   });

    <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">loopback</span> <span class="op">=</span> <span class="ident">future</span>::<span class="ident">loop_fn</span>::<span class="op">&lt;</span><span class="kw">_</span>, (), <span class="kw">_</span>, <span class="kw">_</span><span class="op">&gt;</span>(<span class="ident">serial</span>, <span class="op">|</span><span class="kw-2">mut</span> <span class="ident">serial</span><span class="op">|</span> {
        <span clas