style: Avoid some allocations in selector serialization.

The allocations in display_to_css_identifier show up in the profiles of
bug 1675628.

Differential Revision: https://phabricator.services.mozilla.com/D97856

19 files changed, 220 insertions, 114 deletions

diff --git a/components/style/dom.rs b/components/style/dom.rs
index 351f54af6e3..2edf5ae7dab 100644
--- a/components/style/dom.rs
+++ b/components/style/dom.rs
@@ -20,7 +20,8 @@ use crate::selector_parser::{AttrValue, Lang, PseudoElement, SelectorImpl};
 use crate::shared_lock::{Locked, SharedRwLock};
 use crate::stylist::CascadeData;
 use crate::traversal_flags::TraversalFlags;
-use crate::{Atom, LocalName, Namespace, WeakAtom};
+use crate::values::AtomIdent;
+use crate::{LocalName, Namespace, WeakAtom};
 use atomic_refcell::{AtomicRef, AtomicRefMut};
 use selectors::matching::{ElementSelectorFlags, QuirksMode, VisitedHandlingMode};
 use selectors::sink::Push;
@@ -121,7 +122,7 @@ pub trait TDocument: Sized + Copy + Clone {
     /// return an empty slice.
     fn elements_with_id<'a>(
         &self,
-        _id: &Atom,
+        _id: &AtomIdent,
     ) -> Result<&'a [<Self::ConcreteNode as TNode>::ConcreteElement], ()>
     where
         Self: 'a,
@@ -344,7 +345,7 @@ pub trait TShadowRoot: Sized + Copy + Clone + Debug + PartialEq {
     /// return an empty slice.
     fn elements_with_id<'a>(
         &self,
-        _id: &Atom,
+        _id: &AtomIdent,
     ) -> Result<&'a [<Self::ConcreteNode as TNode>::ConcreteElement], ()>
     where
         Self: 'a,
@@ -520,20 +521,20 @@ pub trait TElement:
     /// Internal iterator for the classes of this element.
     fn each_class<F>(&self, callback: F)
     where
-        F: FnMut(&Atom);
+        F: FnMut(&AtomIdent);
 
     /// Internal iterator for the part names of this element.
     fn each_part<F>(&self, _callback: F)
     where
-        F: FnMut(&Atom),
+        F: FnMut(&AtomIdent),
     {
     }
 
     /// Internal iterator for the part names that this element exports for a
     /// given part name.
-    fn each_exported_part<F>(&self, _name: &Atom, _callback: F)
+    fn each_exported_part<F>(&self, _name: &AtomIdent, _callback: F)
     where
-        F: FnMut(&Atom),
+        F: FnMut(&AtomIdent),
     {
     }
 
diff --git a/components/style/dom_apis.rs b/components/style/dom_apis.rs
index f4e814acd11..a6c50805cbb 100644
--- a/components/style/dom_apis.rs
+++ b/components/style/dom_apis.rs
@@ -10,7 +10,7 @@ use crate::dom::{TDocument, TElement, TNode, TShadowRoot};
 use crate::invalidation::element::invalidation_map::Dependency;
 use crate::invalidation::element::invalidator::{DescendantInvalidationLists, Invalidation};
 use crate::invalidation::element::invalidator::{InvalidationProcessor, InvalidationVector};
-use crate::Atom;
+use crate::values::AtomIdent;
 use selectors::attr::CaseSensitivity;
 use selectors::matching::{self, MatchingContext, MatchingMode};
 use selectors::parser::{Combinator, Component, LocalName, SelectorImpl};
@@ -276,7 +276,7 @@ where
 /// or shadow root that `root` is connected to.
 fn fast_connected_elements_with_id<'a, N>(
     root: N,
-    id: &Atom,
+    id: &AtomIdent,
     quirks_mode: QuirksMode,
 ) -> Result<&'a [N::ConcreteElement], ()>
 where
@@ -305,7 +305,7 @@ where
 /// Collects elements with a given id under `root`, that pass `filter`.
 fn collect_elements_with_id<E, Q, F>(
     root: E::ConcreteNode,
-    id: &Atom,
+    id: &AtomIdent,
     results: &mut Q::Output,
     quirks_mode: QuirksMode,
     mut filter: F,
@@ -398,7 +398,7 @@ where
 }
 
 enum SimpleFilter<'a, Impl: SelectorImpl> {
-    Class(&'a Atom),
+    Class(&'a AtomIdent),
     LocalName(&'a LocalName<Impl>),
 }
 
diff --git a/components/style/gecko/selector_parser.rs b/components/style/gecko/selector_parser.rs
index 6f8ac010e11..c63bc81c95b 100644
--- a/components/style/gecko/selector_parser.rs
+++ b/components/style/gecko/selector_parser.rs
@@ -11,7 +11,7 @@ use crate::invalidation::element::document_state::InvalidationMatchingData;
 use crate::selector_parser::{Direction, SelectorParser};
 use crate::str::starts_with_ignore_ascii_case;
 use crate::string_cache::{Atom, Namespace, WeakAtom, WeakNamespace};
-use crate::values::serialize_atom_identifier;
+use crate::values::{AtomIdent, AtomString};
 use cssparser::{BasicParseError, BasicParseErrorKind, Parser};
 use cssparser::{CowRcStr, SourceLocation, ToCss, Token};
 use selectors::parser::{SelectorParseErrorKind, ParseErrorRecovery};
@@ -39,7 +39,7 @@ bitflags! {
 }
 
