trait FnBox {
    fn call_box(self: Box<Self>);
}

impl<F: FnOnce() + ?Sized> FnBox for F {
    fn call_box(self: Box<F>) {
        (*self)()
    }
}

fn main() {
    let x = "test".to_string();
    let f = Box::new(move || { x; });
    f.call_box();
}