Rust code shared from the playground

playground.rs

#![feature(proc_macro, unboxed_closures)]
use std::any::{Any};

fn main() {
    let any_box_box = Box::new(ff()) as Box<Any + 'static>;
    let g = any_box_box.downcast_ref::<Box<Fn<(u32,), Output = u32>>>().expect("Cast D:");
    let gg = g.as_ref();
    println!("{}", gg(2u32));
}

fn ff() -> Box<Fn<(u32,), Output = u32>> {
    Box::new(|x: u32| x * 2)
}