style: Support lazy pseudo-elements

These can't be supported in Servo as of right now, because I'm not
totally sure the accesses that should be done in layout would be
thread-safe.

It can be revisited later though.

components/layout/wrapper.rs

@@ -71,7 +71,7 @@ use style::element_state::*;
 use style::properties::{ComputedValues, ServoComputedValues};
 use style::properties::{PropertyDeclaration, PropertyDeclarationBlock};
 use style::restyle_hints::ElementSnapshot;
-use style::selector_impl::{NonTSPseudoClass, PseudoElement, ServoSelectorImpl};
+use style::selector_impl::{NonTSPseudoClass, PseudoElement, PseudoElementCascadeType, ServoSelectorImpl};
 use style::servo::{PrivateStyleData, SharedStyleContext};
 use url::Url;
 use util::str::is_whitespace;
@@ -774,13 +774,41 @@ pub trait ThreadSafeLayoutNode : Clone + Copy + Sized + PartialEq {
                 // Precompute non-eagerly-cascaded pseudo-element styles if not
                 // cached before.
                 let style_pseudo = other.style_pseudo_element();
-                if !style_pseudo.is_eagerly_cascaded() &&
-                   !self.borrow_layout_data().unwrap().style_data.per_pseudo.contains_key(&style_pseudo) {
+                match style_pseudo.cascade_type() {
+                    // Already computed during the cascade.
+                    PseudoElementCascadeType::Eager => {},
+                    PseudoElementCascadeType::Precomputed => {
+                        if !self.borrow_layout_data()
+                                .unwrap().style_data
+                                .per_pseudo.contains_key(&style_pseudo) {
                             let mut data = self.mutate_layout_data().unwrap();
-                    let new_style = context.stylist
-                                           .computed_values_for_pseudo(&style_pseudo,
+                            let new_style =
+                                context.stylist
+                                       .precomputed_values_for_pseudo(&style_pseudo,
                                                                       data.style_data.style.as_ref());
-                    data.style_data.per_pseudo.insert(style_pseudo.clone(), new_style.unwrap());
+                            data.style_data.per_pseudo
+                                .insert(style_pseudo.clone(), new_style.unwrap());
+                        }
+                    }
+                    PseudoElementCascadeType::Lazy => {
+                        panic!("Lazy pseudo-elements can't be used in Servo \
+                               since accessing the DOM tree during layout \
+                               could be unsafe.")
+                        // debug_assert!(self.is_element());
+                        // if !self.borrow_layout_data()
+                        //         .unwrap().style_data
+                        //         .per_pseudo.contains_key(&style_pseudo) {
+                        //     let mut data = self.mutate_layout_data().unwrap();
+                        //     let new_style =
+                        //         context.stylist
+                        //                .lazily_compute_pseudo_element_style(
+                        //                    &self.as_element(),
+                        //                    &style_pseudo,
+                        //                    data.style_data.style.as_ref().unwrap());
+                        //     data.style_data.per_pseudo
+                        //         .insert(style_pseudo.clone(), new_style.unwrap())
+                        // }
+                    }
                 }
 
                 Ref::map(self.borrow_layout_data().unwrap(), |data| {
@@ -1252,8 +1280,8 @@ impl<ConcreteNode> Iterator for ThreadSafeLayoutNodeChildrenIterator<ConcreteNod
     }
 }
 
-/// A wrapper around elements that ensures layout can only ever access safe properties and cannot
-/// race on elements.
+/// A wrapper around elements that ensures layout can only
+/// ever access safe properties and cannot race on elements.
 #[derive(Copy, Clone)]
 pub struct ServoThreadSafeLayoutElement<'le> {
     element: &'le Element,

components/style/matching.rs

@@ -540,7 +540,7 @@ pub trait ElementMatchMethods : TElement
             stylist.push_applicable_declarations(self,
                                                  parent_bf,
                                                  None,
-                                                 Some(pseudo.clone()),
+                                                 Some(&pseudo.clone()),
                                                  applicable_declarations.per_pseudo.entry(pseudo).or_insert(vec![]));
         });
 

components/style/media_queries.rs

@@ -117,6 +117,13 @@ impl Device {
             viewport_size: viewport_size,
         }
     }
+
+    #[inline]
+    pub fn au_viewport_size(&self) -> Size2D<Au> {
+        Size2D::new(Au::from_f32_px(self.viewport_size.width.get()),
+                    Au::from_f32_px(self.viewport_size.height.get()))
+    }
+
 }
 
 impl Expression {
@@ -203,8 +210,7 @@ pub fn parse_media_query_list(input: &mut Parser) -> MediaQueryList {
 
 impl MediaQueryList {
     pub fn evaluate(&self, device: &Device) -> bool {
-        let viewport_size = Size2D::new(Au::from_f32_px(device.viewport_size.width.get()),
-                                        Au::from_f32_px(device.viewport_size.height.get()));
+        let viewport_size = device.au_viewport_size();
 
