title: Why define a struct with single private field of unit type?
body: <p>Why does <a href="https://github.com/rust-lang/rust/blob/1.47.0/library/core/src/str/mod.rs#L149-L151" rel="noreferrer"><code>ParseBoolError</code></a> have the <code>_priv</code> field:</p>
  <pre><code>pub struct ParseBoolError {
      _priv: (),
  }
  </code></pre>
  <p>I don't think that the <code>_priv</code> field is used.</p>