Test and fix removal at past-the-end index

src/arrayvec.rs

@@ -384,12 +384,10 @@ impl<A: Array> ArrayVec<A> {
   #[inline]
   pub fn remove(&mut self, index: usize) -> A::Item {
     let targets: &mut [A::Item] = &mut self.deref_mut()[index..];
-    let mut spare = A::Item::default();
-    for target in targets.iter_mut().rev() {
-      spare = replace(target, spare);
-    }
+    let item = replace(&mut targets[0], A::Item::default());
+    targets.rotate_left(1);
     self.len -= 1;
-    spare
+    item
   }
 
   // NIGHTLY: remove_item, https://github.com/rust-lang/rust/issues/40062

tests/arrayvec.rs

@@ -123,6 +123,14 @@ fn ArrayVec_remove() {
   assert_eq!(&av[..], &[1, 3][..]);
 }
 
+#[test]
+#[should_panic]
+fn ArrayVec_remove_invalid() {
+  let mut av: ArrayVec<[i32; 1]> = Default::default();
+  av.push(1);
+  av.remove(1);
+}
+
 #[test]
 fn ArrayVec_swap_remove() {
   let mut av: ArrayVec<[i32; 10]> = Default::default();