Auto merge of #16872 - jryans:matching-context, r=emilio

Create a MatchingContext to group related matching args

https://bugzilla.mozilla.org/show_bug.cgi?id=1364914

Upcoming changes for visited support and others as well need to pass more args through matching, so this creates a MatchingContext to group them together.

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9 files changed, 91 insertions, 83 deletions

diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index aa32370127c..91ea1435302 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -85,7 +85,7 @@ use net_traits::request::CorsSettings;
 use ref_filter_map::ref_filter_map;
 use script_layout_interface::message::ReflowQueryType;
 use script_thread::Runnable;
-use selectors::matching::{ElementSelectorFlags, StyleRelations, matches_selector_list};
+use selectors::matching::{ElementSelectorFlags, MatchingContext, matches_selector_list};
 use selectors::matching::{HAS_EDGE_CHILD_SELECTOR, HAS_SLOW_SELECTOR, HAS_SLOW_SELECTOR_LATER_SIBLINGS};
 use selectors::parser::{AttrSelector, NamespaceConstraint};
 use servo_atoms::Atom;
@@ -2414,7 +2414,7 @@ impl<'a> ::selectors::Element for Root<Element> {
 
     fn match_non_ts_pseudo_class<F>(&self,
                                     pseudo_class: &NonTSPseudoClass,
-                                    _: &mut StyleRelations,
+                                    _: &mut MatchingContext,
                                     _: &mut F)
                                     -> bool
         where F: FnMut(&Self, ElementSelectorFlags),
diff --git a/components/script/layout_wrapper.rs b/components/script/layout_wrapper.rs
index 3d4b90eaeab..5cea48786e3 100644
--- a/components/script/layout_wrapper.rs
+++ b/components/script/layout_wrapper.rs
@@ -50,7 +50,7 @@ use script_layout_interface::{HTMLCanvasData, LayoutNodeType, SVGSVGData, Truste
 use script_layout_interface::{OpaqueStyleAndLayoutData, PartialPersistentLayoutData};
 use script_layout_interface::wrapper_traits::{DangerousThreadSafeLayoutNode, GetLayoutData, LayoutNode};
 use script_layout_interface::wrapper_traits::{PseudoElementType, ThreadSafeLayoutElement, ThreadSafeLayoutNode};
-use selectors::matching::{ElementSelectorFlags, StyleRelations};
+use selectors::matching::{ElementSelectorFlags, MatchingContext};
 use selectors::parser::{AttrSelector, NamespaceConstraint};
 use servo_atoms::Atom;
 use servo_url::ServoUrl;
@@ -654,7 +654,7 @@ impl<'le> ::selectors::Element for ServoLayoutElement<'le> {
 
     fn match_non_ts_pseudo_class<F>(&self,
                                     pseudo_class: &NonTSPseudoClass,
-                                    _: &mut StyleRelations,
+                                    _: &mut MatchingContext,
                                     _: &mut F)
                                     -> bool
         where F: FnMut(&Self, ElementSelectorFlags),
@@ -1152,7 +1152,7 @@ impl<'le> ::selectors::Element for ServoThreadSafeLayoutElement<'le> {
 
     fn match_non_ts_pseudo_class<F>(&self,
                                     _: &NonTSPseudoClass,
-                                    _: &mut StyleRelations,
+                                    _: &mut MatchingContext,
                                     _: &mut F)
                                     -> bool
         where F: FnMut(&Self, ElementSelectorFlags),
diff --git a/components/selectors/matching.rs b/components/selectors/matching.rs
index ada15089061..ebc31fa7b53 100644
--- a/components/selectors/matching.rs
+++ b/components/selectors/matching.rs
@@ -17,6 +17,7 @@ bitflags! {
     /// the selector matching process.
     ///
     /// This is used to implement efficient sharing.
+    #[derive(Default)]
     pub flags StyleRelations: usize {
         /// Whether this element is affected by an ID selector.
         const AFFECTED_BY_ID_SELECTOR = 1 << 0,
@@ -68,6 +69,16 @@ impl ElementSelectorFlags {
     }
 }
 
