title: What does 'let x = x' do in Rust?
body: <p>I saw this code <a href="https://github.com/danburkert/prost/blob/5f54a1be5e4a3c85734f14c3ec23a09bd9cfa609/prost-derive/src/lib.rs#L86" rel="noreferrer">in the wild</a>:</p>
  
  <pre><code>fields.sort_by_key(|&amp;(_, ref field)| field.tags().into_iter().min().unwrap());
  let fields = fields;
  </code></pre>
  
  <p>What does the <code>let fields = fields;</code> line do? Why is it there?</p>