fn takes_dyn_fun(fun: &dyn Fn() -> u32) {
    let x = fun();
}

pub fn unit_to_u32() -> u32 {
    assert!(false); 
    0
}

fn main() {
    takes_dyn_fun(&unit_to_u32)
}

    // Create a nested boxed once-callable closure
    let f: Box<Box<dyn FnOnce(i32)>> = Box::new(Box::new(|x| assert!(x == 1)));
    f(1);

    // Create a pointer to a closure
    let g = |y| assert!(y == 2);
    let p: &dyn Fn(i32) = &g;
    p(2);

    // Additional level of pointer nesting
    let q: &dyn Fn(i32) = &p;
    q(2);