Bug 1663618 - Speed up custom property diffing. r=boris

When entering or leaving fullscreen in youtube, we spend most of the
restyle time diffing custom properties, under IndexMap::eq.

Turns out that IndexMap equality is not order-aware, and thus you
actually need to make a hashmap lookup for each entry in the map, which
is unnecessarily inefficient.

Instead, just compare the iterators.

See indexmap-rs/indexmap#153.

Differential Revision: https://phabricator.services.mozilla.com/D89434

UltraBlame original commit: d46a20917d66d3c7fc9ada2204eae0af92365e08

servo/components/style/gecko/restyle_damage.rs

@@ -231,17 +231,12 @@ if
 reset_only
 &
 &
+!
 old_style
 .
-custom_properties
+custom_properties_equal
 (
-)
-!
-=
 new_style
-.
-custom_properties
-(
 )
 {
 any_style_changed

servo/components/style/properties/properties.mako.rs

@@ -16050,6 +16050,116 @@ as_ref
 (
 )
 }
+pub
+fn
+custom_properties_equal
+(
+&
+self
+other
+:
+&
+Self
+)
+-
+>
+bool
+{
+match
+(
+self
+.
+custom_properties
+(
+)
+other
+.
+custom_properties
+(
+)
+)
+{
+(
+Some
+(
+l
+)
+Some
+(
+r
+)
+)
+=
+>
+{
+l
+.
+len
+(
+)
+=
+=
+r
+.
+len
+(
+)
+&
+&
+l
+.
+iter
+(
+)
+.
+zip
+(
+r
+.
+iter
+(
+)
+)
+.
+all
+(
+|
+(
+(
+k1
+v1
+)
+(
+k2
+v2
+)
+)
+|
+k1
+=
+=
+k2
+&
+&
+v1
+=
+=
+v2
+)
+}
+(
+None
+None
+)
+=
+>
+true
+_
+=
+>
+false
+}
+}
 %
 for
 prop

servo/components/style/servo/restyle_damage.rs

@@ -1066,17 +1066,12 @@ REPAINT
 )
 ;
 if
+!
 old
 .
-custom_properties
+custom_properties_equal
 (
-)
-!
-=
 new
-.
-custom_properties
-(
 )
 {
 damage