Rollup merge of #88507 - atsuzaki:slice-fill-maybeuninit-test, r=RalfJung

Add test case for using `slice::fill` with MaybeUninit

Adds test for #87891

Looks alright? `@RalfJung`
Fixes #87891

Author: m-ou-se

library/core/tests/slice.rs

@@ -1,5 +1,6 @@
 use core::cell::Cell;
 use core::cmp::Ordering;
+use core::mem::MaybeUninit;
 use core::result::Result::{Err, Ok};
 
 #[test]
@@ -2144,3 +2145,10 @@ fn test_slice_run_destructors() {
 
     assert_eq!(x.get(), 1);
 }
+
+#[test]
+fn test_slice_fill_with_uninit() {
+    // This should not UB. See #87891
+    let mut a = [MaybeUninit::<u8>::uninit(); 10];
+    a.fill(MaybeUninit::uninit());
+}