsTasks_DisplayChangedFromNone;
+    }
+}
+
+#[cfg(feature = "gecko")]
+bitflags! {
+    /// Represents which tasks are performed in a SequentialTask as a result of
+    /// animation-only restyle.
+    pub struct PostAnimationTasks: u8 {
+        /// Display property was changed from none in animation-only restyle so
+        /// that we need to resolve styles for descendants in a subsequent
+        /// normal restyle.
+        const DISPLAY_CHANGED_FROM_NONE_FOR_SMIL = 0x01;
+    }
+}
+
+/// A task to be run in sequential mode on the parent (non-worker) thread. This
+/// is used by the style system to queue up work which is not safe to do during
+/// the parallel traversal.
+pub enum SequentialTask<E: TElement> {
+    /// Entry to avoid an unused type parameter error on servo.
+    Unused(SendElement<E>),
+
+    /// Performs one of a number of possible tasks related to updating
+    /// animations based on the |tasks| field. These include updating CSS
+    /// animations/transitions that changed as part of the non-animation style
+    /// traversal, and updating the computed effect properties.
+    #[cfg(feature = "gecko")]
+    UpdateAnimations {
+        /// The target element or pseudo-element.
+        el: SendElement<E>,
+        /// The before-change style for transitions. We use before-change style
+        /// as the initial value of its Keyframe. Required if |tasks| includes
+        /// CSSTransitions.
+        before_change_style: Option<Arc<ComputedValues>>,
+        /// The tasks which are performed in this SequentialTask.
+        tasks: UpdateAnimationsTasks,
+    },
+
+    /// Performs one of a number of possible tasks as a result of animation-only
+    /// restyle.
+    ///
+    /// Currently we do only process for resolving descendant elements that were
+    /// display:none subtree for SMIL animation.
+    #[cfg(feature = "gecko")]
+    PostAnimation {
+        /// The target element.
+        el: SendElement<E>,
+        /// The tasks which are performed in this SequentialTask.
+        tasks: PostAnimationTasks,
+    },
+}
+
+impl<E: TElement> SequentialTask<E> {
+    /// Executes this task.
+    pub fn execute(self) {
+        use self::SequentialTask::*;
+        debug_assert_eq!(thread_state::get(), ThreadState::LAYOUT);
+        match self {
+            Unused(_) => unreachable!(),
+            #[cfg(feature = "gecko")]
+            UpdateAnimations {
+                el,
+                before_change_style,
+                tasks,
+            } => {
+                el.update_animations(before_change_style, tasks);
+            },
+            #[cfg(feature = "gecko")]
+            PostAnimation { el, tasks } => {
+                el.process_post_animation(tasks);
+            },
+        }
+    }
+
+    /// Creates a task to update various animation-related state on a given
+    /// (pseudo-)element.
+    #[cfg(feature = "gecko")]
+    pub fn update_animations(
+        el: E,
+        before_change_style: Option<Arc<ComputedValues>>,
+        tasks: UpdateAnimationsTasks,
+    ) -> Self {
+        use self::SequentialTask::*;
+        UpdateAnimations {
+            el: unsafe { SendElement::new(el) },
+            before_change_style,
+            tasks,
+        }
+    }
+
+    /// Creates a task to do post-process for a given element as a result of
+    /// animation-only restyle.
+    #[cfg(feature = "gecko")]
+    pub fn process_post_animation(el: E, tasks: PostAnimationTasks) -> Self {
+        use self::SequentialTask::*;
+        PostAnimation {
+            el: unsafe { SendElement::new(el) },
+            tasks,
+        }
+    }
+}
+
+/// A list of SequentialTasks that get executed on Drop.
+pub struct SequentialTaskList<E>(Vec<SequentialTask<E>>)
+where
+    E: TElement;
+
+impl<E> ops::Deref for SequentialTaskList<E>
+where
+    E: TElement,
+{
+    type Target = Vec<SequentialTask<E>>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<E> ops::DerefMut for SequentialTaskList<E>
+where
+    E: TElement,
+{
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+impl<E> Drop for SequentialTaskList<E>
+where
+    E: TElement,
+{
+    fn drop(&mut self) {
+        debug_assert_eq!(thread_state::get(), ThreadState::LAYOUT);
+        for task in self.0.drain(..) {
+            task.execute()
+        }
+    }
+}
+
+/// A helper type for stack limit checking.  This assumes that stacks grow
+/// down, which is true for all non-ancient CPU architectures.
+pub struct StackLimitChecker {
+    lower_limit: usize,
+}
+
+impl StackLimitChecker {
+    /// Create a new limit checker, for this thread, allowing further use
+    /// of up to |stack_size| bytes beyond (below) the current stack pointer.
+    #[inline(never)]
+    pub fn new(stack_size_limit: usize) -> Self {
+        StackLimitChecker {
+            lower_limit: StackLimitChecker::get_sp() - stack_size_limit,
+        }
+    }
+
+    /// Checks whether the previously stored stack limit has now been exceeded.
