style: Implement selector-matching for ::part().

Also fairly straight-forward. This may get more complicated when we do part
forwarding, if any.

I've opened https://github.com/w3c/csswg-drafts/issues/3841 in what I think
would be a cleaner model for forwarding.

Differential Revision: https://phabricator.services.mozilla.com/D28063

components/style/gecko/snapshot.rs

@@ -183,13 +183,23 @@ impl ElementSnapshot for GeckoElementSnapshot {
         snapshot_helpers::get_id(&*self.mAttrs)
     }
 
+    #[inline]
+    fn is_part(&self, name: &Atom) -> bool {
+        let attr = match snapshot_helpers::find_attr(&*self.mAttrs, &atom!("part")) {
+            Some(attr) => attr,
+            None => return false,
+        };
+
+        snapshot_helpers::has_class_or_part(name, CaseSensitivity::CaseSensitive, attr)
+    }
+
     #[inline]
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
         if !self.has_any(Flags::MaybeClass) {
             return false;
         }
 
-        snapshot_helpers::has_class(name, case_sensitivity, &self.mClass)
+        snapshot_helpers::has_class_or_part(name, case_sensitivity, &self.mClass)
     }
 
     #[inline]

components/style/gecko/snapshot_helpers.rs

@@ -29,7 +29,7 @@ unsafe fn ptr<T>(attr: &structs::nsAttrValue) -> *const T {
 }
 
 #[inline(always)]
-unsafe fn get_class_from_attr(attr: &structs::nsAttrValue) -> Class {
+unsafe fn get_class_or_part_from_attr(attr: &structs::nsAttrValue) -> Class {
     debug_assert!(bindings::Gecko_AssertClassAttrValueIsSane(attr));
     let base_type = base_type(attr);
     if base_type == structs::nsAttrValue_ValueBaseType_eStringBase {
@@ -82,15 +82,15 @@ pub fn get_id(attrs: &[structs::AttrArray_InternalAttr]) -> Option<&WeakAtom> {
     Some(unsafe { get_id_from_attr(find_attr(attrs, &atom!("id"))?) })
 }
 
-/// Given a class name, a case sensitivity, and an array of attributes, returns
+/// Given a class or part name, a case sensitivity, and an array of attributes,
-/// whether the class has the attribute that name represents.
+/// returns whether the attribute has that name.
 #[inline(always)]
-pub fn has_class(
+pub fn has_class_or_part(
     name: &Atom,
     case_sensitivity: CaseSensitivity,
     attr: &structs::nsAttrValue,
 ) -> bool {
-    match unsafe { get_class_from_attr(attr) } {
+    match unsafe { get_class_or_part_from_attr(attr) } {
         Class::None => false,
         Class::One(atom) => unsafe { case_sensitivity.eq_atom(name, WeakAtom::new(atom)) },
         Class::More(atoms) => match case_sensitivity {
@@ -114,7 +114,7 @@ where
     F: FnMut(&Atom),
 {
     unsafe {
-        match get_class_from_attr(attr) {
+        match get_class_or_part_from_attr(attr) {
             Class::None => {},
             Class::One(atom) => Atom::with(atom, callback),
             Class::More(atoms) => {

components/style/gecko/wrapper.rs

@@ -573,6 +573,11 @@ impl<'le> GeckoElement<'le> {
         }
     }
 
+    #[inline(always)]
+    fn get_part_attr(&self) -> Option<&structs::nsAttrValue> {
+        snapshot_helpers::find_attr(self.attrs(), &atom!("part"))
+    }
+
     #[inline(always)]
     fn get_class_attr(&self) -> Option<&structs::nsAttrValue> {
         if !self.may_have_class() {
@@ -2259,8 +2264,14 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
         case_sensitivity.eq_atom(element_id, id)
     }
 
-    fn is_part(&self, _name: &Atom) -> bool {
+    #[inline]
-        unimplemented!();
+    fn is_part(&self, name: &Atom) -> bool {
+        let attr = match self.get_part_attr() {
+            Some(c) => c,
+            None => return false,
+        };
+
+        snapshot_helpers::has_class_or_part(name, CaseSensitivity::CaseSensitive, attr)
     }
 
     #[inline(always)]
@@ -2270,7 +2281,7 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
             None => return false,
         };
 
-        snapshot_helpers::has_class(name, case_sensitivity, attr)
+        snapshot_helpers::has_class_or_part(name, case_sensitivity, attr)
     }
 
     #[inline]

components/style/invalidation/element/element_wrapper.rs

@@ -58,6 +58,10 @@ pub trait ElementSnapshot: Sized {
     /// if `has_attrs()` returns true.
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool;
 
+    /// Whether this snapshot represents the part named `name`. Should only be
+    /// called if `has_attrs()` returns true.
+    fn is_part(&self, name: &Atom) -> bool;
+
     /// A callback that should be called for each class of the snapshot. Should
     /// only be called if `has_attrs()` returns true.
     fn each_class<F>(&self, _: F)
@@ -340,8 +344,11 @@ where
         }
     }
 
-    fn is_part(&self, _name: &Atom) -> bool {
+    fn is_part(&self, name: &Atom) -> bool {
-        unimplemented!();
+        match self.snapshot() {
+            Some(snapshot) if snapshot.has_attrs() => snapshot.is_part(name),
+            _ => self.element.is_part(name),
+        }
     }
 
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {