style: Add a root bucket to the selector map.

:root can't change without getting unbound from the tree so no fancy stuff other
than that needed.

This removes a lot of revalidation and attribute invalidation matching from the
Chrome, and looks like it should be a good idea in general.

Differential Revision: https://phabricator.services.mozilla.com/D2462

components/style/selector_map.rs

@@ -96,6 +96,8 @@ pub trait SelectorMapEntry: Sized + Clone {
 /// TODO: Tune the initial capacity of the HashMap
 #[derive(Debug, MallocSizeOf)]
 pub struct SelectorMap<T: 'static> {
+    /// Rules that have `:root` selectors.
+    pub root: SmallVec<[T; 1]>,
     /// A hash from an ID to rules which contain that ID selector.
     pub id_hash: MaybeCaseInsensitiveHashMap<Atom, SmallVec<[T; 1]>>,
     /// A hash from a class name to rules which contain that class selector.
@@ -104,7 +106,7 @@ pub struct SelectorMap<T: 'static> {
     pub local_name_hash: PrecomputedHashMap<LocalName, SmallVec<[T; 1]>>,
     /// A hash from namespace to rules which contain that namespace selector.
     pub namespace_hash: PrecomputedHashMap<Namespace, SmallVec<[T; 1]>>,
-    /// Rules that don't have ID, class, or element selectors.
+    /// All other rules.
     pub other: SmallVec<[T; 1]>,
     /// The number of entries in this map.
     pub count: usize,
@@ -124,6 +126,7 @@ impl<T: 'static> SelectorMap<T> {
     /// Trivially constructs an empty `SelectorMap`.
     pub fn new() -> Self {
         SelectorMap {
+            root: SmallVec::new(),
             id_hash: MaybeCaseInsensitiveHashMap::new(),
             class_hash: MaybeCaseInsensitiveHashMap::new(),
             local_name_hash: HashMap::default(),
@@ -135,6 +138,7 @@ impl<T: 'static> SelectorMap<T> {
 
     /// Clears the hashmap retaining storage.
     pub fn clear(&mut self) {
+        self.root.clear();
         self.id_hash.clear();
         self.class_hash.clear();
         self.local_name_hash.clear();
@@ -181,6 +185,19 @@ impl SelectorMap<Rule> {
         // At the end, we're going to sort the rules that we added, so remember
         // where we began.
         let init_len = matching_rules_list.len();
+
+        if rule_hash_target.is_root() {
+            SelectorMap::get_matching_rules(
+                element,
+                &self.root,
+                matching_rules_list,
+                context,
+                flags_setter,
+                cascade_level,
+                shadow_cascade_order,
+            );
+        }
+
         if let Some(id) = rule_hash_target.id() {
             if let Some(rules) = self.id_hash.get(id, quirks_mode) {
                 SelectorMap::get_matching_rules(
@@ -287,6 +304,7 @@ impl<T: SelectorMapEntry> SelectorMap<T> {
         self.count += 1;
 
         let vector = match find_bucket(entry.selector()) {
+            Bucket::Root => &mut self.root,
             Bucket::ID(id) => self.id_hash
                 .try_entry(id.clone(), quirks_mode)?
                 .or_insert_with(SmallVec::new),
@@ -340,6 +358,14 @@ impl<T: SelectorMapEntry> SelectorMap<T> {
         E: TElement,
         F: FnMut(&'a T) -> bool,
     {
+        if element.is_root() {
+            for entry in self.root.iter() {
+                if !f(&entry) {
+                    return false;
+                }
+            }
+        }
+
         if let Some(id) = element.id() {
             if let Some(v) = self.id_hash.get(id, quirks_mode) {
                 for entry in v.iter() {
@@ -444,6 +470,7 @@ impl<T: SelectorMapEntry> SelectorMap<T> {
 }
 
 enum Bucket<'a> {
+    Root,
     ID(&'a Atom),
     Class(&'a Atom),
     LocalName {
@@ -456,6 +483,7 @@ enum Bucket<'a> {
 
 fn specific_bucket_for<'a>(component: &'a Component<SelectorImpl>) -> Bucket<'a> {
     match *component {
+        Component::Root => Bucket::Root,
         Component::ID(ref id) => Bucket::ID(id),
         Component::Class(ref class) => Bucket::Class(class),
         Component::LocalName(ref selector) => Bucket::LocalName {
@@ -498,14 +526,19 @@ fn find_bucket<'a>(mut iter: SelectorIter<'a, SelectorImpl>) -> Bucket<'a> {
         // We basically want to find the most specific bucket,
         // where:
         //
-        //   id > class > local name > namespace > universal.
+        //   root > id > class > local name > namespace > universal.
         //
         for ss in &mut iter {
             let new_bucket = specific_bucket_for(ss);
             match new_bucket {
-                Bucket::ID(..) => return new_bucket,
-                Bucket::Class(..) => {
+                Bucket::Root => return new_bucket,
+                Bucket::ID(..) => {
                     current_bucket = new_bucket;
+                }
+                Bucket::Class(..) => {
+                    if !matches!(current_bucket, Bucket::ID(..)) {
+                        current_bucket = new_bucket;
+                    }
                 },
                 Bucket::LocalName { .. } => {
                     if matches!(current_bucket, Bucket::Universal | Bucket::Namespace(..)) {