title: What is the use of + operator in the function parameter type?
body: <p>I came across this syntax while learning async &amp; await:</p>
  <pre><code>fn spawn(&amp;self, future: impl Future&lt;Output = ()&gt; + 'static + Send)
  </code></pre>
  <p>What is the role of <code>+</code> sign in the <code>future</code> parameter type?</p>
  <p><a href="https://rust-lang.github.io/async-book/02_execution/04_executor.html" rel="nofollow noreferrer">Link</a> to the guide I'm following.</p>