cannot_refer_to_empty

src/rstd/collections/tree.rs

@@ -265,6 +265,13 @@ mod tests {
         assert_eq!(trees.height(&trees.empty()), 0);
     }
 
+    #[test]
+    fn cannot_refer_to_empty() {
+        let trees = BalancedTrees::new(new_trees());
+        let tree = trees.empty();
+        assert!(trees.refer(&tree).is_none());
+    }
+
     #[test]
     fn one_has_height_1() {
         let trees = BalancedTrees::new(new_trees());