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.

2 files changed, 31 insertions, 47 deletions

diff --git a/components/shared/script_layout/lib.rs b/components/shared/script_layout/lib.rs
index aca819980d7..a09e0fcf1aa 100644
--- a/components/shared/script_layout/lib.rs
+++ b/components/shared/script_layout/lib.rs
@@ -47,6 +47,8 @@ use style::stylesheets::Stylesheet;
 use style_traits::CSSPixel;
 use webrender_api::{ExternalScrollId, ImageKey};
 
+pub type GenericLayoutData = dyn Any + Send + Sync;
+
 #[derive(MallocSizeOf)]
 pub struct StyleData {
     /// Data that the style system associates with a node. When the
@@ -69,33 +71,6 @@ impl Default for StyleData {
     }
 }
 
-pub type StyleAndOpaqueLayoutData = StyleAndGenericData<dyn Any + Send + Sync>;
-
-#[derive(MallocSizeOf)]
-pub struct StyleAndGenericData<T>
-where
-    T: ?Sized,
-{
-    /// The style data.
-    pub style_data: StyleData,
-    /// The opaque layout data.
-    #[ignore_malloc_size_of = "Trait objects are hard"]
-    pub generic_data: T,
-}
-
-impl StyleAndOpaqueLayoutData {
-    #[inline]
-    pub fn new<T>(style_data: StyleData, layout_data: T) -> Box<Self>
-    where
-        T: Any + Send + Sync,
-    {
-        Box::new(StyleAndGenericData {
-            style_data,
-            generic_data: layout_data,
-        })
-    }
-}
-
 /// Information that we need stored in each DOM node.
 #[derive(Default, MallocSizeOf)]
 pub struct DomParallelInfo {
diff --git a/components/shared/script_layout/wrapper_traits.rs b/components/shared/script_layout/wrapper_traits.rs
index ad37763c202..c1bd1fc4d95 100644
--- a/components/shared/script_layout/wrapper_traits.rs
+++ b/components/shared/script_layout/wrapper_traits.rs
@@ -25,7 +25,9 @@ use style::selector_parser::{PseudoElement, PseudoElementCascadeType, SelectorIm
 use style::stylist::RuleInclusion;
 use webrender_api::ExternalScrollId;
 
-use crate::{HTMLCanvasData, HTMLMediaData, LayoutNodeType, SVGSVGData, StyleAndOpaqueLayoutData};
+use crate::{
+    GenericLayoutData, HTMLCanvasData, HTMLMediaData, LayoutNodeType, SVGSVGData, StyleData,
+};
 
 pub trait LayoutDataTrait: Default + Send + Sync + 'static {}
 
@@ -70,19 +72,12 @@ impl PseudoElementType {
     }
 }
 
-/// Trait to abstract access to layout data across various data structures.
-pub trait GetStyleAndOpaqueLayoutData<'dom> {
-    fn get_style_and_opaque_layout_data(self) -> Option<&'dom StyleAndOpaqueLayoutData>;
-}
-
 /// A wrapper so that layout can access only the methods that it should have access to. Layout must
 /// only ever see these and must never see instances of `LayoutDom`.
 /// FIXME(mrobinson): `Send + Sync` is required here for Layout 2020, but eventually it
 /// should stop sending LayoutNodes to other threads and rely on ThreadSafeLayoutNode
 /// or some other mechanism to ensure thread safety.
-pub trait LayoutNode<'dom>:
-    Copy + Debug + GetStyleAndOpaqueLayoutData<'dom> + TNode + Send + Sync
-{
+pub trait LayoutNode<'dom>: Copy + Debug + TNode + Send + Sync {
     type ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode<'dom>;
     fn to_threadsafe(&self) -> Self::ConcreteThreadSafeLayoutNode;
 
@@ -96,7 +91,23 @@ pub trait LayoutNode<'dom>:
     /// This method is unsafe because it modifies the given node during
     /// layout. Callers should ensure that no other layout thread is
     /// attempting to read or modify the opaque layout data of this node.
-    unsafe fn initialize_data(&self);
+    unsafe fn initialize_style_and_layout_data<RequestedLayoutDataType: LayoutDataTrait>(&self);
+
+    /// Initialize this node with empty opaque layout data.
+    ///
+    /// # Safety
+    ///
+    /// This method is unsafe because it modifies the given node during
+    /// layout. Callers should ensure that no other layout thread is
+    /// attempting to read or modify the opaque layout data of this node.
+    fn initialize_layout_data<RequestedLayoutDataType: LayoutDataTrait>(&self);
+
+    /// Get the [`StyleData`] for this node. Returns None if the node is unstyled.
+    fn style_data(&self) -> Option<&'dom StyleData>;
+
+    /// Get the layout data of this node, attempting to downcast it to the desired type.
+    /// Returns None if there is no layout data or it isn't of the desired type.
+    fn layout_data(&self) -> Option<&'dom GenericLayoutData>;
 
     fn rev_children(self) -> LayoutIterator<ReverseChildrenIterator<Self>> {
         LayoutIterator(ReverseChildrenIterator {
@@ -162,9 +173,7 @@ where
 
 /// A thread-safe version of `LayoutNode`, used during flow construction. This type of layout
 /// node does not allow any parents or siblings of nodes to be accessed, to avoid races.
-pub trait ThreadSafeLayoutNode<'dom>:
-    Clone + Copy + Debug + GetStyleAndOpaqueLayoutData<'dom> + NodeInfo + PartialEq + Sized
-{
+pub trait ThreadSafeLayoutNode<'dom>: Clone + Copy + Debug + NodeInfo + PartialEq + Sized {
     type ConcreteNode: LayoutNode<'dom, ConcreteThreadSafeLayoutNode = Self>;
     type ConcreteElement: TElement;
 
@@ -227,7 +236,12 @@ pub trait ThreadSafeLayoutNode<'dom>:
             .map_or(PseudoElementType::Normal, |el| el.get_pseudo_element_type())
     }
 
-    fn get_style_and_opaque_layout_data(self) -> Option<&'dom StyleAndOpaqueLayoutData>;
+    /// Get the [`StyleData`] for this node. Returns None if the node is unstyled.
+    fn style_data(&self) -> Option<&'dom StyleData>;
+
+    /// Get the layout data of this node, attempting to downcast it to the desired type.
+    /// Returns None if there is no layout data or it isn't of the desired type.
+    fn layout_data(&self) -> Option<&'dom GenericLayoutData>;
 
     fn style(&self, context: &SharedStyleContext) -> Arc<ComputedValues> {
         if let Some(el) = self.as_element() {
@@ -309,12 +323,7 @@ pub trait ThreadSafeLayoutNode<'dom>:
 }
 
 pub trait ThreadSafeLayoutElement<'dom>:
-    Clone
-    + Copy
-    + Sized
-    + Debug
-    + ::selectors::Element<Impl = SelectorImpl>
-    + GetStyleAndOpaqueLayoutData<'dom>
+    Clone + Copy + Sized + Debug + ::selectors::Element<Impl = SelectorImpl>
 {
     type ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode<
         'dom,