Rollup merge of rust-lang#101237 - RalfJung:into-iter-zst, r=thomcc

fix into_iter on ZST

Fixes rust-lang#101235
Thanks to `@ChrisDenton` for [spotting the problem](rust-lang#100819 (review)).

Author: RalfJung

library/alloc/src/vec/into_iter.rs

@@ -266,7 +266,7 @@ impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
             None
         } else if mem::size_of::<T>() == 0 {
             // See above for why 'ptr.offset' isn't used
-            self.end = self.ptr.wrapping_byte_sub(1);
+            self.end = self.end.wrapping_byte_sub(1);
 
             // Make up a value of this ZST.
             Some(unsafe { mem::zeroed() })

library/alloc/tests/vec.rs

@@ -1104,6 +1104,12 @@ fn test_into_iter_drop_allocator() {
     assert_eq!(drop_count, 2);
 }
 
+#[test]
+fn test_into_iter_zst() {
+    for _ in vec![[0u64; 0]].into_iter() {}
+    for _ in vec![[0u64; 0]; 5].into_iter().rev() {}
+}
+
 #[test]
 fn test_from_iter_specialization() {
     let src: Vec<usize> = vec![0usize; 1];