cons that does not copy objects.

rust/list.rs

    Cons(A, Box<List<A>>)
}
pub fn cons<A>(x: A, xs: List<A>) -> List<A> {
    match xs {
        Nil => Cons(x, box Nil),
        l => Cons(x, box l)
pub fn cons<A>(x: A, xs: Box<List<A>>) -> Box<List<A>> {
    match *xs {
        Nil => box Cons(x, box Nil),
        l => box Cons(x, box l)
    }
}
}
pub fn main() {
	let nil: List<int> = Nil;
	let ns:  List<int> = cons(1i, cons(2, cons(3, Nil)));
	let nil: List<int>      = Nil;
	let ns:  Box<List<int>> = cons(1i, cons(2, cons(3, box Nil)));
	println!("nil = {}, ns = {}", nil, ns);
	println!("first(nil) = {}, first(ns) = {}", first(nil), first(ns));
	println!("first(nil) = {}, first(ns) = {}", first(nil), first(*ns));
}