+/// Data associated with the matching process for a element.  This context is
+/// used across many selectors for an element, so it's not appropriate for
+/// transient data that applies to only a single selector.
+#[derive(Default)]
+pub struct MatchingContext {
+    /// Output that records certains relations between elements noticed during
+    /// matching (and also extended after matching).
+    pub relations: StyleRelations,
+}
+
 pub fn matches_selector_list<E>(selector_list: &[Selector<E::Impl>],
                                 element: &E,
                                 parent_bf: Option<&BloomFilter>)
@@ -79,7 +90,7 @@ pub fn matches_selector_list<E>(selector_list: &[Selector<E::Impl>],
         matches_selector(&selector.inner,
                          element,
                          parent_bf,
-                         &mut StyleRelations::empty(),
+                         &mut MatchingContext::default(),
                          &mut |_, _| {})
     })
 }
@@ -108,7 +119,7 @@ fn may_match<E>(sel: &SelectorInner<E::Impl>,
 pub fn matches_selector<E, F>(selector: &SelectorInner<E::Impl>,
                               element: &E,
                               parent_bf: Option<&BloomFilter>,
-                              relations: &mut StyleRelations,
+                              context: &mut MatchingContext,
                               flags_setter: &mut F)
                               -> bool
     where E: Element,
@@ -122,7 +133,7 @@ pub fn matches_selector<E, F>(selector: &SelectorInner<E::Impl>,
     }
 
     // Match the selector.
-    matches_complex_selector(&selector.complex, element, relations, flags_setter)
+    matches_complex_selector(&selector.complex, element, context, flags_setter)
 }
 
 /// A result of selector matching, includes 3 failure types,
@@ -178,7 +189,7 @@ enum SelectorMatchingResult {
 /// Matches a complex selector.
 pub fn matches_complex_selector<E, F>(selector: &ComplexSelector<E::Impl>,
                                       element: &E,
-                                      relations: &mut StyleRelations,
+                                      context: &mut MatchingContext,
                                       flags_setter: &mut F)
                                       -> bool
      where E: Element,
@@ -186,7 +197,7 @@ pub fn matches_complex_selector<E, F>(selector: &ComplexSelector<E::Impl>,
 {
     match matches_complex_selector_internal(selector.iter(),
                                             element,
-                                            relations,
+                                            context,
                                             flags_setter) {
         SelectorMatchingResult::Matched => true,
         _ => false
@@ -195,14 +206,14 @@ pub fn matches_complex_selector<E, F>(selector: &ComplexSelector<E::Impl>,
 
 fn matches_complex_selector_internal<E, F>(mut selector_iter: SelectorIter<E::Impl>,
                                            element: &E,
-                                           relations: &mut StyleRelations,
+                                           context: &mut MatchingContext,
                                            flags_setter: &mut F)
                                            -> SelectorMatchingResult
      where E: Element,
            F: FnMut(&E, ElementSelectorFlags),
 {
     let matches_all_simple_selectors = selector_iter.all(|simple| {
-        matches_simple_selector(simple, element, relations, flags_setter)
+        matches_simple_selector(simple, element, context, flags_setter)
     });
 
     let combinator = selector_iter.next_sequence();
@@ -233,7 +244,7 @@ fn matches_complex_selector_internal<E, F>(mut selector_iter: SelectorIter<E::Im
                 };
                 let result = matches_complex_selector_internal(selector_iter.clone(),
                                                                &element,
-                                                               relations,
+                                                               context,
                                                                flags_setter);
                 match (result, c) {
                     // Return the status immediately.
@@ -276,7 +287,7 @@ fn matches_complex_selector_internal<E, F>(mut selector_iter: SelectorIter<E::Im
 fn matches_simple_selector<E, F>(
         selector: &Component<E::Impl>,
         element: &E,
-        relations: &mut StyleRelations,
+        context: &mut MatchingContext,
         flags_setter: &mut F)
         -> bool
     where E: Element,
@@ -285,7 +296,7 @@ fn matches_simple_selector<E, F>(
     macro_rules! relation_if {
         ($ex:expr, $flag:ident) => {
             if $ex {
-                *relations |= $flag;
+                context.relations |= $flag;
                 true
             } else {
                 false
@@ -341,7 +352,7 @@ fn matches_simple_selector<E, F>(
             false
         }
         Component::NonTSPseudoClass(ref pc) => {
-            element.match_non_ts_pseudo_class(pc, relations, flags_setter)
+            element.match_non_ts_pseudo_class(pc, context, flags_setter)
         }
         Component::FirstChild => {
             matches_first_child(element, flags_setter)
@@ -385,7 +396,7 @@ fn matches_simple_selector<E, F>(
             matches_generic_nth_child(element, 0, 1, true, true, flags_setter)
         }
         Component::Negation(ref negated) => {
-            !negated.iter().all(|ss| matches_simple_selector(ss, element, relations, flags_setter))
+            !negated.iter().all(|ss| matches_simple_selector(ss, element, context, flags_setter))
         }
     }
 }
diff --git a/components/selectors/tree.rs b/components/selectors/tree.rs
index abd56e85cd4..6514a786673 100644
--- a/components/selectors/tree.rs
+++ b/components/selectors/tree.rs
@@ -5,7 +5,7 @@
 //! Traits that nodes must implement. Breaks the otherwise-cyclic dependency between layout and
 //! style.
 
