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/fragment.rs

@@ -66,8 +66,6 @@ use crate::inline::{
     InlineFragmentContext, InlineFragmentNodeFlags, InlineFragmentNodeInfo, InlineMetrics,
     LineMetrics,
 };
-#[cfg(debug_assertions)]
-use crate::layout_debug;
 use crate::model::{
     self, style_length, IntrinsicISizes, IntrinsicISizesContribution, MaybeAuto, SizeConstraint,
 };
@@ -151,11 +149,6 @@ pub struct Fragment {
     /// Various flags for this fragment.
     pub flags: FragmentFlags,
 
-    /// A debug ID that is consistent for the life of this fragment (via transform etc).
-    /// This ID should not be considered stable across multiple layouts or fragment
-    /// manipulations.
-    debug_id: DebugId,
-
     /// The ID of the StackingContext that contains this fragment. This is initialized
     /// to 0, but it assigned during the collect_stacking_contexts phase of display
     /// list construction.
@@ -170,7 +163,6 @@ pub struct Fragment {
 impl Serialize for Fragment {
     fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
         let mut serializer = serializer.serialize_struct("fragment", 3)?;
-        serializer.serialize_field("id", &self.debug_id)?;
         serializer.serialize_field("border_box", &self.border_box)?;
         serializer.serialize_field("margin", &self.margin)?;
         serializer.end()
@@ -699,7 +691,6 @@ impl Fragment {
             inline_context: None,
             pseudo: node.get_pseudo_element_type(),
             flags,
-            debug_id: DebugId::new(),
             stacking_context_id: StackingContextId::root(),
             established_reference_frame: None,
         }
@@ -730,7 +721,6 @@ impl Fragment {
             inline_context: None,
             pseudo,
             flags: FragmentFlags::empty(),
-            debug_id: DebugId::new(),
             stacking_context_id: StackingContextId::root(),
             established_reference_frame: None,
         }
@@ -757,7 +747,6 @@ impl Fragment {
             inline_context: None,
             pseudo: self.pseudo,
             flags: FragmentFlags::empty(),
-            debug_id: DebugId::new(),
             stacking_context_id: StackingContextId::root(),
             established_reference_frame: None,
         }
@@ -784,7 +773,6 @@ impl Fragment {
             inline_context: self.inline_context.clone(),
             pseudo: self.pseudo,
             flags: FragmentFlags::empty(),
-            debug_id: self.debug_id.clone(),
             stacking_context_id: StackingContextId::root(),
             established_reference_frame: None,
         }
@@ -3274,14 +3262,13 @@ impl fmt::Debug for Fragment {
 
         write!(
             f,
-            "\n{}({}) [{:?}]\
+            "\n{} [{:?}]\
             \nborder_box={:?}\
             {border_padding_string}\
             {margin_string}\
             {damage_string}\
             {flags_string}",
             self.specific.get_type(),
-            self.debug_id,
             self.specific,
             self.border_box,
         )
@@ -3447,53 +3434,3 @@ pub struct SpeculatedInlineContentEdgeOffsets {
     pub start: Au,
     pub end: Au,
 }
-
-#[cfg(not(debug_assertions))]
-#[derive(Clone)]
-struct DebugId;
-
-#[cfg(debug_assertions)]
-#[derive(Clone)]
-struct DebugId(u16);
-
-#[cfg(not(debug_assertions))]
-impl DebugId {
-    pub fn new() -> DebugId {
-        DebugId
-    }
-}
-
-#[cfg(debug_assertions)]
-impl DebugId {
-    pub fn new() -> DebugId {
-        DebugId(layout_debug::generate_unique_debug_id())
-    }
-}
-
-#[cfg(not(debug_assertions))]
-impl fmt::Display for DebugId {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{:p}", &self)
-    }
-}
-
-#[cfg(debug_assertions)]
-impl fmt::Display for DebugId {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "{}", self.0)
-    }
-}
-
-#[cfg(not(debug_assertions))]
-impl Serialize for DebugId {
-    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
-        serializer.serialize_str(&format!("{:p}", &self))
-    }
-}
-
-#[cfg(debug_assertions)]
-impl Serialize for DebugId {
-    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
-        serializer.serialize_u16(self.0)
-    }
-}