Bug 1391198 - Make the order of rules in DevTools be the specificity …

…order.

We insert rules with any important declaration into rule tree twice,
one for normal level and another for important level. And when we fetch
them from rule tree, we skip the important one to make the order be

MozReview-Commit-ID: HewZG6jYVvv

components/style/rule_tree/mod.rs

@@ -193,10 +193,11 @@ impl RuleTree {
         for (source, level) in iter {
             debug_assert!(last_level <= level, "Not really ordered");
             debug_assert!(!level.is_important(), "Important levels handled internally");
-            let (any_normal, any_important) = {
+            let any_important = {
                 let pdb = source.read(level.guard(guards));
-                (pdb.any_normal(), pdb.any_important())
+                pdb.any_important()
             };
+
             if any_important {
                 found_important = true;
                 match level {
@@ -210,19 +211,25 @@ impl RuleTree {
                     _ => {},
                 };
             }
-            // We really want to ensure empty rule nodes appear in the rule tree for
-            // devtools, this condition ensures that if we find an empty rule node, we
-            // insert it at the normal level.
-            if any_normal || !any_important {
-                if matches!(level, Transitions) && found_important {
-                    // There can be at most one transition, and it will come at
-                    // the end of the iterator. Stash it and apply it after
-                    // !important rules.
-                    debug_assert!(transition.is_none());
-                    transition = Some(source);
-                } else {
-                    current = current.ensure_child(self.root.downgrade(), source, level);
-                }
+
+            // We don't optimize out empty rules, even though we could.
+            //
+            // Inspector relies on every rule being inserted in the normal level
+            // at least once, in order to return the rules with the correct
+            // specificity order.
+            //
+            // TODO(emilio): If we want to apply these optimizations without
+            // breaking inspector's expectations, we'd need to run
+            // selector-matching again at the inspector's request. That may or
+            // may not be a better trade-off.
+            if matches!(level, Transitions) && found_important {
+                // There can be at most one transition, and it will come at
+                // the end of the iterator. Stash it and apply it after
+                // !important rules.
+                debug_assert!(transition.is_none());
+                transition = Some(source);
+            } else {
+                current = current.ensure_child(self.root.downgrade(), source, level);
             }
             last_level = level;
         }

ports/geckolib/glue.rs

@@ -1916,9 +1916,6 @@ pub extern "C" fn Servo_ComputedValues_GetStyleRuleList(values: ServoStyleContex
         None => return,
     };
 
-    let global_style_data = &*GLOBAL_STYLE_DATA;
-    let guard = global_style_data.shared_lock.read();
-
     // TODO(emilio): Will benefit from SmallVec.
     let mut result = vec![];
     for node in rule_node.self_and_ancestors() {
@@ -1927,13 +1924,12 @@ pub extern "C" fn Servo_ComputedValues_GetStyleRuleList(values: ServoStyleContex
             _ => continue,
         };
 
+        // For the rules with any important declaration, we insert them into
+        // rule tree twice, one for normal level and another for important
+        // level. So, we skip the important one to keep the specificity order of
+        // rules.
         if node.importance().important() {
-            let block = style_rule.read_with(&guard).block.read_with(&guard);
-            if block.any_normal() {
-                // We'll append it when we find the normal rules in our
-                // parent chain.
-                continue;
-            }
+            continue;
         }
 
         result.push(style_rule);