Rewrite the style sharing candidate cache.

The style candidate cache had regressed a few times (see #12534), and my
intuition is that being able to disable all style sharing with a single rule in
the page is really unfortunate.

This commit redesigns the style sharing cache in order to be a optimistic cache,
but then reject candidates if they match different sibling-affecting selectors
in the page, for example.

So far the numbers have improved, but not so much as I'd wanted (~10%/20% of
non-incremental restyling time in general). The current implementation is really
dumb though (we recompute and re-match a lot of stuff), so we should be able to
optimise it quite a bit.

I have different ideas for improving it (that may or may not work), apart of the
low-hanging fruit like don't re-matching candidates all the time but I have to
measure the real impact.

Also, I need to verify it against try.

components/layout/Cargo.toml

@@ -33,7 +33,7 @@ range = {path = "../range"}
 rustc-serialize = "0.3"
 script_layout_interface = {path = "../script_layout_interface"}
 script_traits = {path = "../script_traits"}
-selectors = {version = "0.8", features = ["heap_size"]}
+selectors = {version = "0.9", features = ["heap_size"]}
 serde_macros = "0.8"
 smallvec = "0.1"
 string_cache = {version = "0.2.23", features = ["heap_size"]}

components/layout/traversal.rs

@@ -13,7 +13,7 @@ use gfx::display_list::OpaqueNode;
 use script_layout_interface::restyle_damage::{BUBBLE_ISIZES, REFLOW, REFLOW_OUT_OF_FLOW, REPAINT, RestyleDamage};
 use script_layout_interface::wrapper_traits::{LayoutNode, ThreadSafeLayoutNode};
 use std::mem;
-use style::context::SharedStyleContext;
+use style::context::{LocalStyleContext, SharedStyleContext, StyleContext};
 use style::dom::TNode;
 use style::selector_impl::ServoSelectorImpl;
 use style::traversal::RestyleResult;
@@ -81,6 +81,10 @@ impl<'lc, N> DomTraversalContext<N> for RecalcStyleAndConstructFlows<'lc>
     fn process_postorder(&self, node: N) {
         construct_flows_at(&self.context, self.root, node);
     }
+
+    fn local_context(&self) -> &LocalStyleContext {
+        self.context.local_context()
+    }
 }
 
 /// A bottom-up, parallelizable traversal.

components/script/Cargo.toml

@@ -65,7 +65,7 @@ regex = "0.1.43"
 rustc-serialize = "0.3"
 script_layout_interface = {path = "../script_layout_interface"}
 script_traits = {path = "../script_traits"}
-selectors = {version = "0.8", features = ["heap_size"]}
+selectors = {version = "0.9", features = ["heap_size"]}
 serde = "0.8"
 smallvec = "0.1"
 string_cache = {version = "0.2.23", features = ["heap_size", "unstable"]}

components/script/layout_wrapper.rs

@@ -444,6 +444,11 @@ impl<'le> TElement for ServoLayoutElement<'le> {
     }
 }
 
+impl<'le> PartialEq for ServoLayoutElement<'le> {
+    fn eq(&self, other: &Self) -> bool {
+        self.as_node() == other.as_node()
+    }
+}
 
 impl<'le> ServoLayoutElement<'le> {
     fn from_layout_js(el: LayoutJS<Element>) -> ServoLayoutElement<'le> {

components/script_layout_interface/Cargo.toml

@@ -27,7 +27,7 @@ plugins = {path = "../plugins"}
 profile_traits = {path = "../profile_traits"}
 range = {path = "../range"}
 script_traits = {path = "../script_traits"}
-selectors = {version = "0.8", features = ["heap_size"]}
+selectors = {version = "0.9", features = ["heap_size"]}
 string_cache = {version = "0.2.23", features = ["heap_size"]}
 style = {path = "../style"}
 url = {version = "1.2", features = ["heap_size"]}

components/style/Cargo.toml

@@ -38,7 +38,7 @@ num-traits = "0.1.32"
 ordered-float = "0.2.2"
 rand = "0.3"
 rustc-serialize = "0.3"
-selectors = "0.8.2"
+selectors = "0.9"
 serde = {version = "0.8", optional = true}
 serde_macros = {version = "0.8", optional = true}
 smallvec = "0.1"

components/style/context.rs

@@ -7,6 +7,7 @@
 use animation::Animation;
 use app_units::Au;
 use dom::OpaqueNode;
+use dom::TElement;
 use error_reporting::ParseErrorReporter;
 use euclid::Size2D;
 use matching::{ApplicableDeclarationsCache, StyleSharingCandidateCache};

components/style/dom.rs

@@ -201,7 +201,7 @@ pub trait PresentationalHintsSynthetizer {
         where V: Push<DeclarationBlock<Vec<PropertyDeclaration>>>;
 }
 
-pub trait TElement : Sized + Copy + Clone + ElementExt + PresentationalHintsSynthetizer {
+pub trait TElement : PartialEq + Sized + Copy + Clone + ElementExt + PresentationalHintsSynthetizer {
     type ConcreteNode: TNode<ConcreteElement = Self, ConcreteDocument = Self::ConcreteDocument>;
     type ConcreteDocument: TDocument<ConcreteNode = Self::ConcreteNode, ConcreteElement = Self>;