+    #[inline(never)]
+    pub fn limit_exceeded(&self) -> bool {
+        let curr_sp = StackLimitChecker::get_sp();
+
+        // Do some sanity-checking to ensure that our invariants hold, even in
+        // the case where we've exceeded the soft limit.
+        //
+        // The correctness of depends on the assumption that no stack wraps
+        // around the end of the address space.
+        if cfg!(debug_assertions) {
+            // Compute the actual bottom of the stack by subtracting our safety
+            // margin from our soft limit. Note that this will be slightly below
+            // the actual bottom of the stack, because there are a few initial
+            // frames on the stack before we do the measurement that computes
+            // the limit.
+            let stack_bottom = self.lower_limit - STACK_SAFETY_MARGIN_KB * 1024;
+
+            // The bottom of the stack should be below the current sp. If it
+            // isn't, that means we've either waited too long to check the limit
+            // and burned through our safety margin (in which case we probably
+            // would have segfaulted by now), or we're using a limit computed for
+            // a different thread.
+            debug_assert!(stack_bottom < curr_sp);
+
+            // Compute the distance between the current sp and the bottom of
+            // the stack, and compare it against the current stack. It should be
+            // no further from us than the total stack size. We allow some slop
+            // to handle the fact that stack_bottom is a bit further than the
+            // bottom of the stack, as discussed above.
+            let distance_to_stack_bottom = curr_sp - stack_bottom;
+            let max_allowable_distance = (STYLE_THREAD_STACK_SIZE_KB + 10) * 1024;
+            debug_assert!(distance_to_stack_bottom <= max_allowable_distance);
+        }
+
+        // The actual bounds check.
+        curr_sp <= self.lower_limit
+    }
+
+    // Technically, rustc can optimize this away, but shouldn't for now.
+    // We should fix this once black_box is stable.
+    #[inline(always)]
+    fn get_sp() -> usize {
+        let mut foo: usize = 42;
+        (&mut foo as *mut usize) as usize
+    }
+}
+
+/// A thread-local style context.
+///
+/// This context contains data that needs to be used during restyling, but is
+/// not required to be unique among worker threads, so we create one per worker
+/// thread in order to be able to mutate it without locking.
+pub struct ThreadLocalStyleContext<E: TElement> {
+    /// A cache to share style among siblings.
+    pub sharing_cache: StyleSharingCache<E>,
+    /// A cache from matched properties to elements that match those.
+    pub rule_cache: RuleCache,
+    /// The bloom filter used to fast-reject selector-matching.
+    pub bloom_filter: StyleBloom<E>,
+    /// A set of tasks to be run (on the parent thread) in sequential mode after
+    /// the rest of the styling is complete. This is useful for
+    /// infrequently-needed non-threadsafe operations.
+    ///
+    /// It's important that goes after the style sharing cache and the bloom
+    /// filter, to ensure they're dropped before we execute the tasks, which
+    /// could create another ThreadLocalStyleContext for style computation.
+    pub tasks: SequentialTaskList<E>,
+    /// Statistics about the traversal.
+    pub statistics: PerThreadTraversalStatistics,
+    /// A checker used to ensure that parallel.rs does not recurse indefinitely
+    /// even on arbitrarily deep trees.  See Gecko bug 1376883.
+    pub stack_limit_checker: StackLimitChecker,
+    /// A cache for nth-index-like selectors.
+    pub nth_index_cache: NthIndexCache,
+}
+
+impl<E: TElement> ThreadLocalStyleContext<E> {
+    /// Creates a new `ThreadLocalStyleContext`
+    pub fn new() -> Self {
+        ThreadLocalStyleContext {
+            sharing_cache: StyleSharingCache::new(),
+            rule_cache: RuleCache::new(),
+            bloom_filter: StyleBloom::new(),
+            tasks: SequentialTaskList(Vec::new()),
+            statistics: PerThreadTraversalStatistics::default(),
+            stack_limit_checker: StackLimitChecker::new(
+                (STYLE_THREAD_STACK_SIZE_KB - STACK_SAFETY_MARGIN_KB) * 1024,
+            ),
+            nth_index_cache: NthIndexCache::default(),
+        }
+    }
+}
+
+/// A `StyleContext` is just a simple container for a immutable reference to a
+/// shared style context, and a mutable reference to a local one.
+pub struct StyleContext<'a, E: TElement + 'a> {
+    /// The shared style context reference.
+    pub shared: &'a SharedStyleContext<'a>,
+    /// The thread-local style context (mutable) reference.
+    pub thread_local: &'a mut ThreadLocalStyleContext<E>,
+}
+
+/// A registered painter
+#[cfg(feature = "servo")]
+pub trait RegisteredSpeculativePainter: SpeculativePainter {
+    /// The name it was registered with
+    fn name(&self) -> Atom;
+    /// The properties it was registered with
+    fn properties(&self) -> &FxHashMap<Atom, PropertyId>;
+}
+
+/// A set of registered painters
+#[cfg(feature = "servo")]
+pub trait RegisteredSpeculativePainters: Sync {
+    /// Look up a speculative painter
+    fn get(&self, name: &Atom) -> Option<&dyn RegisteredSpeculativePainter>;
+}