servo: Merge #17242 - Reduce the size of ApplicableDeclarationBlock (from bzbarsky:smaller-decl-block); r=bholley

The level is a u8, so on 64-bit it packs nicely after the u32 specificity.

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- [X] These changes fix https://bugzilla.mozilla.org/show_bug.cgi?id=1371297

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Source-Repo: https://github.com/servo/servo
Source-Revision: ca8961a93bd5dfb2680ed0cb73a5fcc0ce0f0fdb

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 50e8378984e0b24cde80ee88d52c0a58ab123cc5

servo/components/style/stylist.rs

@@ -1524,9 +1524,9 @@ impl Rule {
                                            -> ApplicableDeclarationBlock {
         ApplicableDeclarationBlock {
             source: StyleSource::Style(self.style_rule.clone()),
-            level: level,
             source_order: self.source_order,
             specificity: self.specificity(),
+            level: level,
         }
     }
 
@@ -1557,12 +1557,12 @@ pub struct ApplicableDeclarationBlock {
     /// The style source, either a style rule, or a property declaration block.
     #[cfg_attr(feature = "servo", ignore_heap_size_of = "Arc")]
     pub source: StyleSource,
-    /// The cascade level this applicable declaration block is in.
-    pub level: CascadeLevel,
     /// The source order of this block.
     pub source_order: usize,
     /// The specificity of the selector this block is represented by.
     pub specificity: u32,
+    /// The cascade level this applicable declaration block is in.
+    pub level: CascadeLevel,
 }
 
 impl ApplicableDeclarationBlock {
@@ -1574,9 +1574,9 @@ impl ApplicableDeclarationBlock {
                              -> Self {
         ApplicableDeclarationBlock {
             source: StyleSource::Declarations(declarations),
-            level: level,
             source_order: 0,
             specificity: 0,
+            level: level,
         }
     }
 }

servo/tests/unit/stylo/size_of.rs

@@ -10,6 +10,7 @@ use style::data::{ComputedStyle, ElementData, ElementStyles};
 use style::gecko::selector_parser as real;
 use style::properties::ComputedValues;
 use style::rule_tree::StrongRuleNode;
+use style::stylist::ApplicableDeclarationBlock;
 
 #[test]
 fn size_of_selectors_dummy_types() {
@@ -36,6 +37,8 @@ size_of_test!(test_size_of_element_data, ElementData, 56);
 
 size_of_test!(test_size_of_property_declaration, style::properties::PropertyDeclaration, 32);
 
+size_of_test!(test_size_of_application_declaration_block, ApplicableDeclarationBlock, 32);
+
 // This is huge, but we allocate it on the stack and then never move it,
 // we only pass `&mut SourcePropertyDeclaration` references around.
 size_of_test!(test_size_of_parsed_declaration, style::properties::SourcePropertyDeclaration, 704);