title: What is this question mark operator about?
body: <p>I'm reading <a href="https://doc.rust-lang.org/std/fs/struct.File.html" rel="noreferrer">the documentation for <code>File</code></a>:</p>
  
  <pre><code>//..
  let mut file = File::create("foo.txt")?;
  //..
  </code></pre>
  
  <p>What is the <code>?</code> in this line? I do not recall seeing it in the Rust Book before.</p>