style: Add parsing support for ::part().

Disabled for now of course. This should be pretty uncontroversial I'd think.

Differential Revision: https://phabricator.services.mozilla.com/D28060

components/style/invalidation/element/element_wrapper.rs

@@ -340,6 +340,10 @@ where
         }
     }
 
+    fn is_part(&self, _name: &Atom) -> bool {
+        unimplemented!();
+    }
+
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
         match self.snapshot() {
             Some(snapshot) if snapshot.has_attrs() => snapshot.has_class(name, case_sensitivity),

components/style/invalidation/element/invalidation_map.rs

@@ -98,6 +98,7 @@ impl Dependency {
             // an eager pseudo, and return only Descendants here if not.
             Some(Combinator::PseudoElement) => DependencyInvalidationKind::ElementAndDescendants,
             Some(Combinator::SlotAssignment) => DependencyInvalidationKind::SlottedElements,
+            Some(Combinator::Part) => unimplemented!("Need to add invalidation for shadow parts"),
         }
     }
 }

components/style/invalidation/element/invalidator.rs

@@ -158,7 +158,10 @@ impl<'a> Invalidation<'a> {
         // We should be able to do better here!
         match self.selector.combinator_at_parse_order(self.offset - 1) {
             Combinator::Descendant | Combinator::LaterSibling | Combinator::PseudoElement => true,
-            Combinator::SlotAssignment | Combinator::NextSibling | Combinator::Child => false,
+            Combinator::Part |
+            Combinator::SlotAssignment |
+            Combinator::NextSibling |
+            Combinator::Child => false,
         }
     }
 
@@ -171,6 +174,9 @@ impl<'a> Invalidation<'a> {
             Combinator::Child | Combinator::Descendant | Combinator::PseudoElement => {
                 InvalidationKind::Descendant(DescendantInvalidationKind::Dom)
             },
+            Combinator::Part => {
+                unimplemented!("Need to add invalidation for shadow parts");
+            },
             Combinator::SlotAssignment => {
                 InvalidationKind::Descendant(DescendantInvalidationKind::Slotted)
             },