rental! {
    mod rentals {
        use super::*;

        #[rental_mut]
        pub struct FrankRental {
            instance: Box<Instance>,
            allocate: Func<'instance, (i32), i32>,
            deallocate: Func<'instance, (i32, i32), ()>,
            invoke: Func<'instance, (i32, i32), i32>,
        }
    }
}