mutability not working?

gistfile1.rs

use std;

fn poke(value: uint, index: uint, data: @mutable [mutable uint]) {
  (*data)[index] = value;
}

fn main(_args: [str]) {
  let vec = [mutable 4u];
  std::io::println(#fmt["%u", vec[0]]);
  poke(3u, 0u, @mutable vec);
  std::io::println(#fmt["%u", vec[0]]);
}