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));
}

playground.rs

fn main() {
    let s = "def".to_string();
    foo(|_| unsafe{as_mut_static(&s)});
}

fn foo<F: Fn(&u32) -> &mut String>(f: F) {
    let x = 4;
    let s = f(&x);
    s.push_str("aaa ");
    s.push_str(&f(&x));