Auto merge of #17128 - emilio:first-line-letter, r=bholley

stylo: Basic support for first-line/letter dynamic changes.

components/style/data.rs

@@ -4,6 +4,7 @@
 
 //! Per-node data used in style calculation.
 
+use arrayvec::ArrayVec;
 use context::SharedStyleContext;
 use dom::TElement;
 use properties::{AnimationRules, ComputedValues, PropertyDeclarationBlock};
@@ -193,8 +194,8 @@ impl EagerPseudoStyles {
     }
 
     /// Returns a list of the pseudo-elements.
-    pub fn keys(&self) -> Vec<PseudoElement> {
+    pub fn keys(&self) -> ArrayVec<[PseudoElement; EAGER_PSEUDO_COUNT]> {
-        let mut v = Vec::new();
+        let mut v = ArrayVec::new();
         if let Some(ref arr) = self.0 {
             for i in 0..EAGER_PSEUDO_COUNT {
                 if arr[i].is_some() {

components/style/dom.rs

@@ -382,6 +382,20 @@ pub trait TElement : Eq + PartialEq + Debug + Hash + Sized + Copy + Clone +
                                              pseudo: Option<&PseudoElement>)
                                              -> Option<&'a PreExistingComputedValues>;
 
+    /// Whether a given element may generate a pseudo-element.
+    ///
+    /// This is useful to avoid computing, for example, pseudo styles for
+    /// `::-first-line` or `::-first-letter`, when we know it won't affect us.
+    ///
+    /// TODO(emilio, bz): actually implement the logic for it.
+    fn may_generate_pseudo(
+        &self,
+        _pseudo: &PseudoElement,
+        _primary_style: &ComputedValues,
+    ) -> bool {
+        true
+    }
+
     /// Returns true if this element may have a descendant needing style processing.
     ///
     /// Note that we cannot guarantee the existence of such an element, because
@@ -444,7 +458,8 @@ pub trait TElement : Eq + PartialEq + Debug + Hash + Sized + Copy + Clone +
         false
     }
 
-    /// Flag that this element has a descendant for animation-only restyle processing.
+    /// Flag that this element has a descendant for animation-only restyle
+    /// processing.
     ///
     /// Only safe to call with exclusive access to the element.
     unsafe fn set_animation_only_dirty_descendants(&self) {

components/style/gecko/generated/pseudo_element_definition.rs

@@ -176,12 +176,14 @@ pub enum PseudoElement {
 
 
 /// The number of eager pseudo-elements.
-pub const EAGER_PSEUDO_COUNT: usize = 2;
+pub const EAGER_PSEUDO_COUNT: usize = 4;
 
 /// The list of eager pseudos.
 pub const EAGER_PSEUDOS: [PseudoElement; EAGER_PSEUDO_COUNT] = [
     PseudoElement::Before,
     PseudoElement::After,
+    PseudoElement::FirstLine,
+    PseudoElement::FirstLetter,
 ];
 
 impl PseudoElement {
@@ -458,7 +460,7 @@ impl PseudoElement {
     #[inline]
     pub fn is_eager(&self) -> bool {
         matches!(*self,
-                 PseudoElement::Before | PseudoElement::After)
+                 PseudoElement::Before | PseudoElement::After | PseudoElement::FirstLine | PseudoElement::FirstLetter)
     }
 
     /// Gets the flags associated to this pseudo-element, or 0 if it's an

components/style/gecko/pseudo_element_definition.mako.rs

@@ -11,7 +11,7 @@ pub enum PseudoElement {
     % endfor
 }
 
-<% EAGER_PSEUDOS = ["Before", "After"] %>
+<% EAGER_PSEUDOS = ["Before", "After", "FirstLine", "FirstLetter"] %>
 
 /// The number of eager pseudo-elements.
 pub const EAGER_PSEUDO_COUNT: usize = ${len(EAGER_PSEUDOS)};

components/style/matching.rs

@@ -357,6 +357,7 @@ trait PrivateMatchMethods: TElement {
             // below like a lazy pseudo.
             let only_default_rules = context.shared.traversal_flags.for_default_styles();
             if pseudo.is_eager() && !only_default_rules {
+                debug_assert!(pseudo.is_before_or_after());
                 let parent = self.parent_element().unwrap();
                 if !parent.may_have_animations() ||
                    primary_style.rules.get_animation_rules().is_empty() {
@@ -1106,6 +1107,10 @@ pub trait MatchMethods : TElement {
                 return
             }
 
+            if !self.may_generate_pseudo(&pseudo, data.styles().primary.values()) {
+                return;
+            }
+
             debug_assert!(applicable_declarations.is_empty());
             // NB: We handle animation rules for ::before and ::after when
             // traversing them.