 /// The type used to store the language argument to the `:lang` pseudo-class.
-pub type Lang = Atom;
+pub type Lang = AtomIdent;
 
 macro_rules! pseudo_class_name {
     ([$(($css:expr, $name:ident, $state:tt, $flags:tt),)*]) => {
@@ -77,7 +77,7 @@ impl ToCss for NonTSPseudoClass {
                     $(NonTSPseudoClass::$name => concat!(":", $css),)*
                     NonTSPseudoClass::Lang(ref s) => {
                         dest.write_str(":lang(")?;
-                        serialize_atom_identifier(s, dest)?;
+                        s.to_css(dest)?;
                         return dest.write_char(')');
                     },
                     NonTSPseudoClass::MozLocaleDir(ref dir) => {
@@ -270,12 +270,10 @@ pub struct SelectorImpl;
 
 impl ::selectors::SelectorImpl for SelectorImpl {
     type ExtraMatchingData = InvalidationMatchingData;
-    type AttrValue = Atom;
-    type Identifier = Atom;
-    type ClassName = Atom;
-    type PartName = Atom;
-    type LocalName = Atom;
-    type NamespacePrefix = Atom;
+    type AttrValue = AtomString;
+    type Identifier = AtomIdent;
+    type LocalName = AtomIdent;
+    type NamespacePrefix = AtomIdent;
     type NamespaceUrl = Namespace;
     type BorrowedNamespaceUrl = WeakNamespace;
     type BorrowedLocalName = WeakAtom;
@@ -386,7 +384,7 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
         let pseudo_class = match_ignore_ascii_case! { &name,
             "lang" => {
                 let name = parser.expect_ident_or_string()?;
-                NonTSPseudoClass::Lang(Atom::from(name.as_ref()))
+                NonTSPseudoClass::Lang(Lang::from(name.as_ref()))
             },
             "-moz-locale-dir" => {
                 NonTSPseudoClass::MozLocaleDir(Direction::parse(parser)?)
@@ -475,7 +473,7 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
         self.namespaces.default.clone()
     }
 
-    fn namespace_for_prefix(&self, prefix: &Atom) -> Option<Namespace> {
+    fn namespace_for_prefix(&self, prefix: &AtomIdent) -> Option<Namespace> {
         self.namespaces.prefixes.get(prefix).cloned()
     }
 }
diff --git a/components/style/gecko/snapshot.rs b/components/style/gecko/snapshot.rs
index 26b3e1393dd..8ff794a4d8c 100644
--- a/components/style/gecko/snapshot.rs
+++ b/components/style/gecko/snapshot.rs
@@ -14,7 +14,10 @@ use crate::gecko_bindings::structs::ServoElementSnapshot;
 use crate::gecko_bindings::structs::ServoElementSnapshotFlags as Flags;
 use crate::gecko_bindings::structs::ServoElementSnapshotTable;
 use crate::invalidation::element::element_wrapper::ElementSnapshot;
+use crate::selector_parser::AttrValue;
 use crate::string_cache::{Atom, Namespace};
+use crate::values::{AtomIdent, AtomString};
+use crate::LocalName;
 use crate::WeakAtom;
 use selectors::attr::{AttrSelectorOperation, AttrSelectorOperator};
 use selectors::attr::{CaseSensitivity, NamespaceConstraint};
@@ -74,10 +77,10 @@ impl GeckoElementSnapshot {
     #[inline]
     pub fn each_attr_changed<F>(&self, mut callback: F)
     where
-        F: FnMut(&Atom),
+        F: FnMut(&AtomIdent),
     {
         for attr in self.mChangedAttrNames.iter() {
-            unsafe { Atom::with(attr.mRawPtr, &mut callback) }
+            unsafe { AtomIdent::with(attr.mRawPtr, &mut callback) }
         }
     }
 
@@ -85,8 +88,8 @@ impl GeckoElementSnapshot {
     pub fn attr_matches(
         &self,
         ns: &NamespaceConstraint<&Namespace>,
-        local_name: &Atom,
-        operation: &AttrSelectorOperation<&Atom>,
+        local_name: &LocalName,
+        operation: &AttrSelectorOperation<&AttrValue>,
     ) -> bool {
         unsafe {
             match *operation {
@@ -188,7 +191,7 @@ impl ElementSnapshot for GeckoElementSnapshot {
     }
 
     #[inline]
-    fn is_part(&self, name: &Atom) -> bool {
+    fn is_part(&self, name: &AtomIdent) -> bool {
         let attr = match snapshot_helpers::find_attr(&*self.mAttrs, &atom!("part")) {
             Some(attr) => attr,
             None => return false,
@@ -198,12 +201,12 @@ impl ElementSnapshot for GeckoElementSnapshot {
     }
 
     #[inline]
-    fn imported_part(&self, name: &Atom) -> Option<Atom> {
+    fn imported_part(&self, name: &AtomIdent) -> Option<AtomIdent> {
         snapshot_helpers::imported_part(&*self.mAttrs, name)
     }
 
