script: Support custom element states (#38564)

Also adds support for `:state`.

Testing: Covered by existing tests

---------

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/script/dom/customstateset.rs

@@ -0,0 +1,107 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use dom_struct::dom_struct;
+use indexmap::IndexSet;
+use script_bindings::codegen::GenericBindings::ElementInternalsBinding::CustomStateSetMethods;
+use script_bindings::like::Setlike;
+use script_bindings::root::{Dom, DomRoot};
+use script_bindings::script_runtime::CanGc;
+use script_bindings::str::DOMString;
+use script_bindings::trace::CustomTraceable;
+use style::values::AtomIdent;
+
+use crate::dom::bindings::cell::DomRefCell;
+use crate::dom::bindings::inheritance::Castable;
+use crate::dom::bindings::reflector::{Reflector, reflect_dom_object};
+use crate::dom::htmlelement::HTMLElement;
+use crate::dom::node::{Node, NodeDamage};
+use crate::dom::window::Window;
+
+/// <https://html.spec.whatwg.org/multipage/#customstateset>
+#[dom_struct]
+pub(crate) struct CustomStateSet {
+    reflector: Reflector,
+    internal: DomRefCell<IndexSet<DOMString>>,
+    owner_element: Dom<HTMLElement>,
+}
+
+impl CustomStateSet {
+    fn new_inherited(element: &HTMLElement) -> Self {
+        Self {
+            reflector: Reflector::new(),
+            internal: DomRefCell::new(Default::default()),
+            owner_element: Dom::from_ref(element),
+        }
+    }
+
+    pub(crate) fn new(window: &Window, element: &HTMLElement, can_gc: CanGc) -> DomRoot<Self> {
+        reflect_dom_object(Box::new(Self::new_inherited(element)), window, can_gc)
+    }
+
+    pub(crate) fn for_each_state<F>(&self, mut callback: F)
+    where
+        F: FnMut(&AtomIdent),
+    {
+        // FIXME: This creates new atoms whenever it is called, which is not optimal.
+        for state in self.internal.borrow().iter() {
+            callback(&AtomIdent::from(state.str()));
+        }
+    }
+
+    fn states_did_change(&self) {
+        self.owner_element.upcast::<Node>().dirty(NodeDamage::Other);
+    }
+}
+
+impl Setlike for CustomStateSet {
+    type Key = DOMString;
+
+    #[inline(always)]
+    fn get_index(&self, index: u32) -> Option<Self::Key> {
+        self.internal.get_index(index)
+    }
+
+    #[inline(always)]
+    fn size(&self) -> u32 {
+        self.internal.size()
+    }
+
+    #[inline(always)]
+    fn add(&self, key: Self::Key) {
+        self.internal.add(key);
+        self.states_did_change();
+    }
+
+    #[inline(always)]
+    fn has(&self, key: Self::Key) -> bool {
+        self.internal.has(key)
+    }
+
+    #[inline(always)]
+    fn clear(&self) {
+        let old_size = self.internal.size();
+        self.internal.clear();
+        if old_size != 0 {
+            self.states_did_change();
+        }
+    }
+
+    #[inline(always)]
+    fn delete(&self, key: Self::Key) -> bool {
+        if self.internal.delete(key) {
+            self.states_did_change();
+            true
+        } else {
+            false
+        }
+    }
+}
+
+impl CustomStateSetMethods<crate::DomTypeHolder> for CustomStateSet {
+    /// <https://html.spec.whatwg.org/multipage/#customstateset>
+    fn Size(&self) -> u32 {
+        self.internal.size()
+    }
+}