The to_hex() is the problem now in assert macro, not when used with println! in another module.

challenge2.rs

use rustc_serialize::hex::ToHex;
use rustc_serialize::hex::FromHex;

pub fn xor(a: &str, b: &str) -> Vec<u8> {
   return a.from_hex().unwrap().iter().zip(b.from_hex().unwrap().iter()).map(|(&x,&y)|x^y).collect();

}

#[cfg(test)]
mod tests {
  extern crate rustc_serialize;
  use super::*;
  use rustc_serialize::hex::ToHex;
  use rustc_serialize::hex::FromHex;

  #[test]
  fn it_works2() {
    let a = "1c0111001f010100061a024b53535009181c";
    let b = "686974207468652062756c6c277320657965";
    let control = "746865206b696420646f6e277420706c6179";
    let result = xor(a,b);
    assert_eq!("746865206b696420646f6e277420706c6179",[result].to_hex());
  }
} // end mod tests