style: Stop special-casing a few attributes for style sharing, and use a visitor to track dependencies.

Also, simplify all the pre-snapshot attribute hacks in the script and style
code.

MozReview-Commit-ID: 6c9ipeb7Tnr
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/selectors/parser.rs

@@ -145,15 +145,16 @@ impl<Impl: SelectorImpl> SelectorMethods for Selector<Impl> {
     }
 }
 
-impl<Impl: SelectorImpl> SelectorMethods for ComplexSelector<Impl> {
+impl<Impl: SelectorImpl> SelectorMethods for Arc<ComplexSelector<Impl>> {
     type Impl = Impl;
 
     fn visit<V>(&self, visitor: &mut V) -> bool
         where V: SelectorVisitor<Impl = Impl>,
     {
         let mut current = self;
+        let mut combinator = None;
         loop {
-            if !visitor.visit_complex_selector(current) {
+            if !visitor.visit_complex_selector(current, combinator) {
                 return false;
             }
 
@@ -164,7 +165,10 @@ impl<Impl: SelectorImpl> SelectorMethods for ComplexSelector<Impl> {
             }
 
             match current.next {
-                Some((ref next, _)) => current = next,
+                Some((ref next, next_combinator)) => {
+                    current = next;
+                    combinator = Some(next_combinator);
+                }
                 None => break,
             }
         }
@@ -222,30 +226,6 @@ pub struct ComplexSelector<Impl: SelectorImpl> {
     pub next: Option<(Arc<ComplexSelector<Impl>>, Combinator)>,  // c.next is left of c
 }
 
-impl<Impl: SelectorImpl> ComplexSelector<Impl> {
-    /// Visits this selectors and all the ones to the left of it, until a
-    /// visitor method returns `false`.
-    pub fn visit<V>(&self, visitor: &mut V) -> bool
-        where V: SelectorVisitor<Impl = Impl>,
-    {
-        let mut current = self;
-        loop {
-            for selector in &current.compound_selector {
-                if !selector.visit(visitor) {
-                    return false;
-                }
-            }
-
-            match current.next {
-                Some((ref next, _)) => current = next,
-                None => break,
-            }
-        }
-
-        true
-    }
-}
-
 #[derive(Eq, PartialEq, Clone, Copy, Debug, Hash)]
 pub enum Combinator {
     Child,  //  >

components/selectors/visitor.rs

@@ -6,7 +6,8 @@
 
 #![deny(missing_docs)]
 
-use parser::{AttrSelector, ComplexSelector, SelectorImpl, SimpleSelector};
+use parser::{AttrSelector, Combinator, ComplexSelector, SelectorImpl, SimpleSelector};
+use std::sync::Arc;
 
 /// A trait to visit selector properties.
 ///
@@ -24,7 +25,13 @@ pub trait SelectorVisitor {
     }
 
     /// Visits a complex selector.
-    fn visit_complex_selector(&mut self, _: &ComplexSelector<Self::Impl>) -> bool {
+    ///
+    /// Gets the combinator to the right of the selector, or `None` if the
+    /// selector is the leftmost one.
+    fn visit_complex_selector(&mut self,
+                              _: &Arc<ComplexSelector<Self::Impl>>,
+                              _combinator_to_right: Option<Combinator>)
+                              -> bool {
         true
     }