Auto merge of #25957 - pcwalton:layout-2020-atomic-refcell, r=<try>

Start using `AtomicRefCell` in layout 2020 as preparation for incremental layout

This makes `BlockLevelBox` and `InlineLevelBox` use `AtomicRefCell` for incremental layout, per @nox's suggestion in #25168.

As part of this, it reworks inline layout to use recursion, per #25950. LLVM should be able to optimize this into a loop (though I have not verified this).

r? @nox

components/layout_2020/Cargo.toml

@@ -14,7 +14,7 @@ doctest = false
 
 [dependencies]
 app_units = "0.7"
-atomic_refcell = "0.1"
+atomic_refcell = "0.1.6"
 canvas_traits = {path = "../canvas_traits"}
 cssparser = "0.27"
 embedder_traits = {path = "../embedder_traits"}

components/layout_2020/cell.rs

@@ -0,0 +1,69 @@
+/* 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/. */
+
+use atomic_refcell::AtomicRefCell;
+use serde::{Serialize, Serializer};
+use servo_arc::Arc;
+use std::fmt;
+use std::ops::Deref;
+
+pub(crate) struct ArcRefCell<T> {
+    value: Arc<AtomicRefCell<T>>,
+}
+
+impl<T> ArcRefCell<T> {
+    pub fn new(value: T) -> Self {
+        Self {
+            value: Arc::new(AtomicRefCell::new(value)),
+        }
+    }
+}
+
+impl<T> Clone for ArcRefCell<T> {
+    fn clone(&self) -> Self {
+        Self {
+            value: self.value.clone(),
+        }
+    }
+}
+
+impl<T> Default for ArcRefCell<T>
+where
+    T: Default,
+{
+    fn default() -> Self {
+        Self {
+            value: Arc::new(AtomicRefCell::new(Default::default())),
+        }
+    }
+}
+
+impl<T> Deref for ArcRefCell<T> {
+    type Target = AtomicRefCell<T>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.value
+    }
+}
+
+impl<T> fmt::Debug for ArcRefCell<T>
+where
+    T: fmt::Debug,
+{
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        self.value.fmt(formatter)
+    }
+}
+
+impl<T> Serialize for ArcRefCell<T>
+where
+    T: Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        self.borrow().serialize(serializer)
+    }
+}

components/layout_2020/dom_traversal.rs

@@ -2,13 +2,14 @@
  * 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/. */
 
+use crate::cell::ArcRefCell;
 use crate::context::LayoutContext;
 use crate::element_data::{LayoutBox, LayoutDataForElement};
 use crate::geom::PhysicalSize;
 use crate::replaced::{CanvasInfo, CanvasSource, ReplacedContent};
 use crate::style_ext::{Display, DisplayGeneratingBox, DisplayInside, DisplayOutside};
 use crate::wrapper::GetRawData;
-use atomic_refcell::{AtomicRefCell, AtomicRefMut};
+use atomic_refcell::AtomicRefMut;
 use html5ever::LocalName;
 use net_traits::image::base::Image as NetImage;
 use script_layout_interface::wrapper_traits::{
@@ -317,12 +318,12 @@ where
 }
 
 pub struct BoxSlot<'dom> {
-    slot: Option<ServoArc<AtomicRefCell<Option<LayoutBox>>>>,
+    slot: Option<ArcRefCell<Option<LayoutBox>>>,
     marker: marker<&'dom ()>,
 }
 
 impl BoxSlot<'_> {
-    pub(crate) fn new(slot: ServoArc<AtomicRefCell<Option<LayoutBox>>>) -> Self {
+    pub(crate) fn new(slot: ArcRefCell<Option<LayoutBox>>) -> Self {
         *slot.borrow_mut() = None;
         let slot = Some(slot);
         Self { slot, marker }

components/layout_2020/element_data.rs

@@ -2,25 +2,24 @@
  * 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/. */
 
+use crate::cell::ArcRefCell;
 use crate::flow::inline::InlineLevelBox;
 use crate::flow::BlockLevelBox;
-use atomic_refcell::AtomicRefCell;
-use servo_arc::Arc;
 
 #[derive(Default)]
 pub struct LayoutDataForElement {
-    pub(super) self_box: Arc<AtomicRefCell<Option<LayoutBox>>>,
+    pub(super) self_box: ArcRefCell<Option<LayoutBox>>,
     pub(super) pseudo_elements: Option<Box<PseudoElementBoxes>>,
 }
 
 #[derive(Default)]
 pub(super) struct PseudoElementBoxes {
-    pub before: Arc<AtomicRefCell<Option<LayoutBox>>>,
-    pub after: Arc<AtomicRefCell<Option<LayoutBox>>>,
+    pub before: ArcRefCell<Option<LayoutBox>>,
+    pub after: ArcRefCell<Option<LayoutBox>>,
 }
 
 pub(super) enum LayoutBox {
     DisplayContents,
-    BlockLevel(Arc<BlockLevelBox>),
-    InlineLevel(Arc<InlineLevelBox>),
+    BlockLevel(ArcRefCell<BlockLevelBox>),
+    InlineLevel(ArcRefCell<InlineLevelBox>),
 }