title: "Unrecognized option 'release'" when compiling with rustc in release mode
body: <p>I have a simple <code>.rs</code> file and want to compile it using <code>rustc</code>. I always heard about how important it is to compile in release mode, because otherwise my Rust program will be slow.</p>
  
  <p>However, if I use the often quoted <code>--release</code> flag, it doesn't work. What is everyone talking about if the flag doesn't even exist?</p>
  
  <pre class="lang-none prettyprint-override"><code>$ rustc --release foo.rs
  error: Unrecognized option: 'release'.
  </code></pre>