Fixes rust-lang/rust#83583 warnings.

I'm a bit confused by these; is their presence a rustc bug? It doesn't
look like the relevant pattern actually occurs.

tests/vec.rs

@@ -3,6 +3,7 @@
 mod particles;
 use self::particles::{Particle, ParticleVec};
 use soa_derive::StructOfArray;
+use std::{cell::Cell, rc::Rc};
 
 #[test]
 fn ty() {
@@ -162,21 +163,19 @@ fn retain() {
     assert_eq!(particles.index(1).name, "C");
 }
 
-#[test]
-fn drop_vec() {
-    use std::{cell::Cell, rc::Rc};
-
-    #[derive(StructOfArray)]
-    struct IncrOnDrop {
-        cell: Rc<Cell<usize>>,
-    }
+#[derive(StructOfArray)]
+struct IncrOnDrop {
+    cell: Rc<Cell<usize>>,
+}
 
-    impl Drop for IncrOnDrop {
-        fn drop(&mut self) {
-            self.cell.set(self.cell.get() + 1);
-        }
+impl Drop for IncrOnDrop {
+    fn drop(&mut self) {
+        self.cell.set(self.cell.get() + 1);
     }
+}
 
+#[test]
+fn drop_vec() {
     let counter = Rc::new(Cell::default());
     let mut vec = IncrOnDropVec::new();
     for _ in 0..5 {