fn main() {
    let answer = run_it(Box::new(|| 1 + 1));
    println!("Answer: {}", answer);
}

trait FnBox {
    type Output;

    fn call(self: Box<Self>) -> Self::Output;
}

impl<F: FnOnce()> FnBox for F {
    type Output = F::Output;

    fn call(self: Box<F>) -> F::Output {
        self()
    }
}

fn run_it(f: Box<dyn FnBox<Output = i64>>) -> i64 {
    f.call()
}