Auto merge of #24217 - tdelacour:ISSUE-23995, r=jdm

ISSUE-23995: lazily generate UniqueIds for Nodes

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Updated `Node` so that its `unique_id` is only generated when `unique_id()` is called. This getter now also has assumed the responsibility for saving the `id` to the global hashset on `ScriptThread`.

Felt like I was fighting the borrow-checker a little bit on this one. Definitely a good learning experience for me, but the result strikes me as a little messy?

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #23995 (GitHub issue number if applicable)

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because ___

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components/script/dom/node.rs

@@ -152,8 +152,6 @@ pub struct Node {
     /// Must be sent back to the layout thread to be destroyed when this
     /// node is finalized.
     style_and_layout_data: Cell<Option<OpaqueStyleAndLayoutData>>,
-
-    unique_id: UniqueId,
 }
 
 bitflags! {
@@ -1006,7 +1004,20 @@ impl Node {
     }
 
     pub fn unique_id(&self) -> String {
-        self.unique_id.borrow().to_simple().to_string()
+        let mut rare_data = self.ensure_rare_data();
+
+        if rare_data.unique_id.is_none() {
+            let id = UniqueId::new();
+            ScriptThread::save_node_id(id.borrow().to_simple().to_string());
+            rare_data.unique_id = Some(id);
+        }
+        rare_data
+            .unique_id
+            .as_ref()
+            .unwrap()
+            .borrow()
+            .to_simple()
+            .to_string()
     }
 
     pub fn summarize(&self) -> NodeInfo {
@@ -1654,7 +1665,7 @@ impl Node {
 
     #[allow(unrooted_must_root)]
     fn new_(flags: NodeFlags, doc: Option<&Document>) -> Node {
-        let node = Node {
+        Node {
             eventtarget: EventTarget::new_inherited(),
 
             parent_node: Default::default(),
@@ -1671,13 +1682,7 @@ impl Node {
             ranges: WeakRangeVec::new(),
 
             style_and_layout_data: Cell::new(None),
-
-            unique_id: UniqueId::new(),
-        };
-
-        ScriptThread::save_node_id(node.unique_id());
-
-        node
+        }
     }
 
     // https://dom.spec.whatwg.org/#concept-node-adopt
@@ -3122,7 +3127,7 @@ impl<'a> UnbindContext<'a> {
 }
 
 /// A node's unique ID, for devtools.
-struct UniqueId {
+pub struct UniqueId {
     cell: UnsafeCell<Option<Box<Uuid>>>,
 }
 

components/script/dom/raredata.rs

@@ -7,6 +7,7 @@ use crate::dom::customelementregistry::{
     CustomElementDefinition, CustomElementReaction, CustomElementState,
 };
 use crate::dom::mutationobserver::RegisteredObserver;
+use crate::dom::node::UniqueId;
 use crate::dom::shadowroot::ShadowRoot;
 use std::rc::Rc;
 
@@ -22,6 +23,8 @@ pub struct NodeRareData {
     pub containing_shadow_root: Option<Dom<ShadowRoot>>,
     /// Registered observers for this node.
     pub mutation_observers: Vec<RegisteredObserver>,
+    /// Lazily-generated Unique Id for this node.
+    pub unique_id: Option<UniqueId>,
 }
 
 #[derive(Default, JSTraceable, MallocSizeOf)]