-use matching::{ElementSelectorFlags, StyleRelations};
+use matching::{ElementSelectorFlags, MatchingContext};
 use parser::{AttrSelector, SelectorImpl};
 use std::ascii::AsciiExt;
 
@@ -141,7 +141,7 @@ pub trait Element: MatchAttr + Sized {
 
     fn match_non_ts_pseudo_class<F>(&self,
                                     pc: &<Self::Impl as SelectorImpl>::NonTSPseudoClass,
-                                    relations: &mut StyleRelations,
+                                    context: &mut MatchingContext,
                                     flags_setter: &mut F) -> bool
         where F: FnMut(&Self, ElementSelectorFlags);
 
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index 41224c0b403..c38d3b73c93 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -64,7 +64,7 @@ use properties::style_structs::Font;
 use rule_tree::CascadeLevel as ServoCascadeLevel;
 use selector_parser::ElementExt;
 use selectors::Element;
-use selectors::matching::{ElementSelectorFlags, StyleRelations};
+use selectors::matching::{ElementSelectorFlags, MatchingContext};
 use selectors::parser::{AttrSelector, NamespaceConstraint};
 use shared_lock::Locked;
 use sink::Push;
@@ -1125,7 +1125,7 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
 
     fn match_non_ts_pseudo_class<F>(&self,
                                     pseudo_class: &NonTSPseudoClass,
-                                    relations: &mut StyleRelations,
+                                    context: &mut MatchingContext,
                                     flags_setter: &mut F)
                                     -> bool
         where F: FnMut(&Self, ElementSelectorFlags),
@@ -1219,7 +1219,7 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
             },
             NonTSPseudoClass::MozAny(ref sels) => {
                 sels.iter().any(|s| {
-                    matches_complex_selector(s, self, relations, flags_setter)
+                    matches_complex_selector(s, self, context, flags_setter)
                 })
             }
             NonTSPseudoClass::MozSystemMetric(ref s) |
@@ -1376,7 +1376,7 @@ impl<'le> ElementExt for GeckoElement<'le> {
     #[inline]
     fn is_link(&self) -> bool {
         self.match_non_ts_pseudo_class(&NonTSPseudoClass::AnyLink,
-                                       &mut StyleRelations::empty(),
+                                       &mut MatchingContext::default(),
                                        &mut |_, _| {})
     }
 
diff --git a/components/style/matching.rs b/components/style/matching.rs
index 764f58ec080..24cb5c2abe7 100644
--- a/components/style/matching.rs
+++ b/components/style/matching.rs
@@ -24,7 +24,7 @@ use restyle_hints::{RESTYLE_STYLE_ATTRIBUTE, RESTYLE_SMIL};
 use rule_tree::{CascadeLevel, RuleTree, StrongRuleNode};
 use selector_parser::{PseudoElement, RestyleDamage, SelectorImpl};
 use selectors::bloom::BloomFilter;
-use selectors::matching::{ElementSelectorFlags, StyleRelations};
+use selectors::matching::{ElementSelectorFlags, MatchingContext, StyleRelations};
 use selectors::matching::AFFECTED_BY_PSEUDO_ELEMENTS;
 use shared_lock::StylesheetGuards;
 use sink::ForgetfulSink;
@@ -895,8 +895,10 @@ pub trait MatchMethods : TElement {
                          sharing: StyleSharingBehavior)
     {
         // Perform selector matching for the primary style.
-        let mut primary_relations = StyleRelations::empty();
-        let _rule_node_changed = self.match_primary(context, data, &mut primary_relations);
+        let mut primary_matching_context = MatchingContext::default();
+        let _rule_node_changed = self.match_primary(context,
+                                                    data,
+                                                    &mut primary_matching_context);
 