     #[inline]
-    fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
+    fn has_class(&self, name: &AtomIdent, case_sensitivity: CaseSensitivity) -> bool {
         if !self.has_any(Flags::MaybeClass) {
             return false;
         }
@@ -214,7 +217,7 @@ impl ElementSnapshot for GeckoElementSnapshot {
     #[inline]
     fn each_class<F>(&self, callback: F)
     where
-        F: FnMut(&Atom),
+        F: FnMut(&AtomIdent),
     {
         if !self.has_any(Flags::MaybeClass) {
             return;
@@ -224,12 +227,12 @@ impl ElementSnapshot for GeckoElementSnapshot {
     }
 
     #[inline]
-    fn lang_attr(&self) -> Option<Atom> {
+    fn lang_attr(&self) -> Option<AtomString> {
         let ptr = unsafe { bindings::Gecko_SnapshotLangValue(self) };
         if ptr.is_null() {
             None
         } else {
-            Some(unsafe { Atom::from_addrefed(ptr) })
+            Some(AtomString(unsafe { Atom::from_addrefed(ptr) }))
         }
     }
 }
diff --git a/components/style/gecko/snapshot_helpers.rs b/components/style/gecko/snapshot_helpers.rs
index bd905870353..ee8d7dfd704 100644
--- a/components/style/gecko/snapshot_helpers.rs
+++ b/components/style/gecko/snapshot_helpers.rs
@@ -6,7 +6,9 @@
 
 use crate::gecko_bindings::bindings;
 use crate::gecko_bindings::structs::{self, nsAtom};
-use crate::string_cache::{Atom, WeakAtom};
+use crate::string_cache::WeakAtom;
+use crate::values::AtomIdent;
+use crate::Atom;
 use crate::CaseSensitivityExt;
 use selectors::attr::CaseSensitivity;
 
@@ -85,8 +87,8 @@ pub fn get_id(attrs: &[structs::AttrArray_InternalAttr]) -> Option<&WeakAtom> {
 #[inline(always)]
 pub(super) fn each_exported_part(
     attrs: &[structs::AttrArray_InternalAttr],
-    name: &Atom,
-    mut callback: impl FnMut(&Atom),
+    name: &AtomIdent,
+    mut callback: impl FnMut(&AtomIdent),
 ) {
     let attr = match find_attr(attrs, &atom!("exportparts")) {
         Some(attr) => attr,
@@ -100,7 +102,7 @@ pub(super) fn each_exported_part(
 
     unsafe {
         for atom in std::slice::from_raw_parts(atoms, length) {
-            Atom::with(*atom, &mut callback)
+            AtomIdent::with(*atom, &mut callback)
         }
     }
 }
@@ -108,21 +110,21 @@ pub(super) fn each_exported_part(
 #[inline(always)]
 pub(super) fn imported_part(
     attrs: &[structs::AttrArray_InternalAttr],
-    name: &Atom,
-) -> Option<Atom> {
+    name: &AtomIdent,
+) -> Option<AtomIdent> {
     let attr = find_attr(attrs, &atom!("exportparts"))?;
     let atom = unsafe { bindings::Gecko_Element_ImportedPart(attr, name.as_ptr()) };
     if atom.is_null() {
         return None;
     }
-    Some(unsafe { Atom::from_raw(atom) })
+    Some(AtomIdent(unsafe { Atom::from_raw(atom) }))
 }
 
 /// Given a class or part name, a case sensitivity, and an array of attributes,
 /// returns whether the attribute has that name.
 #[inline(always)]
 pub fn has_class_or_part(
-    name: &Atom,
+    name: &AtomIdent,
     case_sensitivity: CaseSensitivity,
     attr: &structs::nsAttrValue,
 ) -> bool {
@@ -147,15 +149,15 @@ pub fn has_class_or_part(
 #[inline(always)]
 pub fn each_class_or_part<F>(attr: &structs::nsAttrValue, mut callback: F)
 where
-    F: FnMut(&Atom),
+    F: FnMut(&AtomIdent),
 {
     unsafe {
         match get_class_or_part_from_attr(attr) {
             Class::None => {},
-            Class::One(atom) => Atom::with(atom, callback),
+            Class::One(atom) => AtomIdent::with(atom, callback),
             Class::More(atoms) => {
                 for atom in atoms {
-                    Atom::with(atom.mRawPtr, &mut callback)
+                    AtomIdent::with(atom.mRawPtr, &mut callback)
                 }
             },
         }
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index 847b822b28d..4773bd89997 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -70,7 +70,9 @@ use crate::stylist::CascadeData;
 use crate::values::computed::font::GenericFontFamily;
 use crate::values::computed::Length;
 use crate::values::specified::length::FontBaseSize;
+use crate::values::{AtomIdent, AtomString};
 use crate::CaseSensitivityExt;
+use crate::LocalName;
 use atomic_refcell::{AtomicRef, AtomicRefCell, AtomicRefMut};
 use selectors::attr::{AttrSelectorOperation, AttrSelectorOperator};
 use selectors::attr::{CaseSensitivity, NamespaceConstraint};
@@ -130,7 +132,7 @@ impl<'ld> TDocument for GeckoDocument<'ld> {
     }
 
