style: Use only Origin during the cascade, rather than CascadeLevel.

The micro-benchmark `style-attr-1.html` regressed slightly with my patch, after
the CascadeLevel size increase.

This benchmark is meant to test for the "changing the style attribute doesn't
cause selector-matching" optimization (which, mind you, keeps working).

But in the process it creates 10k rules which form a perfect path in the rule
tree and that we put into a SmallVec during the cascade, and the benchmark
spends most of the time pushing to that SmallVec and iterating the declarations
(as there's only one property to apply).

So we could argue that the regression is minor and is not what the benchark is
supposed to be testing, but given I did the digging... :)

My patch made CascadeLevel bigger, which means that we create a somewhat bigger
vector in this case. Thankfully it also removed the dependency in the
CascadeLevel, so we can stop using that and use just Origin which is one byte to
revert the perf regression.

Differential Revision: https://phabricator.services.mozilla.com/D53181

components/style/animation.rs

@@ -16,8 +16,8 @@ use crate::properties::animated_properties::AnimatedProperty;
 use crate::properties::longhands::animation_direction::computed_value::single_value::T as AnimationDirection;
 use crate::properties::longhands::animation_play_state::computed_value::single_value::T as AnimationPlayState;
 use crate::properties::{self, CascadeMode, ComputedValues, LonghandId};
-use crate::rule_tree::CascadeLevel;
 use crate::stylesheets::keyframes_rule::{KeyframesAnimation, KeyframesStep, KeyframesStepValue};
+use crate::stylesheets::Origin;
 use crate::timer::Timer;
 use crate::values::computed::box_::TransitionProperty;
 use crate::values::computed::Time;
@@ -491,7 +491,7 @@ where
                 guard
                     .normal_declaration_iter()
                     .filter(|declaration| declaration.is_animatable())
-                    .map(|decl| (decl, CascadeLevel::Animations))
+                    .map(|decl| (decl, Origin::Author))
             };
 
             // This currently ignores visited styles, which seems acceptable,

components/style/properties/cascade.rs

@@ -15,7 +15,7 @@ use crate::properties::{LonghandId, LonghandIdSet, CSSWideKeyword};
 use crate::properties::{PropertyDeclaration, PropertyDeclarationId, DeclarationImportanceIterator};
 use crate::properties::CASCADE_PROPERTY;
 use crate::rule_cache::{RuleCache, RuleCacheConditions};
-use crate::rule_tree::{CascadeLevel, StrongRuleNode};
+use crate::rule_tree::StrongRuleNode;
 use crate::selector_parser::PseudoElement;
 use crate::stylesheets::{Origin, PerOrigin};
 use servo_arc::Arc;
@@ -134,11 +134,15 @@ where
     let restriction = pseudo.and_then(|p| p.property_restriction());
     let iter_declarations = || {
         rule_node.self_and_ancestors().flat_map(|node| {
-            let cascade_level = node.cascade_level();
+            let origin = node.cascade_level().origin();
             let node_importance = node.importance();
+            let guard = match origin {
+                Origin::Author => guards.author,
+                Origin::User | Origin::UserAgent => guards.ua_or_user,
+            };
             let declarations = match node.style_source() {
                 Some(source) => source
-                    .read(cascade_level.guard(guards))
+                    .read(guard)
                     .declaration_importance_iter(),
                 None => DeclarationImportanceIterator::new(&[], &empty),
             };
@@ -153,14 +157,14 @@ where
                         // longhands are only allowed if they have our
                         // restriction flag set.
                         if let PropertyDeclarationId::Longhand(id) = declaration.id() {
-                            if !id.flags().contains(restriction) && cascade_level.origin() != Origin::UserAgent {
+                            if !id.flags().contains(restriction) && origin != Origin::UserAgent {
                                 return None;
                             }
                         }
                     }
 
                     if declaration_importance == node_importance {
-                        Some((declaration, cascade_level))
+                        Some((declaration, origin))
                     } else {
                         None
                     }
@@ -223,7 +227,7 @@ pub fn apply_declarations<'a, E, F, I>(
 where
     E: TElement,
     F: Fn() -> I,
-    I: Iterator<Item = (&'a PropertyDeclaration, CascadeLevel)>,
+    I: Iterator<Item = (&'a PropertyDeclaration, Origin)>,
 {
     debug_assert!(layout_parent_style.is_none() || parent_style.is_some());
     debug_assert_eq!(
@@ -242,17 +246,17 @@ where
 
     let inherited_style = parent_style.unwrap_or(device.default_computed_values());
 