         // Cascade properties and compute primary values.
         self.cascade_primary(context, data);
@@ -910,7 +912,7 @@ pub trait MatchMethods : TElement {
 
         // If we have any pseudo elements, indicate so in the primary StyleRelations.
         if !data.styles().pseudos.is_empty() {
-            primary_relations |= AFFECTED_BY_PSEUDO_ELEMENTS;
+            primary_matching_context.relations |= AFFECTED_BY_PSEUDO_ELEMENTS;
         }
 
         // If the style is shareable, add it to the LRU cache.
@@ -930,7 +932,7 @@ pub trait MatchMethods : TElement {
                    .style_sharing_candidate_cache
                    .insert_if_possible(self,
                                        data.styles().primary.values(),
-                                       primary_relations,
+                                       primary_matching_context.relations,
                                        revalidation_match_results);
         }
     }
@@ -950,7 +952,7 @@ pub trait MatchMethods : TElement {
     fn match_primary(&self,
                      context: &mut StyleContext<Self>,
                      data: &mut ElementData,
-                     relations: &mut StyleRelations)
+                     matching_context: &mut MatchingContext)
                      -> bool
     {
         let implemented_pseudo = self.implemented_pseudo_element();
@@ -1021,14 +1023,15 @@ pub trait MatchMethods : TElement {
         };
 
         // Compute the primary rule node.
-        *relations = stylist.push_applicable_declarations(&selector_matching_target,
-                                                          Some(bloom),
-                                                          style_attribute,
-                                                          smil_override,
-                                                          animation_rules,
-                                                          pseudo_and_state,
-                                                          &mut applicable_declarations,
-                                                          &mut set_selector_flags);
+        stylist.push_applicable_declarations(&selector_matching_target,
+                                             Some(bloom),
+                                             style_attribute,
+                                             smil_override,
+                                             animation_rules,
+                                             pseudo_and_state,
+                                             &mut applicable_declarations,
+                                             matching_context,
+                                             &mut set_selector_flags);
 
         let primary_rule_node =
             compute_rule_node::<Self>(&stylist.rule_tree,
@@ -1082,6 +1085,7 @@ pub trait MatchMethods : TElement {
                                                  AnimationRules(None, None),
                                                  Some((&pseudo, ElementState::empty())),
                                                  &mut applicable_declarations,
+                                                 &mut MatchingContext::default(),
                                                  &mut set_selector_flags);
 