     #[inline]
-    fn elements_with_id<'a>(&self, id: &Atom) -> Result<&'a [GeckoElement<'ld>], ()>
+    fn elements_with_id<'a>(&self, id: &AtomIdent) -> Result<&'a [GeckoElement<'ld>], ()>
     where
         Self: 'a,
     {
@@ -186,7 +188,7 @@ impl<'lr> TShadowRoot for GeckoShadowRoot<'lr> {
     }
 
     #[inline]
-    fn elements_with_id<'a>(&self, id: &Atom) -> Result<&'a [GeckoElement<'lr>], ()>
+    fn elements_with_id<'a>(&self, id: &AtomIdent) -> Result<&'a [GeckoElement<'lr>], ()>
     where
         Self: 'a,
     {
@@ -801,7 +803,7 @@ impl<'le> GeckoElement<'le> {
             return false;
         }
         let host = self.containing_shadow_host().unwrap();
-        host.is_svg_element() && host.local_name() == &*local_name!("use")
+        host.is_svg_element() && host.local_name() == &**local_name!("use")
     }
 
     fn css_transitions_info(&self) -> FxHashMap<LonghandId, Arc<AnimationValue>> {
@@ -1270,7 +1272,7 @@ impl<'le> TElement for GeckoElement<'le> {
     }
 
     #[inline]
-    fn has_attr(&self, namespace: &Namespace, attr: &Atom) -> bool {
+    fn has_attr(&self, namespace: &Namespace, attr: &AtomIdent) -> bool {
         unsafe { bindings::Gecko_HasAttr(self.0, namespace.0.as_ptr(), attr.as_ptr()) }
     }
 
@@ -1297,7 +1299,7 @@ impl<'le> TElement for GeckoElement<'le> {
 
     fn each_class<F>(&self, callback: F)
     where
-        F: FnMut(&Atom),
+        F: FnMut(&AtomIdent),
     {
         let attr = match self.get_class_attr() {
             Some(c) => c,
@@ -1308,16 +1310,16 @@ impl<'le> TElement for GeckoElement<'le> {
     }
 
     #[inline]
-    fn each_exported_part<F>(&self, name: &Atom, callback: F)
+    fn each_exported_part<F>(&self, name: &AtomIdent, callback: F)
     where
-        F: FnMut(&Atom),
+        F: FnMut(&AtomIdent),
     {
         snapshot_helpers::each_exported_part(self.attrs(), name, callback)
     }
 
     fn each_part<F>(&self, callback: F)
     where
-        F: FnMut(&Atom),
+        F: FnMut(&AtomIdent),
     {
         let attr = match self.get_part_attr() {
             Some(c) => c,
@@ -1615,7 +1617,7 @@ impl<'le> TElement for GeckoElement<'le> {
         if ptr.is_null() {
             None
         } else {
-            Some(unsafe { Atom::from_addrefed(ptr) })
+            Some(AtomString(unsafe { Atom::from_addrefed(ptr) }))
         }
     }
 
@@ -1623,8 +1625,8 @@ impl<'le> TElement for GeckoElement<'le> {
         // Gecko supports :lang() from CSS Selectors 3, which only accepts a
         // single language tag, and which performs simple dash-prefix matching
         // on it.
-        let override_lang_ptr = match &override_lang {
-            &Some(Some(ref atom)) => atom.as_ptr(),
+        let override_lang_ptr = match override_lang {
+            Some(Some(ref atom)) => atom.as_ptr(),
             _ => ptr::null_mut(),
         };
         unsafe {
@@ -1638,7 +1640,7 @@ impl<'le> TElement for GeckoElement<'le> {
     }
 
     fn is_html_document_body_element(&self) -> bool {
-        if self.local_name() != &*local_name!("body") {
+        if self.local_name() != &**local_name!("body") {
             return false;
         }
 
@@ -1895,8 +1897,8 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
     fn attr_matches(
         &self,
         ns: &NamespaceConstraint<&Namespace>,
-        local_name: &Atom,
-        operation: &AttrSelectorOperation<&Atom>,
+        local_name: &LocalName,
+        operation: &AttrSelectorOperation<&AttrValue>,
     ) -> bool {
         unsafe {
             match *operation {
@@ -2180,7 +2182,7 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
     }
 
     #[inline]
-    fn has_id(&self, id: &Atom, case_sensitivity: CaseSensitivity) -> bool {
+    fn has_id(&self, id: &AtomIdent, case_sensitivity: CaseSensitivity) -> bool {
         if !self.has_id() {
             return false;
         }
@@ -2194,7 +2196,7 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
     }
 
     #[inline]
-    fn is_part(&self, name: &Atom) -> bool {
+    fn is_part(&self, name: &AtomIdent) -> bool {
         let attr = match self.get_part_attr() {
             Some(c) => c,
             None => return false,
@@ -2204,12 +2206,12 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
     }
 
     #[inline]
-    fn imported_part(&self, name: &Atom) -> Option<Atom> {
+    fn imported_part(&self, name: &AtomIdent) -> Option<AtomIdent> {
         snapshot_helpers::imported_part(self.attrs(), name)
     }
 
