title: When does to_owned() not clone?
body: <p>The <a href="https://doc.rust-lang.org/std/borrow/trait.ToOwned.html#tymethod.to_owned" rel="nofollow noreferrer">docs for to_owned()</a> state:</p>
  <blockquote>
  <p>Creates owned data from borrowed data, usually by cloning.</p>
  </blockquote>
  <p>But it is unstated the conditions under which cloning does not occur.  &quot;Usually&quot; is quite vague, and I am trying to remove <code>.clone()</code> calls for performance reasons.</p>
  <p>Can someone clarify?</p>