[Rust lang] 90. fn mut fnonce

fn fn_closure<F>(f: F) 
where
    F: Fn()
{
    f();
}

fn fn_mut_closure<F>(mut f: F)
where 
    F: FnMut(),
{
    f();
}

fn fn_once_closure<F>(f: F) 
where 
    F: FnOnce(),
{
    f();
}

fn main() {
    let mut my_string = String::from("Hello there");

    /*
    let print_it = || {
        drop(my_string);
    };

    print_if();
    */

    fn_once_closure(|| {
        my_string.push('a);
        println!("{my_string}");
        drop(my_string);
    })
}