Make DOM own the style and layout data, in an UnsafeCell

The previous Cell was a lie.

components/script_layout_interface/lib.rs

@@ -26,7 +26,6 @@ use net_traits::image_cache::PendingImageId;
 use script_traits::UntrustedNodeAddress;
 use servo_url::{ImmutableOrigin, ServoUrl};
 use std::any::Any;
-use std::ptr::NonNull;
 use std::sync::atomic::AtomicIsize;
 use style::data::ElementData;
 
@@ -51,12 +50,11 @@ impl StyleData {
     }
 }
 
-#[derive(Clone, Copy, MallocSizeOf)]
+#[derive(MallocSizeOf)]
 pub struct OpaqueStyleAndLayoutData {
     // NB: We really store a `StyleAndLayoutData` here, so be careful!
-    #[ignore_malloc_size_of = "TODO(#6910) Box value that should be counted but \
-                               the type lives in layout"]
-    ptr: NonNull<dyn Any + Send + Sync>,
+    #[ignore_malloc_size_of = "Trait objects are hard"]
+    ptr: Box<dyn Any + Send + Sync>,
 }
 
 impl OpaqueStyleAndLayoutData {
@@ -66,23 +64,17 @@ impl OpaqueStyleAndLayoutData {
         T: Any + Send + Sync,
     {
         Self {
-            ptr: Box::into_raw_non_null(Box::new(value) as Box<dyn Any + Send + Sync>),
+            ptr: Box::new(value) as Box<dyn Any + Send + Sync>,
         }
     }
 
-    #[inline]
-    pub fn as_ptr(&self) -> *mut (dyn Any + Send + Sync) {
-        self.ptr.as_ptr()
-    }
-
     /// Extremely cursed.
-    #[allow(unsafe_code)]
     #[inline]
-    pub unsafe fn downcast_ref<'extended, T>(&self) -> Option<&'extended T>
+    pub fn downcast_ref<T>(&self) -> Option<&T>
     where
         T: Any + Send + Sync,
     {
-        (*self.ptr.as_ptr()).downcast_ref()
+        self.ptr.downcast_ref()
     }
 }
 

components/script_layout_interface/message.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use crate::rpc::LayoutRPC;
-use crate::{OpaqueStyleAndLayoutData, PendingImage, TrustedNodeAddress};
+use crate::{PendingImage, TrustedNodeAddress};
 use app_units::Au;
 use crossbeam_channel::{Receiver, Sender};
 use euclid::default::{Point2D, Rect};
@@ -56,11 +56,6 @@ pub enum Msg {
     /// field is whether animations should be force-ticked.
     AdvanceClockMs(i32, bool, ImmutableOrigin),
 
-    /// Destroys layout data associated with a DOM node.
-    ///
-    /// TODO(pcwalton): Maybe think about batching to avoid message traffic.
-    ReapStyleAndLayoutData(OpaqueStyleAndLayoutData),
-
     /// Requests that the layout thread measure its memory usage. The resulting reports are sent back
     /// via the supplied channel.
     CollectReports(ReportsChan),

components/script_layout_interface/wrapper_traits.rs

@@ -80,7 +80,7 @@ impl PseudoElementType {
 
 /// Trait to abstract access to layout data across various data structures.
 pub trait GetLayoutData<'dom> {
-    fn get_style_and_layout_data(&self) -> Option<OpaqueStyleAndLayoutData>;
+    fn get_style_and_layout_data(self) -> Option<&'dom OpaqueStyleAndLayoutData>;
 }
 
 /// A wrapper so that layout can access only the methods that it should have access to. Layout must
@@ -224,7 +224,7 @@ pub trait ThreadSafeLayoutNode<'dom>:
             .map_or(PseudoElementType::Normal, |el| el.get_pseudo_element_type())
     }
 
-    fn get_style_and_layout_data(&self) -> Option<OpaqueStyleAndLayoutData>;
+    fn get_style_and_layout_data(self) -> Option<&'dom OpaqueStyleAndLayoutData>;
 
     fn style(&self, context: &SharedStyleContext) -> Arc<ComputedValues> {
         if let Some(el) = self.as_element() {