struct GE {
    env: Option<Box<Fn() -> i32>>,
}

struct HasDrop { i: i32 }

impl Drop for HasDrop {
    fn drop(&mut self) {
        println!("Dropping!");
    }
}

impl GE {
    fn add<T>(&mut self, func: T) where T: 'static + Fn() -> i32 {
         self.env = Some(Box::new(func));
    }
}

fn o() {
    let mut g = GE { env: None };
    let fo = HasDrop { i: 20 };
    g.add(move || { fo.i });
}

fn main() {
    println!("A");
    o();
    println!("B");
    o();
    println!("C");
    o();
    println!("D");
}