fix IntoIter::drop on high-alignment ZST

Author: RalfJung

README.md

@@ -639,6 +639,7 @@ Definite bugs found:
 * [Data race in `thread::scope`](https://github.com/rust-lang/rust/issues/98498)
 * [`regex` incorrectly handling unaligned `Vec<u8>` buffers](https://www.reddit.com/r/rust/comments/vq3mmu/comment/ienc7t0?context=3)
 * [Incorrect use of `compare_exchange_weak` in `once_cell`](https://github.com/matklad/once_cell/issues/186)
+* [Dropping with unaligned pointers in `vec::IntoIter`](https://github.com/rust-lang/rust/pull/106084)
 
 Violations of [Stacked Borrows] found that are likely bugs (but Stacked Borrows is currently just an experiment):
 

tests/pass/vec.rs

@@ -37,15 +37,19 @@ fn vec_into_iter() -> u8 {
 }
 
 fn vec_into_iter_rev() -> u8 {
-    vec![1, 2, 3, 4].into_iter().map(|x| x * x).fold(0, |x, y| x + y)
+    vec![1, 2, 3, 4].into_iter().rev().map(|x| x * x).fold(0, |x, y| x + y)
 }
 
-fn vec_into_iter_zst() -> usize {
-    vec![[0u64; 0], [0u64; 0]].into_iter().rev().map(|x| x.len()).sum()
+fn vec_into_iter_zst() {
+    for _ in vec![[0u64; 0]].into_iter() {}
+    let v = vec![[0u64; 0], [0u64; 0]].into_iter().map(|x| x.len()).sum::<usize>();
+    assert_eq!(v, 0);
 }
 
-fn vec_into_iter_rev_zst() -> usize {
-    vec![[0u64; 0], [0u64; 0]].into_iter().rev().map(|x| x.len()).sum()
+fn vec_into_iter_rev_zst() {
+    for _ in vec![[0u64; 0]; 5].into_iter().rev() {}
+    let v = vec![[0u64; 0], [0u64; 0]].into_iter().rev().map(|x| x.len()).sum::<usize>();
+    assert_eq!(v, 0);
 }
 
 fn vec_iter_and_mut() {
@@ -150,8 +154,8 @@ fn main() {
     assert_eq!(vec_into_iter(), 30);
     assert_eq!(vec_into_iter_rev(), 30);
     vec_iter_and_mut();
-    assert_eq!(vec_into_iter_zst(), 0);
-    assert_eq!(vec_into_iter_rev_zst(), 0);
+    vec_into_iter_zst();
+    vec_into_iter_rev_zst();
     vec_iter_and_mut_rev();
 
     assert_eq!(make_vec().capacity(), 4);