script: Split style and layout data in DOM nodes (#31985)

This change splits the style and layout data in DOM nodes that is
populated by style and layout passes. This makes Servo's data design
more like Gecko's. This allows:

1. Removing the various `StyleAndLayout` data structures used by layout.
2. Removing the `GetStyleAndLayoutData` and
   `GetStyleAndOpaqueLayoutData` traits. Accessing style and layout data
   are now just functions on the `LayoutNode` and `ThreadSafeLayoutNode`
   traits.
3. Styling now doesn't populate layout data. This is is postponed until
   layout itself.
4. Allows the DOM wrappers to no longer have to be generic over the
   layout data. This data was already stored using `std::any::Any` and
   the new code just makes layout responsible for downcasting. Cleaning
   up the generic type parameter in the DOM wrappers can happen in a
   followup change.

The main benefit to all of this is that we should be able to remove
unsafe creation of `ServoLayoutNode` in layout and
`TrustedLayoutNodeAddress` entirely, because `ServoLayoutNode` will be
able to be passed directly from script to layout. In addition, this
removes one more abstraction layer from the layout DOM wrappers, making
the code a lot more understandable.

Note: This increases the measured size of DOM types, but the same data
is stored. It's simply that before that data was stored behind a heap
pointer.

components/layout/wrapper.rs

@@ -32,59 +32,20 @@
 
 use atomic_refcell::{AtomicRef, AtomicRefMut};
 use script_layout_interface::wrapper_traits::{
-    GetStyleAndOpaqueLayoutData, ThreadSafeLayoutElement, ThreadSafeLayoutNode,
+    LayoutNode, ThreadSafeLayoutElement, ThreadSafeLayoutNode,
 };
 use style::dom::{NodeInfo, TElement, TNode};
 use style::selector_parser::RestyleDamage;
 use style::values::computed::counters::ContentItem;
 use style::values::generics::counters::Content;
 
-use crate::data::{LayoutData, LayoutDataFlags, StyleAndLayoutData};
+use crate::data::{InnerLayoutData, LayoutData, LayoutDataFlags};
 
-pub trait LayoutNodeLayoutData<'dom> {
-    fn borrow_layout_data(self) -> Option<AtomicRef<'dom, LayoutData>>;
-    fn mutate_layout_data(self) -> Option<AtomicRefMut<'dom, LayoutData>>;
+pub trait ThreadSafeLayoutNodeHelpers<'dom> {
+    fn borrow_layout_data(self) -> Option<AtomicRef<'dom, InnerLayoutData>>;
+    fn mutate_layout_data(self) -> Option<AtomicRefMut<'dom, InnerLayoutData>>;
     fn flow_debug_id(self) -> usize;
-}
 
-impl<'dom, T> LayoutNodeLayoutData<'dom> for T
-where
-    T: GetStyleAndOpaqueLayoutData<'dom>,
-{
-    fn borrow_layout_data(self) -> Option<AtomicRef<'dom, LayoutData>> {
-        self.get_style_and_layout_data()
-            .map(|d| d.layout_data.borrow())
-    }
-
-    fn mutate_layout_data(self) -> Option<AtomicRefMut<'dom, LayoutData>> {
-        self.get_style_and_layout_data()
-            .map(|d| d.layout_data.borrow_mut())
-    }
-
-    fn flow_debug_id(self) -> usize {
-        self.borrow_layout_data()
-            .map_or(0, |d| d.flow_construction_result.debug_id())
-    }
-}
-
-pub trait GetStyleAndLayoutData<'dom> {
-    fn get_style_and_layout_data(self) -> Option<StyleAndLayoutData<'dom>>;
-}
-
-impl<'dom, T> GetStyleAndLayoutData<'dom> for T
-where
-    T: GetStyleAndOpaqueLayoutData<'dom>,
-{
-    fn get_style_and_layout_data(self) -> Option<StyleAndLayoutData<'dom>> {
-        self.get_style_and_opaque_layout_data()
-            .map(|data| StyleAndLayoutData {
-                style_data: &data.style_data,
-                layout_data: data.generic_data.downcast_ref().unwrap(),
-            })
-    }
-}
-
-pub trait ThreadSafeLayoutNodeHelpers {
     /// Returns the layout data flags for this node.
     fn flags(self) -> LayoutDataFlags;
 
@@ -107,10 +68,26 @@ pub trait ThreadSafeLayoutNodeHelpers {
     fn restyle_damage(self) -> RestyleDamage;
 }
 
-impl<'dom, T> ThreadSafeLayoutNodeHelpers for T
+impl<'dom, T> ThreadSafeLayoutNodeHelpers<'dom> for T
 where
     T: ThreadSafeLayoutNode<'dom>,
 {
+    fn borrow_layout_data(self) -> Option<AtomicRef<'dom, InnerLayoutData>> {
+        self.layout_data()
+            .map(|data| data.downcast_ref::<LayoutData>().unwrap().0.borrow())
+    }
+
+    fn mutate_layout_data(self) -> Option<AtomicRefMut<'dom, InnerLayoutData>> {
+        self.layout_data()
+            .and_then(|data| data.downcast_ref::<LayoutData>())
+            .map(|data| data.0.borrow_mut())
+    }
+
+    fn flow_debug_id(self) -> usize {
+        self.borrow_layout_data()
+            .map_or(0, |d| d.flow_construction_result.debug_id())
+    }
+
     fn flags(self) -> LayoutDataFlags {
         self.borrow_layout_data().as_ref().unwrap().flags
     }
@@ -150,17 +127,16 @@ where
         }
 
         let damage = {
-            let data = match node.get_style_and_layout_data() {
-                Some(data) => data,
-                None => panic!(
+            let (layout_data, style_data) = match (node.layout_data(), node.style_data()) {
+                (Some(layout_data), Some(style_data)) => (layout_data, style_data),
+                _ => panic!(
                     "could not get style and layout data for <{}>",
                     node.as_element().unwrap().local_name()
                 ),
             };
 
-            if !data
-                .layout_data
-                .borrow()
+            let layout_data = layout_data.downcast_ref::<LayoutData>().unwrap().0.borrow();
+            if !layout_data
                 .flags
                 .contains(crate::data::LayoutDataFlags::HAS_BEEN_TRAVERSED)
             {
@@ -169,7 +145,7 @@ where
                 // because that's what the code expects.
                 RestyleDamage::rebuild_and_reflow()
             } else {
-                data.style_data.element_data.borrow().damage
+                style_data.element_data.borrow().damage
             }
         };