layout: Cache content block size contributions (#33964)

This is the first part of caching intermediary layout during flexbox
layout. A later change will try to reuse these layouts, when possible,
for actual item layout and re-layout due to stretching.

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

components/layout_2020/flexbox/mod.rs

@@ -2,6 +2,7 @@
  * 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 app_units::Au;
 use geom::{FlexAxis, MainStartCrossStart};
 use serde::Serialize;
 use servo_arc::Arc as ServoArc;
@@ -112,9 +113,17 @@ pub(crate) enum FlexLevelBox {
     OutOfFlowAbsolutelyPositionedBox(ArcRefCell<AbsolutelyPositionedBox>),
 }
 
-#[derive(Debug, Serialize)]
+#[derive(Serialize)]
 pub(crate) struct FlexItemBox {
     independent_formatting_context: IndependentFormattingContext,
+    #[serde(skip)]
+    cached_layout: ArcRefCell<Option<FlexItemLayoutCache>>,
+}
+
+impl std::fmt::Debug for FlexItemBox {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("FlexItemBox")
+    }
 }
 
 impl FlexItemBox {
@@ -126,3 +135,20 @@ impl FlexItemBox {
         self.independent_formatting_context.base_fragment_info()
     }
 }
+
+#[derive(Debug)]
+struct FlexItemLayoutCacheDescriptor {
+    containing_block_inline_size: Au,
+    content_block_size: Au,
+}
+
+impl FlexItemLayoutCacheDescriptor {
+    fn compatible_with_size(&self, inline: Au) -> bool {
+        inline == self.containing_block_inline_size
+    }
+}
+
+/// A cache to avoid multiple layouts during flexbox layout.
+struct FlexItemLayoutCache {
+    descriptor: FlexItemLayoutCacheDescriptor,
+}