title: use of unstable library feature - how can I fix those?
body: <p>I got a bunch of errors again:</p>
  
  <pre><code>$ cargo build
  error: use of unstable library feature 'std_misc'
  use std::time::duration::Duration;
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  error: use of unstable library feature 'convert': waiting on RFC revision
  let my_let1 = aaa(bbb.as_str(), ccc, ddd.eee);
                                                          ^~~~~~~~
  error: use of unstable library feature 'convert': waiting on RFC revision
  let let1 = aaa.as_slice();
                                               ^~~~~~~~~~
  error: use of unstable library feature 'convert': waiting on RFC revision
  let let1 = str::from_utf8(aaa.as_slice()).unwrap();
                                                  ^~~~~~~~~~
  </code></pre>
  
  <p>How to solve them? What does it mean: <code>add #![feature(collections)] to the crate attributes to enable</code> - what crates? I don't have the source code of my crates. How would other people compile my library on their machines then?</p>
  
  <p>And oddly enough this also throws an error:</p>
  
  <pre><code>src/lib.rs:1:1: 1:31 error: unstable feature
  src/lib.rs:1 #![feature(convert, std_misc)]
  </code></pre>
  
  <p>when I add it at the top my library.</p>