box_map_in_place.rs

use std::{mem,ptr};

fn box_map_in_place<T, F>(boxed: Box<T>, f: F) -> Box<T> where F: FnOnce(T) -> T {
    unsafe {
        let p : *mut T = mem::transmute(boxed);
        ptr::write(p, f(ptr::read(p)));
        return mem::transmute(p);
    }
}

fn main() {
    let b = Box::new("foo".to_string());
    let c = box_map_in_place(b, |s| s + "bar");
    println!("{}", c);
}