     #[inline(always)]
-    fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
+    fn has_class(&self, name: &AtomIdent, case_sensitivity: CaseSensitivity) -> bool {
         let attr = match self.get_class_attr() {
             Some(c) => c,
             None => return false,
diff --git a/components/style/gecko_string_cache/mod.rs b/components/style/gecko_string_cache/mod.rs
index e9208a5e32e..a6cf017877d 100644
--- a/components/style/gecko_string_cache/mod.rs
+++ b/components/style/gecko_string_cache/mod.rs
@@ -42,12 +42,6 @@ pub mod namespace;
 
 pub use self::namespace::{Namespace, WeakNamespace};
 
-macro_rules! local_name {
-    ($s:tt) => {
-        atom!($s)
-    };
-}
-
 /// A handle to a Gecko atom. This is a type that can represent either:
 ///
 ///  * A strong reference to a dynamic atom (an `nsAtom` pointer), in which case
diff --git a/components/style/gecko_string_cache/namespace.rs b/components/style/gecko_string_cache/namespace.rs
index 72de229f1d7..1acf04c93ff 100644
--- a/components/style/gecko_string_cache/namespace.rs
+++ b/components/style/gecko_string_cache/namespace.rs
@@ -47,18 +47,9 @@ impl PrecomputedHash for Namespace {
 }
 
 /// A Gecko WeakNamespace is a wrapped WeakAtom.
-#[derive(Hash)]
+#[derive(Hash, Deref)]
 pub struct WeakNamespace(WeakAtom);
 
-impl Deref for WeakNamespace {
-    type Target = WeakAtom;
-
-    #[inline]
-    fn deref(&self) -> &WeakAtom {
-        &self.0
-    }
-}
-
 impl Deref for Namespace {
     type Target = WeakNamespace;
 
diff --git a/components/style/invalidation/element/element_wrapper.rs b/components/style/invalidation/element/element_wrapper.rs
index 2b6f934c8b7..953c07f46c7 100644
--- a/components/style/invalidation/element/element_wrapper.rs
+++ b/components/style/invalidation/element/element_wrapper.rs
@@ -9,7 +9,8 @@ use crate::dom::TElement;
 use crate::element_state::ElementState;
 use crate::selector_parser::{AttrValue, NonTSPseudoClass, PseudoElement, SelectorImpl};
 use crate::selector_parser::{Snapshot, SnapshotMap};
-use crate::{Atom, CaseSensitivityExt, LocalName, Namespace, WeakAtom};
+use crate::values::AtomIdent;
+use crate::{CaseSensitivityExt, LocalName, Namespace, WeakAtom};
 use selectors::attr::{AttrSelectorOperation, CaseSensitivity, NamespaceConstraint};
 use selectors::matching::{ElementSelectorFlags, MatchingContext};
 use selectors::{Element, OpaqueElement};
@@ -56,20 +57,20 @@ pub trait ElementSnapshot: Sized {
 
     /// Whether this snapshot contains the class `name`. Should only be called
     /// if `has_attrs()` returns true.
-    fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool;
+    fn has_class(&self, name: &AtomIdent, 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;
+    fn is_part(&self, name: &AtomIdent) -> bool;
 
     /// See Element::imported_part.
-    fn imported_part(&self, name: &Atom) -> Option<Atom>;
+    fn imported_part(&self, name: &AtomIdent) -> Option<AtomIdent>;
 
     /// 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)
     where
-        F: FnMut(&Atom);
+        F: FnMut(&AtomIdent);
 
     /// The `xml:lang=""` or `lang=""` attribute value per this snapshot.
     fn lang_attr(&self) -> Option<AttrValue>;
@@ -361,7 +362,7 @@ where
         }
     }
 
-    fn has_id(&self, id: &Atom, case_sensitivity: CaseSensitivity) -> bool {
+    fn has_id(&self, id: &AtomIdent, case_sensitivity: CaseSensitivity) -> bool {
         match self.snapshot() {
             Some(snapshot) if snapshot.has_attrs() => snapshot
                 .id_attr()
@@ -370,21 +371,21 @@ where
         }
     }
 
-    fn is_part(&self, name: &Atom) -> bool {
+    fn is_part(&self, name: &AtomIdent) -> bool {
         match self.snapshot() {
             Some(snapshot) if snapshot.has_attrs() => snapshot.is_part(name),
             _ => self.element.is_part(name),
         }
     }
 
-    fn imported_part(&self, name: &Atom) -> Option<Atom> {
+    fn imported_part(&self, name: &AtomIdent) -> Option<AtomIdent> {
         match self.snapshot() {
             Some(snapshot) if snapshot.has_attrs() => snapshot.imported_part(name),
             _ => self.element.imported_part(name),
         }
     }
 
