guage-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span><span class="boring">use std::thread;
</span><span class="boring">use std::time::Duration;
</span><span class="boring">
</span>let expensive_closure = |num: u32| -&gt; u32 {
    println!(&quot;calculating slowly...&quot;);
    thread::sleep(Duration::from_secs(2));
    num
};
<span class="boring">}</span></code></pre></pre>
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<span class="caption">Listing 13-7: Adding optional type annotations of the
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