Rust code shared from the playground

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));
    println!("TESTA {}", s);
}

unsafe fn as_static<T>(t_ref: &T) -> &'static T {
    &*(t_ref as *const T)
}

unsafe fn as_mut_static<T>(t_ref: &T) -> &'static mut T {
    &mut *(t_ref as *const T as *mut T)
}