         // Check if any queries match (OR condition)
         self.media_queries.iter().any(|mq| {

components/style/selector_impl.rs

@@ -8,6 +8,50 @@ use selectors::Element;
 use selectors::parser::{ParserContext, SelectorImpl};
 use stylesheets::Stylesheet;
 
+/// This function determines if a pseudo-element is eagerly cascaded or not.
+///
+/// Eagerly cascaded pseudo-elements are "normal" pseudo-elements (i.e.
+/// `::before` and `::after`). They inherit styles normally as another
+/// selector would do, and they're part of the cascade.
+///
+/// Lazy pseudo-elements are affected by selector matching, but they're only
+/// computed when needed, and not before. They're useful for general
+/// pseudo-elements that are not very common.
+///
+/// Precomputed ones skip the cascade process entirely, mostly as an
+/// optimisation since they are private pseudo-elements (like
+/// `::-servo-details-content`).
+///
+/// This pseudo-elements are resolved on the fly using *only* global rules
+/// (rules of the form `*|*`), and applying them to the parent style.
+///
+/// If you're implementing a public selector that the end-user might customize,
+/// then you probably need doing the whole cascading process and return true in
+/// this function for that pseudo (either as Eager or Lazy).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum PseudoElementCascadeType {
+    Eager,
+    Lazy,
+    Precomputed,
+}
+
+impl PseudoElementCascadeType {
+    #[inline]
+    pub fn is_eager(&self) -> bool {
+        *self == PseudoElementCascadeType::Eager
+    }
+
+    #[inline]
+    pub fn is_lazy(&self) -> bool {
+        *self == PseudoElementCascadeType::Lazy
+    }
+
+    #[inline]
+    pub fn is_precomputed(&self) -> bool {
+        *self == PseudoElementCascadeType::Precomputed
+    }
+}
+
 pub trait ElementExt: Element {
     fn is_link(&self) -> bool;
 }
@@ -15,49 +59,32 @@ pub trait ElementExt: Element {
 pub trait SelectorImplExt : SelectorImpl + Sized {
     type ComputedValues: properties::ComputedValues;
 
-    fn each_pseudo_element<F>(mut fun: F)
-        where F: FnMut(<Self as SelectorImpl>::PseudoElement);
+    fn pseudo_element_cascade_type(pseudo: &Self::PseudoElement) -> PseudoElementCascadeType;
 
-    /// This function determines if a pseudo-element is eagerly cascaded or not.
-    ///
-    /// Eagerly cascaded pseudo-elements are "normal" pseudo-elements (i.e.
-    /// `::before` and `::after`). They inherit styles normally as another
-    /// selector would do.
-    ///
-    /// Non-eagerly cascaded ones skip the cascade process entirely, mostly as
-    /// an optimisation since they are private pseudo-elements (like
-    /// `::-servo-details-content`). This pseudo-elements are resolved on the
-    /// fly using global rules (rules of the form `*|*`), and applying them to
-    /// the parent style.
-    ///
-    /// If you're implementing a public selector that the end-user might
-    /// customize, then you probably need doing the whole cascading process and
-    /// return true in this function for that pseudo.
-    ///
-    /// But if you are implementing a private pseudo-element, please consider if
-    /// it might be possible to skip the cascade for it.
-    fn is_eagerly_cascaded_pseudo_element(pseudo: &<Self as SelectorImpl>::PseudoElement) -> bool;
+    fn each_pseudo_element<F>(mut fun: F)
+        where F: FnMut(Self::PseudoElement);
 
     #[inline]
     fn each_eagerly_cascaded_pseudo_element<F>(mut fun: F)
         where F: FnMut(<Self as SelectorImpl>::PseudoElement) {
         Self::each_pseudo_element(|pseudo| {
-            if Self::is_eagerly_cascaded_pseudo_element(&pseudo) {
+            if Self::pseudo_element_cascade_type(&pseudo).is_eager() {
                 fun(pseudo)
             }
         })
     }
 
