style: Use a HashSet in free-list check.

components/style/rule_tree/mod.rs

@@ -468,18 +468,19 @@ impl StrongRuleNode {
     }
 
     unsafe fn assert_free_list_has_no_duplicates_or_null(&self) {
-        assert!(cfg!(debug_assertions));
+        assert!(cfg!(debug_assertions), "This is an expensive check!");
+        use std::collections::HashSet;
 
         let me = &*self.ptr;
         assert!(me.is_root());
 
         let mut current = self.ptr;
-        let mut seen = vec![];
+        let mut seen = HashSet::new();
         while current != FREE_LIST_SENTINEL {
             let next = (*current).next_free.load(Ordering::SeqCst);
             assert!(!next.is_null());
             assert!(!seen.contains(&next));
-            seen.push(next);
+            seen.insert(next);
 
             current = next;
         }