trait CloneFn: Fn() {
    fn clone_erased(&self) -> Box<dyn CloneFn>;
}

impl<F: Fn() + Clone + 'static> CloneFn for F {
    fn clone_erased(&self) -> Box<dyn CloneFn> {
        Box::new(self.clone())
    }
}

fn foo(f: &dyn CloneFn) {
    f();
    let g = f.clone_erased();
    g();
    let h = g.clone_erased();
    h();
}

fn main() {
    use std::sync::Arc;
    let a = Arc::new(());
    foo(& move || { println!("{}", Arc::strong_count(&a)) })
}