Ick. :)

gistfile1.rs

fn main() {
    let mut input = str::to_chars("abcd");
    while !input.is_empty() {
        let pair = taken(input, 2);
        input = dropn(input, 2);
        io::println(str::from_chars(pair));
    }
}

fn taken<T:Copy>(v: &[T], n: uint) -> ~[T] {
    v.slice(0, n).to_owned()
}

fn dropn<T:Copy>(v: &[T], n: uint) -> ~[T] {
    v.slice(n, v.len()).to_owned()
}