style: Implement a more fine-grained invalidation method.

This commit also removes the old restyle_hints module and splits it into
multiple modules under components/style/invalidation/element/.

The basic approach is to walk down the tree using compound selectors as needed,
in order to do as little selector-matching as possible.

Bug: 1368240
MozReview-Commit-ID: 2YO8fKFygZI

components/style/invalidation/element/restyle_hints.rs

@@ -0,0 +1,212 @@
+/* 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/. */
+
+//! Restyle hints: an optimization to avoid unnecessarily matching selectors.
+
+#[cfg(feature = "gecko")]
+use gecko_bindings::structs::nsRestyleHint;
+
+bitflags! {
+    /// The kind of restyle we need to do for a given element.
+    pub flags RestyleHint: u8 {
+        /// Do a selector match of the element.
+        const RESTYLE_SELF = 1 << 0,
+
+        /// Do a selector match of the element's descendants.
+        const RESTYLE_DESCENDANTS = 1 << 1,
+
+        /// Recascade the current element.
+        const RECASCADE_SELF = 1 << 2,
+
+        /// Recascade all descendant elements.
+        const RECASCADE_DESCENDANTS = 1 << 3,
+
+        /// Replace the style data coming from CSS transitions without updating
+        /// any other style data. This hint is only processed in animation-only
+        /// traversal which is prior to normal traversal.
+        const RESTYLE_CSS_TRANSITIONS = 1 << 4,
+
+        /// Replace the style data coming from CSS animations without updating
+        /// any other style data. This hint is only processed in animation-only
+        /// traversal which is prior to normal traversal.
+        const RESTYLE_CSS_ANIMATIONS = 1 << 5,
+
+        /// Don't re-run selector-matching on the element, only the style
+        /// attribute has changed, and this change didn't have any other
+        /// dependencies.
+        const RESTYLE_STYLE_ATTRIBUTE = 1 << 6,
+
+        /// Replace the style data coming from SMIL animations without updating
+        /// any other style data. This hint is only processed in animation-only
+        /// traversal which is prior to normal traversal.
+        const RESTYLE_SMIL = 1 << 7,
+    }
+}
+
+impl RestyleHint {
+    /// Creates a new `RestyleHint` indicating that the current element and all
+    /// its descendants must be fully restyled.
+    pub fn restyle_subtree() -> Self {
+        RESTYLE_SELF | RESTYLE_DESCENDANTS
+    }
+
+    /// Creates a new `RestyleHint` indicating that the current element and all
+    /// its descendants must be recascaded.
+    pub fn recascade_subtree() -> Self {
+        RECASCADE_SELF | RECASCADE_DESCENDANTS
+    }
+
+    /// Returns a new `CascadeHint` appropriate for children of the current
+    /// element.
+    pub fn propagate_for_non_animation_restyle(&self) -> Self {
+        if self.contains(RESTYLE_DESCENDANTS) {
+            return Self::restyle_subtree()
+        }
+        if self.contains(RECASCADE_DESCENDANTS) {
+            return Self::recascade_subtree()
+        }
+        Self::empty()
+    }
+
+    /// Creates a new `RestyleHint` that indicates the element must be
+    /// recascaded.
+    pub fn recascade_self() -> Self {
+        RECASCADE_SELF
+    }
+
+    /// Returns a hint that contains all the replacement hints.
+    pub fn replacements() -> Self {
+        RESTYLE_STYLE_ATTRIBUTE | Self::for_animations()
+    }
+
+    /// The replacements for the animation cascade levels.
+    #[inline]
+    pub fn for_animations() -> Self {
+        RESTYLE_SMIL | RESTYLE_CSS_ANIMATIONS | RESTYLE_CSS_TRANSITIONS
+    }
+
+    /// Returns whether the hint specifies that some work must be performed on
+    /// the current element.
+    #[inline]
+    pub fn affects_self(&self) -> bool {
+        self.intersects(RESTYLE_SELF | RECASCADE_SELF | Self::replacements())
+    }
+
+    /// Returns whether the hint specifies that the currently element must be
+    /// recascaded.
+    pub fn has_recascade_self(&self) -> bool {
+        self.contains(RECASCADE_SELF)
+    }
+
+    /// Returns whether the hint specifies that an animation cascade level must
+    /// be replaced.
+    #[inline]