-    fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
+    fn has_class(&self, name: &AtomIdent, case_sensitivity: CaseSensitivity) -> bool {
         match self.snapshot() {
             Some(snapshot) if snapshot.has_attrs() => snapshot.has_class(name, case_sensitivity),
             _ => self.element.has_class(name, case_sensitivity),
diff --git a/components/style/invalidation/element/invalidation_map.rs b/components/style/invalidation/element/invalidation_map.rs
index ea08bb4f4fb..29e0962a897 100644
--- a/components/style/invalidation/element/invalidation_map.rs
+++ b/components/style/invalidation/element/invalidation_map.rs
@@ -478,7 +478,7 @@ impl<'a> SelectorVisitor for SelectorDependencyCollector<'a> {
                     Component::Class(..) => &mut self.map.class_to_selector,
                     _ => unreachable!(),
                 };
-                let entry = match map.try_entry(atom.clone(), self.quirks_mode) {
+                let entry = match map.try_entry(atom.0.clone(), self.quirks_mode) {
                     Ok(entry) => entry,
                     Err(err) => {
                         *self.alloc_error = Some(err);
diff --git a/components/style/invalidation/element/state_and_attributes.rs b/components/style/invalidation/element/state_and_attributes.rs
index 79f1b53ada7..128feae76be 100644
--- a/components/style/invalidation/element/state_and_attributes.rs
+++ b/components/style/invalidation/element/state_and_attributes.rs
@@ -216,13 +216,13 @@ where
             // TODO(emilio): Do this more efficiently!
             snapshot.each_class(|c| {
                 if !element.has_class(c, CaseSensitivity::CaseSensitive) {
-                    classes_removed.push(c.clone())
+                    classes_removed.push(c.0.clone())
                 }
             });
 
             element.each_class(|c| {
                 if !snapshot.has_class(c, CaseSensitivity::CaseSensitive) {
-                    classes_added.push(c.clone())
+                    classes_added.push(c.0.clone())
                 }
             })
         }
diff --git a/components/style/invalidation/stylesheets.rs b/components/style/invalidation/stylesheets.rs
index 8ac2b57b34e..9854387ccad 100644
--- a/components/style/invalidation/stylesheets.rs
+++ b/components/style/invalidation/stylesheets.rs
@@ -17,6 +17,7 @@ use crate::selector_parser::{SelectorImpl, Snapshot, SnapshotMap};
 use crate::shared_lock::SharedRwLockReadGuard;
 use crate::stylesheets::{CssRule, StylesheetInDocument};
 use crate::stylesheets::{EffectiveRules, EffectiveRulesIterator};
+use crate::values::AtomIdent;
 use crate::Atom;
 use crate::LocalName as SelectorLocalName;
 use selectors::parser::{Component, LocalName, Selector};
@@ -43,9 +44,9 @@ pub enum RuleChangeKind {
 #[derive(Debug, Eq, Hash, MallocSizeOf, PartialEq)]
 enum Invalidation {
     /// An element with a given id.
-    ID(Atom),
+    ID(AtomIdent),
     /// An element with a given class name.
-    Class(Atom),
+    Class(AtomIdent),
     /// An element with a given local name.
     LocalName {
         name: SelectorLocalName,
@@ -468,14 +469,14 @@ impl StylesheetInvalidationSet {
     ) -> bool {
         match invalidation {
             Invalidation::Class(c) => {
-                let entry = match self.classes.try_entry(c, quirks_mode) {
+                let entry = match self.classes.try_entry(c.0, quirks_mode) {
                     Ok(e) => e,
                     Err(..) => return false,
                 };
                 *entry.or_insert(InvalidationKind::None) |= kind;
             },
             Invalidation::ID(i) => {
-                let entry = match self.ids.try_entry(i, quirks_mode) {
+                let entry = match self.ids.try_entry(i.0, quirks_mode) {
                     Ok(e) => e,
                     Err(..) => return false,
                 };
diff --git a/components/style/lib.rs b/components/style/lib.rs
index 8b8af1073ae..f1d36b2722c 100644
--- a/components/style/lib.rs
+++ b/components/style/lib.rs
@@ -131,10 +131,10 @@ pub use crate::gecko_string_cache as string_cache;
 pub use crate::gecko_string_cache::Atom;
 /// The namespace prefix type for Gecko, which is just an atom.
 #[cfg(feature = "gecko")]
-pub type Prefix = crate::gecko_string_cache::Atom;
+pub type Prefix = crate::values::AtomIdent;
 /// The local name of an element for Gecko, which is just an atom.
 #[cfg(feature = "gecko")]
-pub type LocalName = crate::gecko_string_cache::Atom;
+pub type LocalName = crate::values::AtomIdent;
 #[cfg(feature = "gecko")]
 pub use crate::gecko_string_cache::Namespace;
 
diff --git a/components/style/macros.rs b/components/style/macros.rs
index b4f4f1f7caf..e60be8efc5f 100644
--- a/components/style/macros.rs
+++ b/components/style/macros.rs
@@ -133,3 +133,10 @@ macro_rules! profiler_label {
 macro_rules! profiler_label {
     ($label_type:ident) => {};
 }
+
+#[cfg(feature = "gecko")]
+macro_rules! local_name {
+    ($s:tt) => {
+        $crate::values::AtomIdent(atom!($s))
+    };
+}
diff --git a/components/style/rule_collector.rs b/components/style/rule_collector.rs
index 884abc07c63..86293e5c785 100644
--- a/components/style/rule_collector.rs
+++ b/components/style/rule_collector.rs
@@ -13,7 +13,6 @@ use crate::selector_parser::PseudoElement;
 use crate::shared_lock::Locked;
 use crate::stylesheets::Origin;
 use crate::stylist::{AuthorStylesEnabled, Rule, RuleInclusion, Stylist};
-use crate::Atom;
 use selectors::matching::{ElementSelectorFlags, MatchingContext, MatchingMode};
 use servo_arc::ArcBorrow;
 use smallvec::SmallVec;
@@ -43,7 +42,7 @@ pub fn containing_shadow_ignoring_svg_use<E: TElement>(
     loop {
         let host = shadow.host();
         let host_is_svg_use_element =
-            host.is_svg_element() && host.local_name() == &*local_name!("use");
+            host.is_svg_element() && host.local_name() == &**local_name!("use");
         if !host_is_svg_use_element {
             return Some(shadow);
         }
@@ -319,7 +318,7 @@ where
             };
 
