Add support for :scope pseudo-class

components/selectors/builder.rs

@@ -284,7 +284,7 @@ fn complex_selector_specificity<Impl>(mut iter: slice::Iter<Component<Impl>>)
 
             Component::FirstChild | Component::LastChild |
             Component::OnlyChild | Component::Root |
-            Component::Empty |
+            Component::Empty | Component::Scope |
             Component::NthChild(..) |
             Component::NthLastChild(..) |
             Component::NthOfType(..) |

components/selectors/context.rs

@@ -5,6 +5,7 @@
 use attr::CaseSensitivity;
 use bloom::BloomFilter;
 use nth_index_cache::NthIndexCache;
+use tree::OpaqueElement;
 
 /// What kind of selector matching mode we should use.
 ///
@@ -88,6 +89,19 @@ pub struct MatchingContext<'a> {
     /// only.)
     pub relevant_link_found: bool,
 
+    /// The element which is going to match :scope pseudo-class. It can be
+    /// either one :scope element, or the scoping element.
+    ///
+    /// Note that, although in theory there can be multiple :scope elements,
+    /// in current specs, at most one is specified, and when there is one,
+    /// scoping element is not relevant anymore, so we use a single field for
+    /// them.
+    ///
+    /// When this is None, :scope will match the root element.
+    ///
+    /// See https://drafts.csswg.org/selectors-4/#scope-pseudo
+    pub scope_element: Option<OpaqueElement>,
+
     quirks_mode: QuirksMode,
     classes_and_ids_case_sensitivity: CaseSensitivity,
 }
@@ -125,6 +139,7 @@ impl<'a> MatchingContext<'a> {
             quirks_mode,
             relevant_link_found: false,
             classes_and_ids_case_sensitivity: quirks_mode.classes_and_ids_case_sensitivity(),
+            scope_element: None,
         }
     }
 

components/selectors/matching.rs

@@ -724,6 +724,12 @@ fn matches_simple_selector<E, F>(
             flags_setter(element, HAS_EMPTY_SELECTOR);
             element.is_empty()
         }
+        Component::Scope => {
+            match context.shared.scope_element {
+                Some(ref scope_element) => element.opaque() == *scope_element,
+                None => element.is_root(),
+            }
+        }
         Component::NthChild(a, b) => {
             matches_generic_nth_child(element, context, a, b, false, false, flags_setter)
         }

components/selectors/parser.rs

@@ -673,6 +673,7 @@ pub enum Component<Impl: SelectorImpl> {
     FirstChild, LastChild, OnlyChild,
     Root,
     Empty,
+    Scope,
     NthChild(i32, i32),
     NthLastChild(i32, i32),
     NthOfType(i32, i32),
@@ -969,6 +970,7 @@ impl<Impl: SelectorImpl> ToCss for Component<Impl> {
             OnlyChild => dest.write_str(":only-child"),
             Root => dest.write_str(":root"),
             Empty => dest.write_str(":empty"),
+            Scope => dest.write_str(":scope"),
             FirstOfType => dest.write_str(":first-of-type"),
             LastOfType => dest.write_str(":last-of-type"),
             OnlyOfType => dest.write_str(":only-of-type"),
@@ -1699,6 +1701,7 @@ fn parse_simple_pseudo_class<'i, P, E, Impl>(parser: &P, name: CowRcStr<'i>)
         "only-child"  => Ok(Component::OnlyChild),
         "root" => Ok(Component::Root),
         "empty" => Ok(Component::Empty),
+        "scope" => Ok(Component::Scope),
         "first-of-type" => Ok(Component::FirstOfType),
         "last-of-type"  => Ok(Component::LastOfType),
         "only-of-type"  => Ok(Component::OnlyOfType),