Rollup merge of rust-lang#147258 - hkBst:panic-last-repeat, r=scottmcm

iter repeat: panic on last

rust-lang#146660 (comment)

Author: matthiaskrgr

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

@@ -101,8 +101,9 @@ impl<A: Clone> Iterator for Repeat<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));
 }