title: Swift: #warning equivalent
body: <p>Does Swift have a #warning equivalent?
  It's simply used to show a warning in Xcode's own GUI</p>
  
  <p>I'm also interested in whether there's a #error equivalent.</p>
  
  <p>Apple has said #pragma mark is coming soon, it could possibly be the same with this.</p>
  
  <p><img src="https://i.stack.imgur.com/0a7FV.png" alt="enter image description here"></p>