layout: Make `Fragment` hold `ArcRefCell` inside (#34923)

Push the interior mutability into enum variants of `Fragment`, so that
they can be cloned. This saves memory in the `Fragment` tree as the
`Fragment` enum is now a relatively wee 16 bytes and the interior parts
can be a variety of sizes. Before, every `Fragment` was the size of the
biggest kind (`BoxFragment` - 248 bytes).

This a step on the way toward incremental layout.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

1 files changed, 10 insertions, 9 deletions

diff --git a/components/layout_2020/fragment_tree/fragment_tree.rs b/components/layout_2020/fragment_tree/fragment_tree.rs
index f2a440ecf9b..35718c708b9 100644
--- a/components/layout_2020/fragment_tree/fragment_tree.rs
+++ b/components/layout_2020/fragment_tree/fragment_tree.rs
@@ -13,7 +13,6 @@ use webrender_api::units;
 use webrender_traits::display_list::ScrollSensitivity;
 
 use super::{ContainingBlockManager, Fragment, Tag};
-use crate::cell::ArcRefCell;
 use crate::display_list::StackingContext;
 use crate::flow::CanvasBackground;
 use crate::geom::PhysicalRect;
@@ -28,7 +27,7 @@ pub struct FragmentTree {
     /// * The first fragment is generated by the root element.
     /// * There may be additional fragments generated by positioned boxes
     ///   that have the initial containing block.
-    pub(crate) root_fragments: Vec<ArcRefCell<Fragment>>,
+    pub(crate) root_fragments: Vec<Fragment>,
 
     /// The scrollable overflow rectangle for the entire tree
     /// <https://drafts.csswg.org/css-overflow/#scrollable>
@@ -63,7 +62,7 @@ impl FragmentTree {
     pub fn print(&self) {
         let mut print_tree = PrintTree::new("Fragment Tree".to_string());
         for fragment in &self.root_fragments {
-            fragment.borrow().print(&mut print_tree);
+            fragment.print(&mut print_tree);
         }
     }
 
@@ -85,7 +84,7 @@ impl FragmentTree {
         };
         self.root_fragments
             .iter()
-            .find_map(|child| child.borrow().find(&info, 0, &mut process_func))
+            .find_map(|child| child.find(&info, 0, &mut process_func))
     }
 
     pub fn remove_nodes_in_fragment_tree_from_set(&self, set: &mut FxHashSet<AnimationSetKey>) {
@@ -110,9 +109,11 @@ impl FragmentTree {
             }
 
             let fragment_relative_rect = match fragment {
-                Fragment::Box(fragment) | Fragment::Float(fragment) => fragment.border_rect(),
-                Fragment::Positioning(fragment) => fragment.rect,
-                Fragment::Text(fragment) => fragment.rect,
+                Fragment::Box(fragment) | Fragment::Float(fragment) => {
+                    fragment.borrow().border_rect()
+                },
+                Fragment::Positioning(fragment) => fragment.borrow().rect,
+                Fragment::Text(fragment) => fragment.borrow().rect,
                 Fragment::AbsoluteOrFixedPositioned(_) |
                 Fragment::Image(_) |
                 Fragment::IFrame(_) => return None,
@@ -140,6 +141,7 @@ impl FragmentTree {
                     //   CSS layout box is inline, return zero." For this check we
                     // also explicitly ignore the list item portion of the display
                     // style.
+                    let fragment = fragment.borrow();
                     if fragment.is_inline_box() {
                         return Some(Rect::zero());
                     }
@@ -151,7 +153,7 @@ impl FragmentTree {
                         Size2D::new(padding_rect.size.width, padding_rect.size.height),
                     )
                 },
-                Fragment::Positioning(fragment) => fragment.rect.cast_unit(),
+                Fragment::Positioning(fragment) => fragment.borrow().rect.cast_unit(),
                 _ => return None,
             };
 
@@ -168,7 +170,6 @@ impl FragmentTree {
         let mut scroll_area = self.initial_containing_block;
         for fragment in self.root_fragments.iter() {
             scroll_area = fragment
-                .borrow()
                 .scrolling_area(&self.initial_containing_block)
                 .union(&scroll_area);
         }