Fix: Clean history and address all reviewer comments

Create ZST-nthback.rs

Delete tests/ui/iterators/ZST-nthback.rs

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);

src/tools/miri/tests/fail/unaligned_pointers/ZST-nthback.rs

@@ -0,0 +1,27 @@
+//@ compile-flags: -Zmiri-symbolic-alignment-check
+//@ run-pass
+
+use std::ptr;
+
+// This is a ZST (Zero-Sized Type) that requires high alignment (alignment 8).
+// The original bug occurred because IntoIter::nth_back tried to calculate an 
+// unaligned pointer for drop_in_place based on this high-aligned ZST, leading to UB in Miri.
+#[repr(align(8))]
+struct Thing;
+
+fn main() {
+    // Create a Vec containing two high-aligned ZSTs.
+    let v = vec![Thing, Thing];
+    
+    // Calling IntoIter::nth_back(1) triggers the problematic drop_in_place logic 
+    // within Vec::IntoIter. The fix (using ptr::NonNull::dangling()) prevents the UB.
+    let mut iter = v.into_iter();
+    let _ = iter.nth_back(1);
+    
+    // Also test nth_back(0) for completeness.
+    let v2 = vec![Thing, Thing];
+    let mut iter2 = v2.into_iter();
+    let _ = iter2.nth_back(0);
+    
+    // The main assertion is that the program runs to completion without Miri reporting UB.
+}