-    let mut declarations = SmallVec::<[(&_, CascadeLevel); 32]>::new();
+    let mut declarations = SmallVec::<[(&_, Origin); 32]>::new();
     let custom_properties = {
         let mut builder = CustomPropertiesBuilder::new(
             inherited_style.custom_properties(),
             device.environment(),
         );
 
-        for (declaration, cascade_level) in iter_declarations() {
-            declarations.push((declaration, cascade_level));
+        for (declaration, origin) in iter_declarations() {
+            declarations.push((declaration, origin));
             if let PropertyDeclaration::Custom(ref declaration) = *declaration {
-                builder.cascade(declaration, cascade_level.origin());
+                builder.cascade(declaration, origin);
             }
         }
 
@@ -332,16 +336,15 @@ where
 fn should_ignore_declaration_when_ignoring_document_colors(
     device: &Device,
     longhand_id: LonghandId,
-    cascade_level: CascadeLevel,
+    origin: Origin,
     pseudo: Option<&PseudoElement>,
     declaration: &mut Cow<PropertyDeclaration>,
 ) -> bool {
     if !longhand_id.ignored_when_document_colors_disabled() {
         return false;
     }
 
-    let is_ua_or_user_rule =
-        matches!(cascade_level.origin(), Origin::User | Origin::UserAgent);
+    let is_ua_or_user_rule = matches!(origin, Origin::User | Origin::UserAgent);
     if is_ua_or_user_rule {
         return false;
     }
@@ -446,7 +449,7 @@ impl<'a, 'b: 'a> Cascade<'a, 'b> {
         declarations: I,
     ) where
         Phase: CascadePhase,
-        I: Iterator<Item = (&'decls PropertyDeclaration, CascadeLevel)>,
+        I: Iterator<Item = (&'decls PropertyDeclaration, Origin)>,
     {
         let apply_reset = apply_reset == ApplyResetProperties::Yes;
 
@@ -459,9 +462,8 @@ impl<'a, 'b: 'a> Cascade<'a, 'b> {
 
         let ignore_colors = !self.context.builder.device.use_document_colors();
 
-        for (declaration, cascade_level) in declarations {
+        for (declaration, origin) in declarations {
             let declaration_id = declaration.id();
-            let origin = cascade_level.origin();
             let longhand_id = match declaration_id {
                 PropertyDeclarationId::Longhand(id) => id,
                 PropertyDeclarationId::Custom(..) => continue,
@@ -509,7 +511,7 @@ impl<'a, 'b: 'a> Cascade<'a, 'b> {
                 let should_ignore = should_ignore_declaration_when_ignoring_document_colors(
                     self.context.builder.device,
                     longhand_id,
-                    cascade_level,
+                    origin,
                     self.context.builder.pseudo,
                     &mut declaration,
                 );

components/style/stylist.rs

@@ -1322,16 +1322,17 @@ impl Stylist {
         let iter_declarations = || {
             block
                 .declaration_importance_iter()
-                .map(|(declaration, importance)| {
-                    debug_assert!(!importance.important());
-                    (declaration, CascadeLevel::same_tree_author_normal())
-                })
+                .map(|(declaration, _)| (declaration, Origin::Author))
         };
 
         let metrics = get_metrics_provider_for_product();
 
         // We don't bother inserting these declarations in the rule tree, since
         // it'd be quite useless and slow.
+        //
+        // TODO(emilio): Now that we fixed bug 1493420, we should consider
+        // reversing this as it shouldn't be slow anymore, and should avoid
+        // generating two instantiations of apply_declarations.
         properties::apply_declarations::<E, _, _>(
             &self.device,
             /* pseudo = */ None,