Move OpaqueNode to style_traits

components/style_traits/dom.rs

@@ -0,0 +1,26 @@
+/* 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/. */
+
+//! Types used to access the DOM from style calculation.
+
+/// An opaque handle to a node, which, unlike UnsafeNode, cannot be transformed
+/// back into a non-opaque representation. The only safe operation that can be
+/// performed on this node is to compare it to another opaque handle or to another
+/// OpaqueNode.
+///
+/// Layout and Graphics use this to safely represent nodes for comparison purposes.
+/// Because the script task's GC does not trace layout, node data cannot be safely stored in layout
+/// data structures. Also, layout code tends to be faster when the DOM is not being accessed, for
+/// locality reasons. Using `OpaqueNode` enforces this invariant.
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf, Deserialize, Serialize))]
+pub struct OpaqueNode(pub usize);
+
+impl OpaqueNode {
+    /// Returns the address of this node, for debugging purposes.
+    #[inline]
+    pub fn id(&self) -> usize {
+        self.0
+    }
+}