An attempt to make a simple tree implementation in rust

tree.rs

use std::util::swap;

#[deriving(Eq)]
pub enum Tree {
  Text(~str),
  Cons(~Tree, ~Tree),
  Nil
}

pub fn first<'r>(tree: &'r Tree) -> &'r Tree {
  match *tree {
    Cons(ref first, _) => {
      let node: &Tree = *first;
      node
    }
    _ => &Nil
  }
}

pub fn rest<'r>(tree: &'r Tree) -> &'r Tree {
  match *tree {
    Cons(_, ref rest) => {
      let node: &Tree = *rest;
      node
    }
    _ => &Nil
  }
}

pub fn next<'r>(tree: &'r Tree) -> &'r Tree {
  first(rest(tree))
}

pub fn append(tree: &mut Tree, node: Tree) {
  match *tree {
    Cons(_, ref mut tail @ ~Nil) => {
      let end: &mut Tree = *tail;
      swap(end, ~Cons(~node, ~Nil))
    }
    Cons(_, ref mut rest) => {
      let next: &mut Tree = *rest;
      append(next, node)
    },
    _ => fail!("appending() on non Cons node")
  }
}

#[main]
fn main() {
  let mut tree = ~Cons(~Nil, ~Nil);

  assert!(first(tree) == &Nil);
  assert!(rest(tree) == &Nil);

  let head = Text(~"hello");
  append(tree, head);

  println!("{:?}", tree)
  //assert!(rest(tree) == &head);
  //assert!(next(tree) == &Nil);

  let tail = Text(~"world");
  append(tree, tail);

  println!("{:?}", tree)
  //assert!(rest(tree) == &head);
  //assert!(next(tree) == &tail);
}