+    pub fn has_animation_hint(&self) -> bool {
+        self.intersects(Self::for_animations())
+    }
+
+    /// Returns whether the hint specifies some restyle work other than an
+    /// animation cascade level replacement.
+    #[inline]
+    pub fn has_non_animation_hint(&self) -> bool {
+        !(*self & !Self::for_animations()).is_empty()
+    }
+
+    /// Returns whether the hint specifies that selector matching must be re-run
+    /// for the element.
+    #[inline]
+    pub fn match_self(&self) -> bool {
+        self.intersects(RESTYLE_SELF)
+    }
+
+    /// Returns whether the hint specifies that some cascade levels must be
+    /// replaced.
+    #[inline]
+    pub fn has_replacements(&self) -> bool {
+        self.intersects(Self::replacements())
+    }
+
+    /// Removes all of the animation-related hints.
+    #[inline]
+    pub fn remove_animation_hints(&mut self) {
+        self.remove(Self::for_animations());
+
+        // While RECASCADE_SELF is not animation-specific, we only ever add and
+        // process it during traversal.  If we are here, removing animation
+        // hints, then we are in an animation-only traversal, and we know that
+        // any RECASCADE_SELF flag must have been set due to changes in
+        // inherited values after restyling for animations, and thus we want to
+        // remove it so that we don't later try to restyle the element during a
+        // normal restyle.  (We could have separate RECASCADE_SELF_NORMAL and
+        // RECASCADE_SELF_ANIMATIONS flags to make it clear, but this isn't
+        // currently necessary.)
+        self.remove(RECASCADE_SELF);
+    }
+}
+
+#[cfg(feature = "gecko")]
+impl From<nsRestyleHint> for RestyleHint {
+    fn from(raw: nsRestyleHint) -> Self {
+        use gecko_bindings::structs::nsRestyleHint_eRestyle_ForceDescendants as eRestyle_ForceDescendants;
+        use gecko_bindings::structs::nsRestyleHint_eRestyle_LaterSiblings as eRestyle_LaterSiblings;
+        use gecko_bindings::structs::nsRestyleHint_eRestyle_Self as eRestyle_Self;
+        use gecko_bindings::structs::nsRestyleHint_eRestyle_SomeDescendants as eRestyle_SomeDescendants;
+        use gecko_bindings::structs::nsRestyleHint_eRestyle_Subtree as eRestyle_Subtree;
+
+        let mut hint = RestyleHint::empty();
+
+        debug_assert!(raw.0 & eRestyle_LaterSiblings.0 == 0,
+                      "Handle later siblings manually if necessary plz.");
+
+        if (raw.0 & (eRestyle_Self.0 | eRestyle_Subtree.0)) != 0 {
+            hint.insert(RESTYLE_SELF);
+        }
+
+        if (raw.0 & (eRestyle_Subtree.0 | eRestyle_SomeDescendants.0)) != 0 {
+            hint.insert(RESTYLE_DESCENDANTS);
+        }
+
+        if (raw.0 & eRestyle_ForceDescendants.0) != 0 {
+            hint.insert(RECASCADE_DESCENDANTS);
+        }
+
+        hint.insert(RestyleHint::from_bits_truncate(raw.0 as u8));
+
+        hint
+    }
+}
+
+#[cfg(feature = "servo")]
+impl ::heapsize::HeapSizeOf for RestyleHint {
+    fn heap_size_of_children(&self) -> usize { 0 }
+}
+
+/// Asserts that all replacement hints have a matching nsRestyleHint value.
+#[cfg(feature = "gecko")]
+#[inline]
+pub fn assert_restyle_hints_match() {
+    use gecko_bindings::structs;
+
+    macro_rules! check_restyle_hints {
+        ( $( $a:ident => $b:ident ),*, ) => {
+            if cfg!(debug_assertions) {
+                let mut replacements = RestyleHint::replacements();
+                $(
+                    assert_eq!(structs::$a.0 as usize, $b.bits() as usize, stringify!($b));
+                    replacements.remove($b);
+                )*
+                assert_eq!(replacements, RestyleHint::empty(),
+                           "all RestyleHint replacement bits should have an \
+                            assertion");
+            }
+        }
+    }
+
+    check_restyle_hints! {
+        nsRestyleHint_eRestyle_CSSTransitions => RESTYLE_CSS_TRANSITIONS,
+        nsRestyleHint_eRestyle_CSSAnimations => RESTYLE_CSS_ANIMATIONS,
+        nsRestyleHint_eRestyle_StyleAttribute => RESTYLE_STYLE_ATTRIBUTE,
+        nsRestyleHint_eRestyle_StyleAttribute_Animations => RESTYLE_SMIL,
+    }
+}