repeat iter: tests + panic in fold

library/core/src/iter/adapters/map.rs

@@ -112,6 +112,13 @@ where
         self.iter.size_hint()
     }
 
+    fn last(mut self) -> Option<Self::Item>
+    where
+        Self: Sized,
+    {
+        self.iter.last().map(&mut self.f)
+    }
+
     fn try_fold<Acc, G, R>(&mut self, init: Acc, g: G) -> R
     where
         Self: Sized,

library/core/src/iter/sources/repeat.rs

@@ -102,7 +102,11 @@ impl<A: Clone> Iterator for Repeat<A> {
     }
 
     #[track_caller]
-    fn count(self) -> usize {
+    fn fold<B, F>(self, _init: B, _f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
         panic!("iterator is infinite");
     }
 }

library/coretests/tests/iter/sources.rs

@@ -30,6 +30,22 @@ fn test_repeat_take_collect() {
     assert_eq!(v, vec![42, 42, 42]);
 }
 
+#[test]
+#[should_panic = "iterator is infinite"]
+fn test_repeat_count() {
+    repeat(42).count();
+}
+
+#[test]
+fn test_repeat_last() {
+    assert_eq!(repeat(42).last(), Some(42));
+}
+
+#[test]
+fn test_repeat_map_double_last() {
+    assert_eq!(repeat(42).map(|e| 2 * e).last(), Some(2 * 42));
+}
+
 #[test]
 fn test_repeat_with() {
     #[derive(PartialEq, Debug)]