title: How to attach possible_values to a struct with structopt?
body: <p><a href="https://docs.rs/clap/2.33.0/clap/" rel="noreferrer"><code>clap</code></a> allows you to provide list of accepted values using <a href="https://kbknapp.github.io/clap-rs/clap/struct.Arg.html#method.possible_values" rel="noreferrer"><code>possible_values</code></a> like this.</p>
  
  <pre class="lang-rust prettyprint-override"><code>let mode_vals = ["fast", "slow"];
  .possible_values(&amp;mode_vals)
  </code></pre>
  
  <p>How to do this with <a href="https://docs.rs/structopt/0.3.5/structopt/" rel="noreferrer"><code>structopt</code></a>?</p>