Auto merge of #13202 - emilio:rule-tree, r=SimonSapin,Manishearth,emilio

Rule tree, v1

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [x] There are tests for these changes OR

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This patch introduces infrastructure for the rule tree, and constructs it.

We don't use it yet, nor have good heuristics for GC'ing it, but this should not
happen anymore once we store the rule node reference in the node.

I haven't messed up with memory orders because I want to do a try run with it,
then mess with them.

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tests/unit/style/selector_matching.rs

@@ -11,6 +11,7 @@ use std::sync::Arc;
 use style::properties::{PropertyDeclarationBlock, PropertyDeclaration, DeclaredValue};
 use style::properties::{longhands, Importance};
 use style::selector_matching::{Rule, SelectorMap};
+use style::stylesheets::StyleRule;
 use style::thread_state;
 
 /// Helper method to get some Rules from selector strings.
@@ -18,18 +19,26 @@ use style::thread_state;
 fn get_mock_rules(css_selectors: &[&str]) -> Vec<Vec<Rule>> {
     css_selectors.iter().enumerate().map(|(i, selectors)| {
         let context = ParserContext::new();
-        parse_selector_list(&context, &mut Parser::new(*selectors))
-        .unwrap().into_iter().map(|s| {
+        let selectors =
+            parse_selector_list(&context, &mut Parser::new(*selectors)).unwrap();
+
+        let rule = Arc::new(RwLock::new(StyleRule {
+            selectors: selectors,
+            block: Arc::new(RwLock::new(PropertyDeclarationBlock {
+                declarations: vec![
+                    (PropertyDeclaration::Display(DeclaredValue::Value(
+                        longhands::display::SpecifiedValue::block)),
+                     Importance::Normal),
+                ],
+                important_count: 0,
+            })),
+        }));
+
+        let guard = rule.read();
+        guard.selectors.iter().map(|s| {
             Rule {
                 selector: s.complex_selector.clone(),
-                declarations: Arc::new(RwLock::new(PropertyDeclarationBlock {
-                    declarations: vec![
-                        (PropertyDeclaration::Display(DeclaredValue::Value(
-                            longhands::display::SpecifiedValue::block)),
-                         Importance::Normal),
-                    ],
-                    important_count: 0,
-                })),
+                style_rule: rule.clone(),
                 specificity: s.specificity,
                 source_order: i,
             }