Auto merge of #13172 - bholley:display_enum, r=emilio

stylo: avoid traversing non element/text nodes in style and layout

r? @emilio

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components/style/dom.rs

@@ -64,7 +64,33 @@ pub trait TRestyleDamage : Debug + PartialEq + BitOr<Output=Self> + Copy {
     fn rebuild_and_reflow() -> Self;
 }
 
-pub trait TNode : Sized + Copy + Clone {
+/// Simple trait to provide basic information about the type of an element.
+///
+/// We avoid exposing the full type id, since computing it in the general case
+/// would be difficult for Gecko nodes.
+pub trait NodeInfo {
+    fn is_element(&self) -> bool;
+    fn is_text_node(&self) -> bool;
+
+    // Comments, doctypes, etc are ignored by layout algorithms.
+    fn needs_layout(&self) -> bool { self.is_element() || self.is_text_node() }
+}
+
+pub struct LayoutIterator<T>(pub T);
+impl<T, I> Iterator for LayoutIterator<T> where T: Iterator<Item=I>, I: NodeInfo {
+    type Item = I;
+    fn next(&mut self) -> Option<I> {
+        loop {
+            // Filter out nodes that layout should ignore.
+            let n = self.0.next();
+            if n.is_none() || n.as_ref().unwrap().needs_layout() {
+                return n
+            }
+        }
+    }
+}
+
+pub trait TNode : Sized + Copy + Clone + NodeInfo {
     type ConcreteElement: TElement<ConcreteNode = Self, ConcreteDocument = Self::ConcreteDocument>;
     type ConcreteDocument: TDocument<ConcreteNode = Self, ConcreteElement = Self::ConcreteElement>;
     type ConcreteRestyleDamage: TRestyleDamage;
@@ -73,19 +99,12 @@ pub trait TNode : Sized + Copy + Clone {
     fn to_unsafe(&self) -> UnsafeNode;
     unsafe fn from_unsafe(n: &UnsafeNode) -> Self;
 
-    /// Returns whether this is a text node. It turns out that this is all the style system cares
-    /// about, and thus obviates the need to compute the full type id, which would be expensive in
-    /// Gecko.
-    fn is_text_node(&self) -> bool;
-
-    fn is_element(&self) -> bool;
-
     fn dump(self);
 
     fn dump_style(self);
 
     /// Returns an iterator over this node's children.
-    fn children(self) -> Self::ConcreteChildrenIterator;
+    fn children(self) -> LayoutIterator<Self::ConcreteChildrenIterator>;
 
     /// Converts self into an `OpaqueNode`.
     fn opaque(&self) -> OpaqueNode;

components/style/matching.rs

@@ -12,7 +12,7 @@ use cache::{LRUCache, SimpleHashCache};
 use cascade_info::CascadeInfo;
 use context::{SharedStyleContext, StyleContext};
 use data::PrivateStyleData;
-use dom::{TElement, TNode, TRestyleDamage, UnsafeNode};
+use dom::{NodeInfo, TElement, TNode, TRestyleDamage, UnsafeNode};
 use properties::{ComputedValues, PropertyDeclarationBlock, cascade};
 use properties::longhands::display::computed_value as display;
 use selector_impl::{PseudoElement, TheSelectorImpl};