#[cfg(test)]

mod tests {

    #[test]

    fn test() {
        #[derive(Debug)]

        struct Data {
            value: usize,
        }

        let a = Data { value: 1 };

        let mut b = Box::new(a);

        b.value += 1;

        println!("{:?}", b);
        // let mut c = Box::new(a);
        // compile error because a has been removed.
    }
}