title: What is the canonical way to implement is_empty for Iterator?
body: <p>I have something that implements <code>std::iter::Iterator</code> and I want to know if there are <code>&gt; 0</code> elements. What is the standard way to do it? <code>count() &gt; 0</code> looks too expensive. </p>
  
  <p>I see two candidates: <code>any(|_| true)</code> and <code>nth(0).is_some()</code>, but which one should I pick so a future reader can understand on sight what I'm checking here?</p>