layout: Remove the obsolete layout tracing functionality (#35001)

There were two kinds of layout tracing controlled by the same debugging
option:

 - modern layout: Functionality that dumped a JSON serialization of the
   layout tree before and after layout.
 - legacy layout: A scope based tracing that reported the process of
   layout in a structured way.

I don't think anyone working on layout is using either of these two
features. For modern layout requiring data structure to implement
`serde` serialization is incredibly inconvenient and also generates a
lot of extra code.

We also have a more modern tracing functionality based on perfetto that
we have started to use for layout and IMO it's actually being used and
more robust.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

2 files changed, 5 insertions, 10 deletions

diff --git a/components/layout_2020/flexbox/geom.rs b/components/layout_2020/flexbox/geom.rs
index 9e18561d8bd..6a302e27486 100644
--- a/components/layout_2020/flexbox/geom.rs
+++ b/components/layout_2020/flexbox/geom.rs
@@ -4,7 +4,6 @@
 
 //! <https://drafts.csswg.org/css-flexbox/#box-model>
 
-use serde::Serialize;
 use style::properties::longhands::flex_direction::computed_value::T as FlexDirection;
 
 use crate::geom::{LogicalRect, LogicalSides, LogicalVec2};
@@ -68,7 +67,7 @@ impl<T> FlexRelativeSides<T> {
 
 /// One of the two bits set by the `flex-direction` property
 /// (The other is "forward" v.s. reverse.)
-#[derive(Clone, Copy, Debug, PartialEq, Serialize)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub(super) enum FlexAxis {
     /// The main axis is the inline axis of the container (not necessarily of flex items!),
     /// cross is block.
@@ -79,7 +78,7 @@ pub(super) enum FlexAxis {
 
 /// Which flow-relative sides map to the main-start and cross-start sides, respectively.
 /// See <https://drafts.csswg.org/css-flexbox/#box-model>
-#[derive(Clone, Copy, Debug, Serialize)]
+#[derive(Clone, Copy, Debug)]
 pub(super) enum MainStartCrossStart {
     InlineStartBlockStart,
     InlineStartBlockEnd,
diff --git a/components/layout_2020/flexbox/mod.rs b/components/layout_2020/flexbox/mod.rs
index 06cffac3df9..32c1023214f 100644
--- a/components/layout_2020/flexbox/mod.rs
+++ b/components/layout_2020/flexbox/mod.rs
@@ -4,7 +4,6 @@
 
 use app_units::Au;
 use geom::{FlexAxis, MainStartCrossStart};
-use serde::Serialize;
 use servo_arc::Arc as ServoArc;
 use style::logical_geometry::WritingMode;
 use style::properties::longhands::align_items::computed_value::T as AlignItems;
@@ -30,7 +29,7 @@ mod layout;
 
 /// A structure to hold the configuration of a flex container for use during layout
 /// and preferred width calculation.
-#[derive(Clone, Debug, Serialize)]
+#[derive(Clone, Debug)]
 pub(crate) struct FlexContainerConfig {
     container_is_single_line: bool,
     writing_mode: WritingMode,
@@ -88,11 +87,10 @@ impl FlexContainerConfig {
     }
 }
 
-#[derive(Debug, Serialize)]
+#[derive(Debug)]
 pub(crate) struct FlexContainer {
     children: Vec<ArcRefCell<FlexLevelBox>>,
 
-    #[serde(skip_serializing)]
     style: ServoArc<ComputedValues>,
 
     /// The configuration of this [`FlexContainer`].
@@ -143,16 +141,14 @@ impl FlexContainer {
     }
 }
 
-#[derive(Debug, Serialize)]
+#[derive(Debug)]
 pub(crate) enum FlexLevelBox {
     FlexItem(FlexItemBox),
     OutOfFlowAbsolutelyPositionedBox(ArcRefCell<AbsolutelyPositionedBox>),
 }
 
-#[derive(Serialize)]
 pub(crate) struct FlexItemBox {
     independent_formatting_context: IndependentFormattingContext,
-    #[serde(skip)]
     block_content_size_cache: ArcRefCell<Option<CachedBlockSizeContribution>>,
 }