title: "use of unstable library feature 'collections'" using nightly
body: <p>This</p>
  
  <pre><code>fn main() {
  
      let test = "Foo".to_string();
      test.to_lowercase();
  
  }
  </code></pre>
  
  <p>produces an error</p>
  
  <pre><code>error: use of unstable library feature 'collections'
         test.to_lowercase();
              ^~~~~~~~~~~~~~
  </code></pre>
  
  <p>but I am using</p>
  
  <pre><code>rustc 1.2.0-nightly (f76d9bcfc 2015-05-28) (built 2015-05-28)
  </code></pre>
  
  <p>and according to <a href="http://doc.rust-lang.org/1.0.0/book/release-channels.html" rel="noreferrer">http://doc.rust-lang.org/1.0.0/book/release-channels.html</a> unstable features are enabled on nightly. I've also tried stable and beta, but the error is exactly the same. So what's the issue here?</p>