annel such as a memoization combinator.</p>

<p>The caller may not mutate <code class="code">s</code> while the string is borrowed (it has
   temporarily given up ownership). This affects concurrent programs,
   but also higher-order functions: if <a href="String.html#VALlength"><code class="code"><span class="constructor">String</span>.length</code></a> returned
   a closure to be called later, <code class="code">s</code> should not be mutated until this
   closure is fully applied and returns ownership.</