servo: Merge #14436 - Make restyle tracking more granular (from bholley:granular_restyle); r=emilio

The primary idea of this patch is to ditch the rigid enum of Previous/Current
styles, and replace it with a series of indicators for the various types of
work that needs to be performed (expanding snapshots, rematching, recascading,
and damage processing). This loses us a little bit of sanity checking (since
the up-to-date-ness of our style is no longer baked into the type system), but
gives us a lot more flexibility that we'll need going forward (especially when
we separate matching from cascading). We also eliminate get_styling_mode in
favor of a method on the traversal.

This patch does a few other things as ridealongs:
* Temporarily eliminates the handling for transfering ownership of styles to the
  frame. We'll need this again at some point, but for now it's causing too much
  complexity for a half-implemented feature.
* Ditches TRestyleDamage, which is no longer necessary post-crate-merge, and is
  a constant source of compilation failures from either needing to be imported
  or being unnecessarily imported (which varies between gecko and servo).
* Expands Snapshots for the traversal root, which was missing before.
* Fixes up the skip_root stuff to avoid visiting the skipped root.
* Unifies parallel traversal and avoids spawning for a single work item.
* Adds an explicit pre_traverse step do any pre-processing and determine whether
      we need to traverse at all.

Source-Repo: https://github.com/servo/servo
Source-Revision: b9a8ccd775c3192e3810a1730b1d0bc2b5c9dfb6

UltraBlame original commit: b5b63ec333ddcb40ac2c321e7e9dda9c79a81bc0

Author: marco-c

servo/components/layout/animation.rs

@@ -155,15 +155,6 @@ use
 style
 :
 :
-dom
-:
-:
-TRestyleDamage
-;
-use
-style
-:
-:
 selector_parser
 :
 :

servo/components/layout/context.rs

@@ -177,6 +177,15 @@ use
 std
 :
 :
+borrow
+:
+:
+Borrow
+;
+use
+std
+:
+:
 cell
 :
 :
@@ -643,6 +652,31 @@ FnvHasher
 >
 >
 }
+impl
+Borrow
+<
+SharedStyleContext
+>
+for
+SharedLayoutContext
+{
+fn
+borrow
+(
+&
+self
+)
+-
+>
+&
+SharedStyleContext
+{
+&
+self
+.
+style_context
+}
+}
 pub
 struct
 LayoutContext

servo/components/layout/flow.rs

@@ -581,15 +581,6 @@ use
 style
 :
 :
-dom
-:
-:
-TRestyleDamage
-;
-use
-style
-:
-:
 logical_geometry
 :
 :

servo/components/layout/fragment.rs

@@ -438,15 +438,6 @@ use
 style
 :
 :
-dom
-:
-:
-TRestyleDamage
-;
-use
-style
-:
-:
 logical_geometry
 :
 :

servo/components/layout/generated_content.rs

@@ -228,15 +228,6 @@ use
 style
 :
 :
-dom
-:
-:
-TRestyleDamage
-;
-use
-style
-:
-:
 properties
 :
 :

servo/components/layout/incremental.rs

@@ -81,15 +81,6 @@ use
 style
 :
 :
-dom
-:
-:
-TRestyleDamage
-;
-use
-style
-:
-:
 selector_parser
 :
 :

servo/components/layout/traversal.rs

@@ -178,7 +178,6 @@ dom
 :
 :
 {
-StylingMode
 TElement
 TNode
 }
@@ -814,7 +813,7 @@ self
 node
 :
 N
-data
+traversal_data
 :
 &
 mut
@@ -872,6 +871,20 @@ unwrap
 (
 )
 ;
+let
+mut
+data
+=
+el
+.
+mutate_data
+(
+)
+.
+unwrap
+(
+)
+;
 recalc_style_at
 :
 :
@@ -885,8 +898,11 @@ Self
 self
 .
 context
-data
+traversal_data
 el
+&
+mut
+data
 )
 ;
 }
@@ -915,77 +931,16 @@ node
 ;
 }
 fn
-should_traverse_child
+text_node_needs_traversal
 (
-child
+node
 :
 N
 )
 -
 >
 bool
 {
-match
-child
-.
-as_element
-(
-)
-{
-/
-/
-Elements
-should
-be
-traversed
-if
-they
-need
-styling
-or
-flow
-construction
-.
-Some
-(
-el
-)
-=
->
-el
-.
-styling_mode
-(
-)
-!
-=
-StylingMode
-:
-:
-Stop
-|
-|
-el
-.
-as_node
-(
-)
-.
-to_threadsafe
-(
-)
-.
-restyle_damage
-(
-)
-!
-=
-RestyleDamage
-:
-:
-empty
-(
-)
 /
 /
 Text
@@ -1049,10 +1004,7 @@ needs
 fixup
 )
 .
-None
-=
->
-child
+node
 .
 get_raw_data
 (
@@ -1063,7 +1015,7 @@ is_none
 )
 |
 |
-child
+node
 .
 parent_node
 (
@@ -1089,7 +1041,6 @@ empty
 (
 )
 }
-}
 unsafe
 fn
 ensure_element_data