     #[inline]
-    fn each_non_eagerly_cascaded_pseudo_element<F>(mut fun: F)
+    fn each_precomputed_pseudo_element<F>(mut fun: F)
         where F: FnMut(<Self as SelectorImpl>::PseudoElement) {
         Self::each_pseudo_element(|pseudo| {
-            if !Self::is_eagerly_cascaded_pseudo_element(&pseudo) {
+            if Self::pseudo_element_cascade_type(&pseudo).is_precomputed() {
                 fun(pseudo)
             }
         })
     }
 
+
     fn pseudo_class_state_flag(pc: &Self::NonTSPseudoClass) -> ElementState;
 
     fn get_user_or_user_agent_stylesheets() -> &'static [Stylesheet<Self>];
@@ -76,13 +103,21 @@ pub enum PseudoElement {
 
 impl PseudoElement {
     #[inline]
-    pub fn is_eagerly_cascaded(&self) -> bool {
+    pub fn cascade_type(&self) -> PseudoElementCascadeType {
+        // TODO: Make PseudoElementCascadeType::Lazy work for Servo.
+        //
+        // This can't be done right now since it would require
+        // ServoThreadSafeLayoutElement to implement ::selectors::Element,
+        // and it might not be thread-safe.
+        //
+        // After that, we'd probably want ::selection and
+        // ::-servo-details-summary to be lazy.
         match *self {
             PseudoElement::Before |
             PseudoElement::After |
             PseudoElement::Selection |
-            PseudoElement::DetailsSummary => true,
-            PseudoElement::DetailsContent => false,
+            PseudoElement::DetailsSummary => PseudoElementCascadeType::Eager,
+            PseudoElement::DetailsContent => PseudoElementCascadeType::Precomputed,
         }
     }
 }
@@ -191,8 +226,8 @@ impl SelectorImplExt for ServoSelectorImpl {
     type ComputedValues = ServoComputedValues;
 
     #[inline]
-    fn is_eagerly_cascaded_pseudo_element(pseudo: &PseudoElement) -> bool {
-        pseudo.is_eagerly_cascaded()
+    fn pseudo_element_cascade_type(pseudo: &PseudoElement) -> PseudoElementCascadeType {
+        pseudo.cascade_type()
     }
 
     #[inline]

components/style/selector_matching.rs

@@ -8,7 +8,6 @@
 use dom::TElement;
 use element_state::*;
 use error_reporting::{ParseErrorReporter, StdoutErrorReporter};
-use euclid::Size2D;
 use media_queries::{Device, MediaType};
 use properties::{self, ComputedValues, PropertyDeclaration, PropertyDeclarationBlock};
 use restyle_hints::{ElementSnapshot, RestyleHint, DependencySet};
@@ -127,7 +126,7 @@ pub struct Stylist<Impl: SelectorImplExt> {
     /// Applicable declarations for a given non-eagerly cascaded pseudo-element.
     /// These are eagerly computed once, and then used to resolve the new
     /// computed values on the fly on layout.
-    non_eagerly_cascaded_pseudo_element_decls: HashMap<Impl::PseudoElement,
+    precomputed_pseudo_element_decls: HashMap<Impl::PseudoElement,
                                               Vec<DeclarationBlock>,
                                               BuildHasherDefault<::fnv::FnvHasher>>,
 
@@ -148,7 +147,7 @@ impl<Impl: SelectorImplExt> Stylist<Impl> {
 
             element_map: PerPseudoElementSelectorMap::new(),
             pseudos_map: HashMap::with_hasher(Default::default()),
-            non_eagerly_cascaded_pseudo_element_decls: HashMap::with_hasher(Default::default()),
+            precomputed_pseudo_element_decls: HashMap::with_hasher(Default::default()),
             rules_source_order: 0,
             state_deps: DependencySet::new(),
         };
@@ -175,7 +174,7 @@ impl<Impl: SelectorImplExt> Stylist<Impl> {
             self.pseudos_map.insert(pseudo, PerPseudoElementSelectorMap::new());
         });
 
-        self.non_eagerly_cascaded_pseudo_element_decls = HashMap::with_hasher(Default::default());
+        self.precomputed_pseudo_element_decls = HashMap::with_hasher(Default::default());
         self.rules_source_order = 0;
         self.state_deps.clear();
 
@@ -242,30 +241,32 @@ impl<Impl: SelectorImplExt> Stylist<Impl> {
 
         self.rules_source_order = rules_source_order;
 
