Create ZST-nthback.rs

Author: DrAsu33

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

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+//@ 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.
+}