use std::any::Any;

fn test1<T: 'static>(f: fn(&mut T)) -> Box<Any + 'static> {
    Box::new(f)
}

struct Foo<T>(Box<dyn for<'a> Fn(&'a mut T) + 'static>);

fn test2<T: 'static, F: Fn(&mut T) + 'static>(f: F) -> Box<Any + 'static> {
    Box::new(Foo(Box::new(f)))
}



fn thing(i: &mut i32) {
    println!("{}", i);
}

fn main() {
    let x = test1(thing);
    let y = test2(thing);

    let a = x.downcast_ref::<fn(&mut i32)>().expect("cast x");
    a(&mut 3);

    let b = y.downcast_ref::<Foo<i32>>().expect("cast y");
    (b.0)(&mut 3);
}