-        Impl::each_non_eagerly_cascaded_pseudo_element(|pseudo| {
-            // TODO: Don't precompute this, compute it on demand instead and
-            // cache it.
-            //
-            // This is actually kind of hard, because the stylist is shared
-            // between threads.
+        Impl::each_precomputed_pseudo_element(|pseudo| {
+            // TODO: Consider not doing this and just getting the rules on the
+            // fly. It should be a bit slower, but we'd take rid of the
+            // extra field, and avoid this precomputation entirely.
             if let Some(map) = self.pseudos_map.remove(&pseudo) {
                 let mut declarations = vec![];
 
                 map.user_agent.normal.get_universal_rules(&mut declarations);
                 map.user_agent.important.get_universal_rules(&mut declarations);
 
-                self.non_eagerly_cascaded_pseudo_element_decls.insert(pseudo, declarations);
+                self.precomputed_pseudo_element_decls.insert(pseudo, declarations);
             }
         })
     }
 
-    pub fn computed_values_for_pseudo(&self,
+    /// Computes the style for a given "precomputed" pseudo-element, taking the
+    /// universal rules and applying them.
+    pub fn precomputed_values_for_pseudo(&self,
                                          pseudo: &Impl::PseudoElement,
-                                      parent: Option<&Arc<Impl::ComputedValues>>) -> Option<Arc<Impl::ComputedValues>> {
-        debug_assert!(!Impl::is_eagerly_cascaded_pseudo_element(pseudo));
-        if let Some(declarations) = self.non_eagerly_cascaded_pseudo_element_decls.get(pseudo) {
+                                         parent: Option<&Arc<Impl::ComputedValues>>)
+                                         -> Option<Arc<Impl::ComputedValues>> {
+        debug_assert!(Impl::pseudo_element_cascade_type(pseudo).is_precomputed());
+        if let Some(declarations) = self.precomputed_pseudo_element_decls.get(pseudo) {
+
             let (computed, _) =
-                properties::cascade::<Impl::ComputedValues>(Size2D::zero(),
+                properties::cascade(self.device.au_viewport_size(),
                                     &declarations, false,
                                     parent.map(|p| &**p), None,
                                     box StdoutErrorReporter);
@@ -275,6 +276,35 @@ impl<Impl: SelectorImplExt> Stylist<Impl> {
         }
     }
 
+    pub fn lazily_compute_pseudo_element_style<E>(&self,
+                                                  element: &E,
+                                                  pseudo: &Impl::PseudoElement,
+                                                  parent: &Arc<Impl::ComputedValues>)
+                                                  -> Option<Arc<Impl::ComputedValues>>
+                                                  where E: Element<Impl=Impl> + TElement {
+        debug_assert!(Impl::pseudo_element_cascade_type(pseudo).is_lazy());
+        if self.pseudos_map.get(pseudo).is_none() {
+            return None;
+        }
+
+        let mut declarations = vec![];
+
+        // NB: This being cached could be worth it, maybe allow an optional
+        // ApplicableDeclarationsCache?.
+        self.push_applicable_declarations(element,
+                                          None,
+                                          None,
+                                          Some(pseudo),
+                                          &mut declarations);
+
+        let (computed, _) =
+            properties::cascade(self.device.au_viewport_size(),
+                                &declarations, false,
+                                Some(&**parent), None,
+                                box StdoutErrorReporter);
+        Some(Arc::new(computed))
+    }
+
     pub fn compute_restyle_hint<E>(&self, element: &E,
                                    snapshot: &ElementSnapshot,
                                    // NB: We need to pass current_state as an argument because
@@ -325,7 +355,7 @@ impl<Impl: SelectorImplExt> Stylist<Impl> {
                                         element: &E,
                                         parent_bf: Option<&BloomFilter>,
                                         style_attribute: Option<&PropertyDeclarationBlock>,
-                                        pseudo_element: Option<Impl::PseudoElement>,
+                                        pseudo_element: Option<&Impl::PseudoElement>,
                                         applicable_declarations: &mut V)
                                         -> bool
                                         where E: Element<Impl=Impl> + TElement,
@@ -334,7 +364,7 @@ impl<Impl: SelectorImplExt> Stylist<Impl> {
         assert!(style_attribute.is_none() || pseudo_element.is_none(),
                 "Style attributes do not apply to pseudo-elements");
         debug_assert!(pseudo_element.is_none() ||
-                      Impl::is_eagerly_cascaded_pseudo_element(pseudo_element.as_ref().unwrap()));
+                      !Impl::pseudo_element_cascade_type(pseudo_element.as_ref().unwrap()).is_precomputed());
 
         let map = match pseudo_element {
             Some(ref pseudo) => self.pseudos_map.get(pseudo).unwrap(),

ports/geckolib/glue.rs

@@ -267,7 +267,7 @@ pub extern "C" fn Servo_GetComputedValuesForAnonymousBox(parent_style_or_null: *
     type Helpers = ArcHelpers<ServoComputedValues, GeckoComputedValues>;
 