             hash_target.each_part(|part| {
-                if let Some(part_rules) = part_rules.get(part) {
+                if let Some(part_rules) = part_rules.get(&part.0) {
                     collector.collect_rules_in_list(part_rules, cascade_level);
                 }
             });
@@ -377,7 +376,7 @@ where
 
         let mut shadow_cascade_order = ShadowCascadeOrder::for_innermost_containing_tree();
 
-        let mut parts = SmallVec::<[Atom; 3]>::new();
+        let mut parts = SmallVec::<[_; 3]>::new();
         self.rule_hash_target.each_part(|p| parts.push(p.clone()));
 
         loop {
@@ -405,7 +404,7 @@ where
                 };
                 self.in_tree(containing_host, |collector| {
                     for p in &parts {
-                        if let Some(part_rules) = part_rules.get(p) {
+                        if let Some(part_rules) = part_rules.get(&p.0) {
                             collector.collect_rules_in_list(part_rules, cascade_level);
                         }
                     }
diff --git a/components/style/sharing/mod.rs b/components/style/sharing/mod.rs
index fab21901bd9..9adb7afb8cd 100644
--- a/components/style/sharing/mod.rs
+++ b/components/style/sharing/mod.rs
@@ -73,7 +73,7 @@ use crate::properties::ComputedValues;
 use crate::rule_tree::StrongRuleNode;
 use crate::style_resolver::{PrimaryStyle, ResolvedElementStyles};
 use crate::stylist::Stylist;
-use crate::Atom;
+use crate::values::AtomIdent;
 use atomic_refcell::{AtomicRefCell, AtomicRefMut};
 use owning_ref::OwningHandle;
 use selectors::matching::{ElementSelectorFlags, VisitedHandlingMode};
@@ -126,13 +126,13 @@ pub struct ValidationData {
     ///
     /// TODO(emilio): Maybe check whether rules for these classes apply to the
     /// element?
-    class_list: Option<SmallVec<[Atom; 5]>>,
+    class_list: Option<SmallVec<[AtomIdent; 5]>>,
 
     /// The part list of this element.
     ///
     /// TODO(emilio): Maybe check whether rules with these part names apply to
     /// the element?
-    part_list: Option<SmallVec<[Atom; 5]>>,
+    part_list: Option<SmallVec<[AtomIdent; 5]>>,
 
     /// The list of presentational attributes of the element.
     pres_hints: Option<SmallVec<[ApplicableDeclarationBlock; 5]>>,
@@ -168,7 +168,7 @@ impl ValidationData {
     }
 
     /// Get or compute the part-list associated with this element.
-    pub fn part_list<E>(&mut self, element: E) -> &[Atom]
+    pub fn part_list<E>(&mut self, element: E) -> &[AtomIdent]
     where
         E: TElement,
     {
@@ -176,7 +176,7 @@ impl ValidationData {
             return &[];
         }
         self.part_list.get_or_insert_with(|| {
-            let mut list = SmallVec::<[Atom; 5]>::new();
+            let mut list = SmallVec::<[_; 5]>::new();
             element.each_part(|p| list.push(p.clone()));
             // See below for the reasoning.
             if !list.spilled() {
@@ -187,12 +187,12 @@ impl ValidationData {
     }
 
     /// Get or compute the class-list associated with this element.
-    pub fn class_list<E>(&mut self, element: E) -> &[Atom]
+    pub fn class_list<E>(&mut self, element: E) -> &[AtomIdent]
     where
         E: TElement,
     {
         self.class_list.get_or_insert_with(|| {
-            let mut list = SmallVec::<[Atom; 5]>::new();
+            let mut list = SmallVec::<[_; 5]>::new();
             element.each_class(|c| list.push(c.clone()));
             // Assuming there are a reasonable number of classes (we use the
             // inline capacity as "reasonable number"), sort them to so that
@@ -294,12 +294,12 @@ impl<E: TElement> Deref for StyleSharingCandidate<E> {
 
 impl<E: TElement> StyleSharingCandidate<E> {
     /// Get the classlist of this candidate.
-    fn class_list(&mut self) -> &[Atom] {
+    fn class_list(&mut self) -> &[AtomIdent] {
         self.validation_data.class_list(self.element)
     }
 
     /// Get the part list of this candidate.
-    fn part_list(&mut self) -> &[Atom] {
+    fn part_list(&mut self) -> &[AtomIdent] {
         self.validation_data.part_list(self.element)
     }
 
@@ -361,11 +361,11 @@ impl<E: TElement> StyleSharingTarget<E> {
         }
     }
 
