selectors: Allow defining an implementation-dependent field in the matching context.

components/selectors/context.rs

@@ -130,6 +130,9 @@ where
     /// should match when matching_mode is ForStatelessPseudoElement.
     pub pseudo_element_matching_fn: Option<&'a Fn(&Impl::PseudoElement) -> bool>,
 
+    /// Extra implementation-dependent matching data.
+    pub extra_data: Option<Impl::ExtraMatchingData>,
+
     quirks_mode: QuirksMode,
     classes_and_ids_case_sensitivity: CaseSensitivity,
     _impl: ::std::marker::PhantomData<Impl>,
@@ -173,6 +176,7 @@ where
             scope_element: None,
             nesting_level: 0,
             pseudo_element_matching_fn: None,
+            extra_data: None,
             _impl: ::std::marker::PhantomData,
         }
     }

components/selectors/parser.rs

@@ -84,6 +84,7 @@ macro_rules! with_all_bounds {
         /// are parameterized on SelectorImpl. See
         /// <https://github.com/rust-lang/rust/issues/26925>
         pub trait SelectorImpl: Clone + Sized + 'static {
+            type ExtraMatchingData: Sized + 'static;
             type AttrValue: $($InSelector)*;
             type Identifier: $($InSelector)* + PrecomputedHash;
             type ClassName: $($InSelector)* + PrecomputedHash;
@@ -2027,6 +2028,7 @@ pub mod tests {
     }
 
     impl SelectorImpl for DummySelectorImpl {
+        type ExtraMatchingData = ();
         type AttrValue = DummyAtom;
         type Identifier = DummyAtom;
         type ClassName = DummyAtom;

components/selectors/size_of_tests.rs

@@ -24,6 +24,7 @@ impl parser::PseudoElement for gecko_like_types::PseudoElement {
 // Boilerplate
 
 impl SelectorImpl for Impl {
+    type ExtraMatchingData = u64;
     type AttrValue = Atom;
     type Identifier = Atom;
     type ClassName = Atom;

components/style/gecko/selector_parser.rs

@@ -278,6 +278,7 @@ impl NonTSPseudoClass {
 pub struct SelectorImpl;
 
 impl ::selectors::SelectorImpl for SelectorImpl {
+    type ExtraMatchingData = ();
     type AttrValue = Atom;
     type Identifier = Atom;
     type ClassName = Atom;

components/style/lib.rs

@@ -26,6 +26,8 @@
 #![deny(warnings)]
 #![deny(missing_docs)]
 
+#![cfg_attr(feature = "servo", feature(never_type))]
+
 #![recursion_limit = "500"]  // For define_css_keyword_enum! in -moz-appearance
 
 extern crate app_units;

components/style/servo/selector_parser.rs

@@ -377,6 +377,7 @@ impl ::selectors::SelectorImpl for SelectorImpl {
     type PseudoElement = PseudoElement;
     type NonTSPseudoClass = NonTSPseudoClass;
 
+    type ExtraMatchingData = !;
     type AttrValue = String;
     type Identifier = Atom;
     type ClassName = Atom;