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>

components/layout_2020/flow/inline/inline_box.rs

@@ -6,7 +6,6 @@ use std::vec::IntoIter;
 
 use app_units::Au;
 use fonts::FontMetrics;
-use serde::Serialize;
 use servo_arc::Arc;
 use style::properties::ComputedValues;
 
@@ -19,10 +18,9 @@ use crate::fragment_tree::BaseFragmentInfo;
 use crate::style_ext::{ComputedValuesExt, PaddingBorderMargin};
 use crate::ContainingBlock;
 
-#[derive(Debug, Serialize)]
+#[derive(Debug)]
 pub(crate) struct InlineBox {
     pub base_fragment_info: BaseFragmentInfo,
-    #[serde(skip_serializing)]
     pub style: Arc<ComputedValues>,
     /// The identifier of this inline box in the containing [`super::InlineFormattingContext`].
     pub(super) identifier: InlineBoxIdentifier,
@@ -56,7 +54,7 @@ impl InlineBox {
     }
 }
 
-#[derive(Debug, Default, Serialize)]
+#[derive(Debug, Default)]
 pub(crate) struct InlineBoxes {
     /// A collection of all inline boxes in a particular [`super::InlineFormattingContext`].
     inline_boxes: Vec<ArcRefCell<InlineBox>>,
@@ -158,7 +156,7 @@ impl InlineBoxes {
     }
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Serialize)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub(super) enum InlineBoxTreePathToken {
     Start(InlineBoxIdentifier),
     End(InlineBoxIdentifier),
@@ -179,7 +177,7 @@ impl InlineBoxTreePathToken {
 /// [`u32`] is used for the index, in order to save space. The value refers to the token
 /// in the start tree data structure which can be fetched to find the actual index of
 /// of the [`InlineBox`] in [`InlineBoxes::inline_boxes`].
-#[derive(Clone, Copy, Debug, Default, Eq, Hash, PartialEq, Serialize)]
+#[derive(Clone, Copy, Debug, Default, Eq, Hash, PartialEq)]
 pub(crate) struct InlineBoxIdentifier {
     pub index_of_start_in_tree: u32,
     pub index_in_inline_boxes: u32,

components/layout_2020/flow/inline/mod.rs

@@ -88,7 +88,6 @@ use line::{
     TextRunLineItem,
 };
 use line_breaker::LineBreaker;
-use serde::Serialize;
 use servo_arc::Arc;
 use style::computed_values::text_wrap_mode::T as TextWrapMode;
 use style::computed_values::vertical_align::T as VerticalAlign;
@@ -134,7 +133,7 @@ use crate::{ConstraintSpace, ContainingBlock};
 static FONT_SUBSCRIPT_OFFSET_RATIO: f32 = 0.20;
 static FONT_SUPERSCRIPT_OFFSET_RATIO: f32 = 0.34;
 
-#[derive(Debug, Serialize)]
+#[derive(Debug)]
 pub(crate) struct InlineFormattingContext {
     /// All [`InlineItem`]s in this [`InlineFormattingContext`] stored in a flat array.
     /// [`InlineItem::StartInlineBox`] and [`InlineItem::EndInlineBox`] allow representing
@@ -171,14 +170,14 @@ pub(crate) struct InlineFormattingContext {
 }
 
 /// A collection of data used to cache [`FontMetrics`] in the [`InlineFormattingContext`]
-#[derive(Debug, Serialize)]
+#[derive(Debug)]
 pub(crate) struct FontKeyAndMetrics {
     pub key: FontInstanceKey,
     pub pt_size: Au,
     pub metrics: FontMetrics,
 }
 
-#[derive(Debug, Serialize)]
+#[derive(Debug)]
 pub(crate) enum InlineItem {
     StartInlineBox(ArcRefCell<InlineBox>),
     EndInlineBox,

components/layout_2020/flow/inline/text_run.rs

@@ -13,7 +13,6 @@ use fonts::{
 use fonts_traits::ByteIndex;
 use log::warn;
 use range::Range as ServoRange;
-use serde::Serialize;
 use servo_arc::Arc;
 use style::computed_values::text_rendering::T as TextRendering;
 use style::computed_values::white_space_collapse::T as WhiteSpaceCollapse;
@@ -38,10 +37,9 @@ pub(crate) const XI_LINE_BREAKING_CLASS_WJ: u8 = 30;
 pub(crate) const XI_LINE_BREAKING_CLASS_ZWJ: u8 = 42;
 
 /// <https://www.w3.org/TR/css-display-3/#css-text-run>
-#[derive(Debug, Serialize)]
+#[derive(Debug)]
 pub(crate) struct TextRun {
     pub base_fragment_info: BaseFragmentInfo,
-    #[serde(skip_serializing)]
     pub parent_style: Arc<ComputedValues>,
     pub text_range: Range<usize>,
 
@@ -64,14 +62,13 @@ enum SegmentStartSoftWrapPolicy {
     FollowLinebreaker,
 }
 
-#[derive(Debug, Serialize)]
+#[derive(Debug)]
 pub(crate) struct TextRunSegment {
     /// The index of this font in the parent [`super::InlineFormattingContext`]'s collection of font
     /// information.
     pub font_index: usize,
 
     /// The [`Script`] of this segment.
-    #[serde(skip_serializing)]
     pub script: Script,
 
     /// The bidi Level of this segment.