add a test demonstrating double-free in extend() for ZSTs

tests/tests.rs

@@ -716,6 +716,32 @@ fn test_extend_zst() {
     assert_eq!(array.len(), 5);
 }
 
+#[test]
+fn test_extend_zst_with_drop_is_not_dropped_twice_via_safe_api() {
+    use std::sync::atomic::{AtomicUsize, Ordering};
+
+    static DROP_COUNT: AtomicUsize = AtomicUsize::new(0);
+
+    struct ZstWithSafeDrop;
+
+    impl Drop for ZstWithSafeDrop {
+        fn drop(&mut self) {
+            let previous = DROP_COUNT.fetch_add(1, Ordering::SeqCst);
+            // Extending with a single ZST moves one logical value into the ArrayVec.
+            // Its Drop implementation must run exactly once, when the ArrayVec drops.
+            if previous != 0 {
+                panic!("ZST value dropped more than once");
+            }
+        }
+    }
+
+    DROP_COUNT.store(0, Ordering::SeqCst);
+
+    let mut vec = ArrayVec::<ZstWithSafeDrop, 1>::new();
+    vec.extend(std::iter::once(ZstWithSafeDrop));
+    drop(vec);
+}
+
 #[test]
 fn test_try_from_argument() {
     use core::convert::TryFrom;