ngs 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 safe is if the
    produced <code class="code">bytes</code> is shared -- used as an immutable byte
    sequence. This is possibly useful for incremental migration of
    low-level programs that manipulate immutable sequences of bytes
    (for example <a href="Marshal