     Helpers::maybe_with(parent_style_or_null, |maybe_parent| {
-        let new_computed = data.stylist.computed_values_for_pseudo(&pseudo, maybe_parent);
+        let new_computed = data.stylist.precomputed_values_for_pseudo(&pseudo, maybe_parent);
         new_computed.map_or(ptr::null_mut(), |c| Helpers::from(c))
     })
 }

ports/geckolib/selector_impl.rs

@@ -6,7 +6,7 @@ use properties::GeckoComputedValues;
 use selectors::parser::{ParserContext, SelectorImpl};
 use style;
 use style::element_state::ElementState;
-use style::selector_impl::SelectorImplExt;
+use style::selector_impl::{PseudoElementCascadeType, SelectorImplExt};
 
 pub type Stylist = style::selector_matching::Stylist<GeckoSelectorImpl>;
 pub type Stylesheet = style::stylesheets::Stylesheet<GeckoSelectorImpl>;
@@ -89,6 +89,36 @@ pub enum PseudoElement {
     MozSVGText,
 }
 
+impl PseudoElement {
+    fn is_anon_box_pseudo(&self) -> bool {
+        use self::PseudoElement::*;
+        match *self {
+            MozNonElement | MozAnonymousBlock | MozAnonymousPositionedBlock |
+            MozMathMLAnonymousBlock | MozXULAnonymousBlock |
+            MozHorizontalFramesetBorder | MozVerticalFramesetBorder |
+            MozLineFrame | MozButtonContent | MozButtonLabel | MozCellContent |
+            MozDropdownList | MozFieldsetContent | MozFramesetBlank |
+            MozDisplayComboboxControlFrame |
+            MozHTMLCanvasContent | MozInlineTable | MozTable | MozTableCell |
+            MozTableColumnGroup | MozTableColumn | MozTableOuter |
+            MozTableRowGroup | MozTableRow | MozCanvas | MozPageBreak |
+            MozPage | MozPageContent | MozPageSequence | MozScrolledContent |
+            MozScrolledCanvas | MozScrolledPageSequence | MozColumnContent |
+            MozViewport | MozViewportScroll | MozAnonymousFlexItem |
+            MozAnonymousGridItem | MozRuby | MozRubyBase |
+            MozRubyBaseContainer | MozRubyText | MozRubyTextContainer |
+            MozTreeColumn | MozTreeRow | MozTreeSeparator | MozTreeCell |
+            MozTreeIndentation | MozTreeLine | MozTreeTwisty | MozTreeImage |
+            MozTreeCellText | MozTreeCheckbox | MozTreeProgressMeter |
+            MozTreeDropFeedback | MozSVGMarkerAnonChild |
+            MozSVGOuterSVGAnonChild | MozSVGForeignContent |
+            MozSVGText => true,
+            _ => false,
+        }
+    }
+}
+
+
 #[derive(Clone, Debug, PartialEq, Eq, HeapSizeOf, Hash)]
 pub enum NonTSPseudoClass {
     AnyLink,
@@ -243,12 +273,17 @@ impl SelectorImplExt for GeckoSelectorImpl {
     type ComputedValues = GeckoComputedValues;
 
     #[inline]
-    fn is_eagerly_cascaded_pseudo_element(pseudo: &PseudoElement) -> bool {
+    fn pseudo_element_cascade_type(pseudo: &PseudoElement) -> PseudoElementCascadeType {
         match *pseudo {
             PseudoElement::Before |
-            PseudoElement::After |
-            PseudoElement::FirstLine => true,
-            _ => false,
+            PseudoElement::After => PseudoElementCascadeType::Eager,
+            _ => {
+                if pseudo.is_anon_box_pseudo() {
+                    PseudoElementCascadeType::Precomputed
+                } else {
+                    PseudoElementCascadeType::Lazy
+                }
+            }
         }
     }