-    fn class_list(&mut self) -> &[Atom] {
+    fn class_list(&mut self) -> &[AtomIdent] {
         self.validation_data.class_list(self.element)
     }
 
-    fn part_list(&mut self) -> &[Atom] {
+    fn part_list(&mut self) -> &[AtomIdent] {
         self.validation_data.part_list(self.element)
     }
 
diff --git a/components/style/style_adjuster.rs b/components/style/style_adjuster.rs
index db950397c37..cc0c4a21d72 100644
--- a/components/style/style_adjuster.rs
+++ b/components/style/style_adjuster.rs
@@ -806,7 +806,7 @@ impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
                 return;
             }
             let is_html_select_element = element.map_or(false, |e| {
-                e.is_html_element() && e.local_name() == &*local_name!("select")
+                e.is_html_element() && e.local_name() == &*atom!("select")
             });
             if !is_html_select_element {
                 return;
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index 8cf9efb3927..418a86bc5e0 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -1651,7 +1651,7 @@ impl<'a> SelectorVisitor for StylistSelectorVisitor<'a> {
                 //
                 // NOTE(emilio): See the comment regarding on when this may
                 // break in visit_complex_selector.
-                self.mapped_ids.insert(id.clone());
+                self.mapped_ids.insert(id.0.clone());
             },
             _ => {},
         }
@@ -2078,7 +2078,7 @@ impl CascadeData {
                             self.part_rules
                                 .get_or_insert_with(|| Box::new(Default::default()))
                                 .for_insertion(pseudo_element)
-                                .try_entry(parts.last().unwrap().clone())?
+                                .try_entry(parts.last().unwrap().clone().0)?
                                 .or_insert_with(SmallVec::new)
                                 .try_push(rule)?;
                         } else {
diff --git a/components/style/values/mod.rs b/components/style/values/mod.rs
index 36ea909ab87..6c088535d7f 100644
--- a/components/style/values/mod.rs
+++ b/components/style/values/mod.rs
@@ -13,6 +13,7 @@ use crate::values::distance::{ComputeSquaredDistance, SquaredDistance};
 use crate::Atom;
 pub use cssparser::{serialize_identifier, serialize_name, CowRcStr, Parser};
 pub use cssparser::{SourceLocation, Token, RGBA};
+use precomputed_hash::PrecomputedHash;
 use selectors::parser::SelectorParseErrorKind;
 use std::fmt::{self, Debug, Write};
 use std::hash;
@@ -84,6 +85,112 @@ where
     serialize_name(&ident, dest)
 }
 
+/// A CSS string stored as an `Atom`.
+#[repr(transparent)]
+#[derive(
+    Clone,
+    Debug,
+    Default,
+    Deref,
+    Eq,
+    Hash,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToResolvedValue,
+    ToShmem,
+)]
+pub struct AtomString(pub Atom);
+
+impl cssparser::ToCss for AtomString {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+    where
+        W: Write,
+    {
+        self.0
+            .with_str(|s| cssparser::CssStringWriter::new(dest).write_str(s))
+    }
+}
+
+impl PrecomputedHash for AtomString {
+    #[inline]
+    fn precomputed_hash(&self) -> u32 {
+        self.0.precomputed_hash()
+    }
+}
+
+impl<'a> From<&'a str> for AtomString {
+    #[inline]
+    fn from(string: &str) -> Self {
+        Self(Atom::from(string))
+    }
+}
+
+/// A CSS `<ident>` stored as an `Atom`.
+#[repr(transparent)]
+#[derive(
+    Clone,
+    Debug,
+    Default,
+    Deref,
+    Eq,
+    Hash,
+    MallocSizeOf,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToResolvedValue,
+    ToShmem,
+)]
+pub struct AtomIdent(pub Atom);
+
+impl cssparser::ToCss for AtomIdent {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+    where
+        W: Write,
+    {
+        serialize_atom_identifier(&self.0, dest)
+    }
+}
+
+impl PrecomputedHash for AtomIdent {
+    #[inline]
+    fn precomputed_hash(&self) -> u32 {
+        self.0.precomputed_hash()
+    }
+}
+
+impl<'a> From<&'a str> for AtomIdent {
+    #[inline]
+    fn from(string: &str) -> Self {
+        Self(Atom::from(string))
+    }
+}
+
+impl AtomIdent {
+    /// Like `Atom::with` but for `AtomIdent`.
+    #[cfg(feature = "gecko")]
+    pub unsafe fn with<F, R>(ptr: *const crate::gecko_bindings::structs::nsAtom, callback: F) -> R
+    where
+        F: FnOnce(&Self) -> R,
+    {
+        Atom::with(ptr, |atom: &Atom| {
+            // safety: repr(transparent)
+            let atom = atom as *const Atom as *const AtomIdent;
+            callback(&*atom)
+        })
+    }
+}
+
+#[cfg(feature = "gecko")]
+impl std::borrow::Borrow<crate::gecko_string_cache::WeakAtom> for AtomIdent {
+    #[inline]
+    fn borrow(&self) -> &crate::gecko_string_cache::WeakAtom {
+        self.0.borrow()
+    }
+}
+
 /// Serialize a normalized value into percentage.
 pub fn serialize_percentage<W>(value: CSSFloat, dest: &mut CssWriter<W>) -> fmt::Result
 where