title: What does Rust's unary || (parallel pipe) mean?
body: <p>In <a href="http://smallcultfollowing.com/babysteps/blog/2016/04/27/non-lexical-lifetimes-introduction/" rel="noreferrer">Non-Lexical Lifetimes: Introduction</a>, Niko includes the following snippet:</p>
  
  <pre><code>fn get_default3&lt;'m,K,V:Default&gt;(map: &amp;'m mut HashMap&lt;K,V&gt;,
                                  key: K)
                                  -&gt; &amp;'m mut V {
      map.entry(key)
         .or_insert_with(|| V::default())
  }
  </code></pre>
  
  <p>What does the <code>|| V::default()</code> mean here?</p>