             if !applicable_declarations.is_empty() {
diff --git a/components/style/restyle_hints.rs b/components/style/restyle_hints.rs
index 54d92151dbd..d0b9cd665f8 100644
--- a/components/style/restyle_hints.rs
+++ b/components/style/restyle_hints.rs
@@ -16,7 +16,7 @@ use gecko_bindings::structs::nsRestyleHint;
 use heapsize::HeapSizeOf;
 use selector_parser::{AttrValue, NonTSPseudoClass, PseudoElement, SelectorImpl, Snapshot, SnapshotMap};
 use selectors::{Element, MatchAttr};
-use selectors::matching::{ElementSelectorFlags, StyleRelations};
+use selectors::matching::{ElementSelectorFlags, MatchingContext};
 use selectors::matching::matches_selector;
 use selectors::parser::{AttrSelector, Combinator, Component, Selector};
 use selectors::parser::{SelectorInner, SelectorMethods};
@@ -348,7 +348,7 @@ impl<'a, E> Element for ElementWrapper<'a, E>
 {
     fn match_non_ts_pseudo_class<F>(&self,
                                     pseudo_class: &NonTSPseudoClass,
-                                    relations: &mut StyleRelations,
+                                    context: &mut MatchingContext,
                                     _setter: &mut F)
                                     -> bool
         where F: FnMut(&Self, ElementSelectorFlags),
@@ -360,7 +360,7 @@ impl<'a, E> Element for ElementWrapper<'a, E>
             use selectors::matching::matches_complex_selector;
             if let NonTSPseudoClass::MozAny(ref selectors) = *pseudo_class {
                 return selectors.iter().any(|s| {
-                    matches_complex_selector(s, self, relations, _setter)
+                    matches_complex_selector(s, self, context, _setter)
                 })
             }
         }
@@ -393,14 +393,14 @@ impl<'a, E> Element for ElementWrapper<'a, E>
         let flag = pseudo_class.state_flag();
         if flag.is_empty() {
             return self.element.match_non_ts_pseudo_class(pseudo_class,
-                                                          relations,
+                                                          context,
                                                           &mut |_, _| {})
         }
         match self.snapshot().and_then(|s| s.state()) {
             Some(snapshot_state) => snapshot_state.intersects(flag),
             None => {
                 self.element.match_non_ts_pseudo_class(pseudo_class,
-                                                       relations,
+                                                       context,
                                                        &mut |_, _| {})
             }
         }
@@ -780,7 +780,7 @@ impl DependencySet {
             // either (or it doesn't matter because our parent posted a restyle
             // for us above).
             if !matches_selector(&dep.selector.inner, &selector_matching_target,
-                                 None, &mut StyleRelations::empty(),
+                                 None, &mut MatchingContext::default(),
                                  &mut |_, _| {}) {
                 return true;
             }
@@ -857,11 +857,11 @@ impl DependencySet {
             // change its matching behavior here.
             let matched_then =
                 matches_selector(&dep.selector, &snapshot_el, None,
-                                 &mut StyleRelations::empty(),
+                                 &mut MatchingContext::default(),
                                  &mut |_, _| {});
             let matches_now =
                 matches_selector(&dep.selector, el, None,
-                                 &mut StyleRelations::empty(),
+                                 &mut MatchingContext::default(),
                                  &mut |_, _| {});
             if matched_then != matches_now {
                 hint.insert(dep.hint);
diff --git a/components/style/servo/selector_parser.rs b/components/style/servo/selector_parser.rs
index e801738866c..6f9ce19f40e 100644
--- a/components/style/servo/selector_parser.rs
+++ b/components/style/servo/selector_parser.rs
@@ -15,7 +15,7 @@ use fnv::FnvHashMap;
 use restyle_hints::ElementSnapshot;
 use selector_parser::{ElementExt, PseudoElementCascadeType, SelectorParser};
 use selectors::{Element, MatchAttrGeneric};
-use selectors::matching::StyleRelations;
+use selectors::matching::MatchingContext;
 use selectors::parser::{AttrSelector, SelectorMethods};
 use selectors::visitor::SelectorVisitor;
 use std::borrow::Cow;
@@ -580,7 +580,7 @@ impl MatchAttrGeneric for ServoElementSnapshot {
 impl<E: Element<Impl=SelectorImpl> + Debug> ElementExt for E {
     fn is_link(&self) -> bool {
         self.match_non_ts_pseudo_class(&NonTSPseudoClass::AnyLink,
-                                       &mut StyleRelations::empty(),
+                                       &mut MatchingContext::default(),
                                        &mut |_, _| {})
     }
 
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index d8b76bd01b0..74221045b11 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -29,7 +29,7 @@ use selector_parser::{SelectorImpl, PseudoElement, SnapshotMap};
 use selectors::Element;
 use selectors::bloom::BloomFilter;
 use selectors::matching::{AFFECTED_BY_STYLE_ATTRIBUTE, AFFECTED_BY_PRESENTATIONAL_HINTS};
-use selectors::matching::{ElementSelectorFlags, StyleRelations, matches_selector};
+use selectors::matching::{ElementSelectorFlags, matches_selector, MatchingContext};
 use selectors::parser::{AttrSelector, Combinator, Component, Selector, SelectorInner, SelectorIter};
 use selectors::parser::{SelectorMethods, LocalName as LocalNameSelector};
 use selectors::visitor::SelectorVisitor;
@@ -707,7 +707,6 @@ impl Stylist {
             }
         };
 
