Fix: Clean history and address all reviewer comments

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);

tests/ui/iterators/ZST-nthback.rs

@@ -0,0 +1,10 @@
+//@ run-pass
+// test Into_Iter::nth_back does not cause UB for ZSTs with high alignment
+
+#[repr(align(8))]
+struct Thing;
+
+fn main() {
+    let v = vec![Thing, Thing];
+    let _ = v.into_iter().nth_back(1);
+}