Add core::iter::once_with

src/libcore/tests/iter.rs

@@ -1906,6 +1906,23 @@ fn test_once() {
     assert_eq!(it.next(), None);
 }
 
+#[test]
+fn test_once_with() {
+    let mut count = 0;
+    let mut it = once_with(|| {
+        count += 1;
+        42
+    });
+
+    assert_eq!(count, 0);
+    assert_eq!(it.next(), Some(42));
+    assert_eq!(count, 1);
+    assert_eq!(it.next(), None);
+    assert_eq!(count, 1);
+    assert_eq!(it.next(), None);
+    assert_eq!(count, 1);
+}
+
 #[test]
 fn test_empty() {
     let mut it = empty::<i32>();