-
         let mut declarations = ApplicableDeclarationList::new();
         self.push_applicable_declarations(element,
                                           None,
@@ -716,6 +715,7 @@ impl Stylist {
                                           AnimationRules(None, None),
                                           Some((pseudo, pseudo_state)),
                                           &mut declarations,
+                                          &mut MatchingContext::default(),
                                           &mut set_selector_flags);
         if declarations.is_empty() {
             return None
@@ -834,8 +834,8 @@ impl Stylist {
     ///
     /// This corresponds to `ElementRuleCollector` in WebKit.
     ///
-    /// The returned `StyleRelations` indicate hints about which kind of rules
-    /// have matched.
+    /// The `StyleRelations` recorded in `MatchingContext` indicate hints about
+    /// which kind of rules have matched.
     pub fn push_applicable_declarations<E, V, F>(
                                         &self,
                                         element: &E,
@@ -845,8 +845,8 @@ impl Stylist {
                                         animation_rules: AnimationRules,
                                         pseudo_element: Option<(&PseudoElement, ElementState)>,
                                         applicable_declarations: &mut V,
+                                        context: &mut MatchingContext,
                                         flags_setter: &mut F)
-                                        -> StyleRelations
         where E: TElement,
               V: Push<ApplicableDeclarationBlock> + VecLike<ApplicableDeclarationBlock>,
               F: FnMut(&E, ElementSelectorFlags),
@@ -864,8 +864,6 @@ impl Stylist {
             None => &self.element_map,
         };
 
-        let mut relations = StyleRelations::empty();
-
         debug!("Determining if style is shareable: pseudo: {}",
                pseudo_element.is_some());
 
@@ -874,10 +872,10 @@ impl Stylist {
                                               pseudo_element,
                                               parent_bf,
                                               applicable_declarations,
-                                              &mut relations,
+                                              context,
                                               flags_setter,
                                               CascadeLevel::UANormal);
-        debug!("UA normal: {:?}", relations);
+        debug!("UA normal: {:?}", context.relations);
 
         if pseudo_element.is_none() {
             // Step 2: Presentational hints.
@@ -890,9 +888,9 @@ impl Stylist {
                     }
                 }
                 // Never share style for elements with preshints
-                relations |= AFFECTED_BY_PRESENTATIONAL_HINTS;
+                context.relations |= AFFECTED_BY_PRESENTATIONAL_HINTS;
             }
-            debug!("preshints: {:?}", relations);
+            debug!("preshints: {:?}", context.relations);
         }
 
         if element.matches_user_and_author_rules() {
@@ -901,29 +899,29 @@ impl Stylist {
                                             pseudo_element,
                                             parent_bf,
                                             applicable_declarations,
-                                            &mut relations,
+                                            context,
                                             flags_setter,
                                             CascadeLevel::UserNormal);
-            debug!("user normal: {:?}", relations);
+            debug!("user normal: {:?}", context.relations);
             map.author.get_all_matching_rules(element,
                                               pseudo_element,
                                               parent_bf,
                                               applicable_declarations,
-                                              &mut relations,
+                                              context,
                                               flags_setter,
                                               CascadeLevel::AuthorNormal);
-            debug!("author normal: {:?}", relations);
+            debug!("author normal: {:?}", context.relations);
 
             // Step 4: Normal style attributes.
             if let Some(sa) = style_attribute {
-                relations |= AFFECTED_BY_STYLE_ATTRIBUTE;
+                context.relations |= AFFECTED_BY_STYLE_ATTRIBUTE;
                 Push::push(
                     applicable_declarations,
                     ApplicableDeclarationBlock::from_declarations(sa.clone(),
                                                                   CascadeLevel::StyleAttributeNormal));
             }
 
-            debug!("style attr: {:?}", relations);
+            debug!("style attr: {:?}", context.relations);
 
             // Step 5: SMIL override.
             // Declarations from SVG SMIL animation elements.
@@ -933,7 +931,7 @@ impl Stylist {
                     ApplicableDeclarationBlock::from_declarations(so.clone(),
                                                                   CascadeLevel::SMILOverride));
             }
-            debug!("SMIL: {:?}", relations);
+            debug!("SMIL: {:?}", context.relations);
 
             // Step 6: Animations.
             // The animations sheet (CSS animations, script-generated animations,
@@ -944,7 +942,7 @@ impl Stylist {
                     ApplicableDeclarationBlock::from_declarations(anim,
                                                                   CascadeLevel::Animations));
             }
-            debug!("animation: {:?}", relations);
+            debug!("animation: {:?}", context.relations);
         } else {
             debug!("skipping non-agent rules");
         }
@@ -961,11 +959,9 @@ impl Stylist {
                 applicable_declarations,
                 ApplicableDeclarationBlock::from_declarations(anim, CascadeLevel::Transitions));
         }
-        debug!("transition: {:?}", relations);
+        debug!("transition: {:?}", context.relations);
 
