Auto merge of #22743 - ferjm:shadowdom, r=emilio

Partial ShadowDOM support

This is just an early WIP, not to take it very seriously yet.

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [x] These changes fix #22719
- [x] There are tests for these changes

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1 files changed, 42 insertions, 0 deletions

diff --git a/components/script/dom/raredata.rs b/components/script/dom/raredata.rs
new file mode 100644
index 00000000000..2372b0b89ab
--- /dev/null
+++ b/components/script/dom/raredata.rs
@@ -0,0 +1,42 @@
+/* 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 crate::dom::bindings::root::Dom;
+use crate::dom::customelementregistry::{
+    CustomElementDefinition, CustomElementReaction, CustomElementState,
+};
+use crate::dom::mutationobserver::RegisteredObserver;
+use crate::dom::shadowroot::ShadowRoot;
+use std::rc::Rc;
+
+//XXX(ferjm) Ideally merge NodeRareData and ElementRareData so they share
+//           storage.
+
+#[derive(Default, JSTraceable, MallocSizeOf)]
+#[must_root]
+pub struct NodeRareData {
+    /// The shadow root the node belongs to.
+    /// This is None if the node is not in a shadow tree or
+    /// if it is a ShadowRoot.
+    pub containing_shadow_root: Option<Dom<ShadowRoot>>,
+    /// Registered observers for this node.
+    pub mutation_observers: Vec<RegisteredObserver>,
+}
+
+#[derive(Default, JSTraceable, MallocSizeOf)]
+#[must_root]
+pub struct ElementRareData {
+    /// https://dom.spec.whatwg.org/#dom-element-shadowroot
+    /// The ShadowRoot this element is host of.
+    /// XXX This is currently not exposed to web content. Only for
+    ///     internal use.
+    pub shadow_root: Option<Dom<ShadowRoot>>,
+    /// <https://html.spec.whatwg.org/multipage/#custom-element-reaction-queue>
+    pub custom_element_reaction_queue: Vec<CustomElementReaction>,
+    /// <https://dom.spec.whatwg.org/#concept-element-custom-element-definition>
+    #[ignore_malloc_size_of = "Rc"]
+    pub custom_element_definition: Option<Rc<CustomElementDefinition>>,
+    /// <https://dom.spec.whatwg.org/#concept-element-custom-element-state>
+    pub custom_element_state: CustomElementState,
+}