title: What is #[warn(unstable)] about in Rust?
body: <p>I have a really simple cat function written in the Rust 1.0 alpha.</p>
  
  <pre><code>use std::io;
  
  fn main(){
      let mut reader = io::stdin();
      loop {
          let input = reader.read_line().ok().expect("Failed to read line");
          print!("{}", input);
      }
  }
  </code></pre>
  
  <p>When I compile it, I get the following warnings:</p>
  
  <pre><code>bindings.rs:5:26: 5:35 warning: use of unstable item, #[warn(unstable)] on by default
  bindings.rs:5         let mut reader = io::stdin();
                                         ^~~~~~~~~
  bindings.rs:6:28: 6:39 warning: use of unstable item, #[warn(unstable)] on by default
  bindings.rs:6         let input = reader.read_line().ok().expect("Failed to read line");
                                           ^~~~~~~~~~~
  </code></pre>
  
  <p>Is there a way to remedy these warnings? </p>