-        debug!("push_applicable_declarations: shareable: {:?}", relations);
-
-        relations
+        debug!("push_applicable_declarations: shareable: {:?}", context.relations);
     }
 
     /// Return whether the device is dirty, that is, whether the screen size or
@@ -997,9 +993,6 @@ impl Stylist {
         where E: TElement,
               F: FnMut(&E, ElementSelectorFlags),
     {
-        use selectors::matching::StyleRelations;
-        use selectors::matching::matches_selector;
-
         // Note that, by the time we're revalidating, we're guaranteed that the
         // candidate and the entry have the same id, classes, and local name.
         // This means we're guaranteed to get the same rulehash buckets for all
@@ -1010,7 +1003,7 @@ impl Stylist {
             results.push(matches_selector(selector,
                                           element,
                                           Some(bloom),
-                                          &mut StyleRelations::empty(),
+                                          &mut MatchingContext::default(),
                                           flags_setter));
             true
         });
@@ -1293,7 +1286,7 @@ impl SelectorMap<Rule> {
                                            pseudo_element: Option<(&PseudoElement, ElementState)>,
                                            parent_bf: Option<&BloomFilter>,
                                            matching_rules_list: &mut V,
-                                           relations: &mut StyleRelations,
+                                           context: &mut MatchingContext,
                                            flags_setter: &mut F,
                                            cascade_level: CascadeLevel)
         where E: Element<Impl=SelectorImpl>,
@@ -1313,7 +1306,7 @@ impl SelectorMap<Rule> {
                                                       &self.id_hash,
                                                       &id,
                                                       matching_rules_list,
-                                                      relations,
+                                                      context,
                                                       flags_setter,
                                                       cascade_level)
         }
@@ -1325,7 +1318,7 @@ impl SelectorMap<Rule> {
                                                       &self.class_hash,
                                                       class,
                                                       matching_rules_list,
-                                                      relations,
+                                                      context,
                                                       flags_setter,
                                                       cascade_level);
         });
@@ -1336,7 +1329,7 @@ impl SelectorMap<Rule> {
                                                   &self.local_name_hash,
                                                   element.get_local_name(),
                                                   matching_rules_list,
-                                                  relations,
+                                                  context,
                                                   flags_setter,
                                                   cascade_level);
 
@@ -1345,7 +1338,7 @@ impl SelectorMap<Rule> {
                                         parent_bf,
                                         &self.other,
                                         matching_rules_list,
-                                        relations,
+                                        context,
                                         flags_setter,
                                         cascade_level);
 
@@ -1384,7 +1377,7 @@ impl SelectorMap<Rule> {
         hash: &FnvHashMap<Str, Vec<Rule>>,
         key: &BorrowedStr,
         matching_rules: &mut Vector,
-        relations: &mut StyleRelations,
+        context: &mut MatchingContext,
         flags_setter: &mut F,
         cascade_level: CascadeLevel)
         where E: Element<Impl=SelectorImpl>,
@@ -1399,7 +1392,7 @@ impl SelectorMap<Rule> {
                                             parent_bf,
                                             rules,
                                             matching_rules,
-                                            relations,
+                                            context,
                                             flags_setter,
                                             cascade_level)
         }
@@ -1411,7 +1404,7 @@ impl SelectorMap<Rule> {
                                    parent_bf: Option<&BloomFilter>,
                                    rules: &[Rule],
                                    matching_rules: &mut V,
-                                   relations: &mut StyleRelations,
+                                   context: &mut MatchingContext,
                                    flags_setter: &mut F,
                                    cascade_level: CascadeLevel)
         where E: Element<Impl=SelectorImpl>,
@@ -1443,7 +1436,7 @@ impl SelectorMap<Rule> {
             if matches_selector(&rule.selector.inner,
                                 element,
                                 parent_bf,
-                                relations,
+                                context,
                                 flags_setter) {
                 matching_rules.push(
                     rule.to_applicable_declaration_block(cascade_level));