Fix(alloc): Correctly handle ZST alignment for IntoIter::nth_back (#148682)

This commit consolidates all changes, including the core logic fix for IntoIter::nth_back and the addition of the Miri regression test in `library/alloctests/tests/vec.rs`, to prevent Undefined Behavior (UB) when dealing with highly-aligned Zero-Sized Types.

Author: DrAsu33

library/alloc/src/vec/into_iter.rs

@@ -411,7 +411,12 @@ impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
             // SAFETY: same as for advance_by()
             self.end = unsafe { self.end.sub(step_size) };
         }
-        let to_drop = ptr::slice_from_raw_parts_mut(self.end as *mut T, step_size);
+        let to_drop = if T::IS_ZST {
+            // ZST may cause unalignment
+            ptr::slice_from_raw_parts_mut(ptr::NonNull::<T>::dangling().as_ptr(), step_size)
+        } else {
+            ptr::slice_from_raw_parts_mut(self.end as *mut T, step_size)
+        };
         // SAFETY: same as for advance_by()
         unsafe {
             ptr::drop_in_place(to_drop);

library/alloctests/tests/vec.rs

@@ -2717,3 +2717,17 @@ fn vec_null_ptr_roundtrip() {
     let new = roundtripped.with_addr(ptr.addr());
     unsafe { new.read() };
 }
+
+// Regression test for Undefined Behavior (UB) caused by IntoIter::nth_back (#148682)
+// when dealing with high-aligned Zero-Sized Types (ZSTs).
+#[test]
+fn zst_vec_iter_nth_back_regression() {
+    #[repr(align(8))]
+    struct Thing;
+    let v = vec![Thing, Thing];
+    let mut iter = v.into_iter();
+    let _ = iter.nth_back(1);
+    let v2 = vec![Thing, Thing];
+    let mut iter2 = v2.into_iter();
+    let _ = iter2.nth_back(0);
+}