title: What does "dyn" mean in a type?
body: <p>I have recently seen code using the <code>dyn</code> keyword:</p>
  
  <pre><code>fn foo(arg: &amp;dyn Display) {}
  
  fn bar() -&gt; Box&lt;dyn Display&gt; {}
  </code></pre>
  
  <p>What does this syntax mean?</p>