style: Rewrite the restyle hints code to allow different kinds of element snapshots, and use it for Gecko.

This is a rewrite for how style interfaces with its consumers in order to allow
different representations for an element snapshot.

This also changes the requirements of an element snapshot, requiring them to
only implement MatchAttr, instead of MatchAttrGeneric. This is important for
stylo since implementing MatchAttrGeneric is way more difficult for us given the
atom limitations. This also allows for more performant implementations in the
Gecko side of things.

components/style/selector_matching.rs

@@ -10,12 +10,12 @@ use error_reporting::StdoutErrorReporter;
 use keyframes::KeyframesAnimation;
 use media_queries::{Device, MediaType};
 use properties::{self, PropertyDeclaration, PropertyDeclarationBlock, ComputedValues};
-use restyle_hints::{ElementSnapshot, RestyleHint, DependencySet};
-use selector_impl::{SelectorImplExt, TheSelectorImpl, PseudoElement, AttrString};
+use restyle_hints::{RestyleHint, DependencySet};
+use selector_impl::{ElementExt, SelectorImplExt, TheSelectorImpl, PseudoElement, AttrString};
+use selectors::Element;
 use selectors::bloom::BloomFilter;
 use selectors::matching::DeclarationBlock as GenericDeclarationBlock;
 use selectors::matching::{Rule, SelectorMap};
-use selectors::{Element, MatchAttrGeneric};
 use sink::Push;
 use smallvec::VecLike;
 use std::collections::HashMap;
@@ -402,15 +402,15 @@ impl Stylist {
     }
 
     pub fn compute_restyle_hint<E>(&self, element: &E,
-                                   snapshot: &ElementSnapshot,
+                                   snapshot: &E::Snapshot,
                                    // NB: We need to pass current_state as an argument because
                                    // selectors::Element doesn't provide access to ElementState
                                    // directly, and computing it from the ElementState would be
                                    // more expensive than getting it directly from the caller.
                                    current_state: ElementState)
                                    -> RestyleHint
-                                   where E: Element<Impl=TheSelectorImpl, AttrString=AttrString>
-                                        + Clone + MatchAttrGeneric {
+        where E: ElementExt + Clone
+    {
         self.state_deps.compute_hint(element, snapshot, current_state)
     }
 }