rect. For example, mutating <code class="code">unsafe_of_string&nbsp;("foo"&nbsp;^&nbsp;s)</code>
    could mutate the shared string <code class="code">"foo"</code> -- assuming a rope-like
    representation of strings. More generally, functions operating on
    strings will assume shared ownership, they do not preserve unique
    ownership. It is thus incorrect to assume unique ownership of the
    result of <code class="code">unsafe_of_string</code>.</p>

<p>The only case we have reasonable confidence is 