e mutated
   after the call to <code class="code">bytes_length&nbsp;s</code>. The <a href="String.html#VALlength"><code class="code">String.length</code></a> function
   temporarily borrows unique ownership of the byte sequence
   (and sees it as a <code class="code">string</code>), but returns this ownership back to
   the caller, which may assume that <code class="code">s</code> is still a valid byte
   sequence after the call. Note that this is only correct because we
   know that <a href="String.html#VA