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, 28 insertions, 0 deletions

diff --git a/components/script/dom/macros.rs b/components/script/dom/macros.rs
index a566fee4fb6..021527a286a 100644
--- a/components/script/dom/macros.rs
+++ b/components/script/dom/macros.rs
@@ -632,3 +632,31 @@ macro_rules! handle_potential_webgl_error {
         handle_potential_webgl_error!($context, $call, ());
     };
 }
+
+macro_rules! impl_rare_data (
+    ($type:ty) => (
+        fn rare_data(&self) -> Ref<Option<Box<$type>>> {
+            self.rare_data.borrow()
+        }
+
+        #[allow(dead_code)]
+        fn rare_data_mut(&self) -> RefMut<Option<Box<$type>>> {
+            self.rare_data.borrow_mut()
+        }
+
+        fn ensure_rare_data(&self) -> RefMut<Box<$type>> {
+            let mut rare_data = self.rare_data.borrow_mut();
+            if rare_data.is_none() {
+                *rare_data = Some(Default::default());
+            }
+            RefMut::map(rare_data, |rare_data| {
+                rare_data.as_mut().unwrap()
+            })
+        }
+
+        #[allow(unsafe_code)]
+        fn rare_data_for_layout(&self) -> &Option<Box<$type>> {
+            unsafe { self.rare_data.borrow_for_layout() }
+        }
+    );
+);