iter repeat: panic on last

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

@@ -97,8 +97,9 @@ fn nth(&mut self, n: usize) -> Option<A> {
         Some(self.element.clone())
     }
 
+    #[track_caller]
     fn last(self) -> Option<A> {
-        Some(self.element)
+        panic!("iterator is infinite");
     }
 
     #[track_caller]

library/coretests/tests/iter/sources.rs

@@ -37,6 +37,7 @@ fn test_repeat_count() {
 }
 
 #[test]
+#[should_panic = "iterator is infinite"]
 fn test_repeat_last() {
     assert_eq!(repeat(42).last(), Some(42));
 }