Fix into_iter of empty map and add test

Author: dtolnay

src/lib.rs

@@ -1029,7 +1029,9 @@ impl<K: Hash + Eq, V, S: BuildHasher> IntoIterator for LinkedHashMap<K, V, S> {
         };
         let len = self.len();
 
-        unsafe { drop_empty_entry_box(self.head) }
+        if !self.head.is_null() {
+            unsafe { drop_empty_entry_box(self.head) }
+        }
         self.clear_free_list();
         // drop the HashMap but not the LinkedHashMap
         self.map = unsafe { mem::uninitialized() };

tests/test.rs

@@ -199,6 +199,29 @@ fn test_consuming_iter() {
     }
 }
 
+#[test]
+fn test_consuming_iter_empty() {
+    let map = LinkedHashMap::<&str, i32>::new();
+    let mut iter = map.into_iter();
+    assert_eq!(None, iter.next());
+    let mut clone = iter.clone();
+    assert_eq!(None, clone.next());
+}
+
+#[test]
+fn test_consuming_iter_with_free_list() {
+    let mut map = LinkedHashMap::new();
+    map.insert("a", 10);
+    map.insert("c", 30);
+    map.insert("b", 20);
+    map.remove("a");
+    map.remove("b");
+
+    let mut iter = map.into_iter();
+    assert_eq!(Some(("c", 30)), iter.next());
+    assert_eq!(None, iter.next());
+}
+
 #[test]
 fn test_borrow() {
     #[derive(PartialEq, Eq, Hash)] struct Foo(Bar);