Assert against permanently-leaked rule nodes.

MozReview-Commit-ID: CK5iEWWHFSr

components/style/rule_tree/mod.rs

@@ -122,6 +122,11 @@ impl RuleTree {
         self.root.clone()
     }
 
+    /// Returns true if there are no nodes in the tree aside from the rule node.
+    pub fn is_empty(&self) -> bool {
+        self.root.get().first_child.load(Ordering::Relaxed).is_null()
+    }
+
     fn dump<W: Write>(&self, guards: &StylesheetGuards, writer: &mut W) {
         let _ = writeln!(writer, " + RuleTree");
         self.root.get().dump(guards, writer, 0);

components/style/stylist.rs

@@ -1283,10 +1283,10 @@ impl Drop for Stylist {
         // dropped all strong rule node references before now, then we will
         // leak them, since there will be no way to call gc() on the rule tree
         // after this point.
-        //
-        // TODO(emilio): We can at least assert all the elements in the free
-        // list are indeed free.
         unsafe { self.rule_tree.gc(); }
+
+        // Assert against leaks.
+        debug_assert!(self.rule_tree.is_empty());
     }
 }