title: In Rust, what is the purpose of a mod.rs file?
body: <p>In some Rust projects I've seen (i.e <a href="https://github.com/pczarn/rustboot" rel="noreferrer">pczarn/rustboot</a>), I've seen <code>mod.rs</code> files in directories for whatever reason. I've not been able to find documentation about this, and I've seen it in many other Rust projects.
  What is the purpose of a mod.rs file, and when should I use it?</p>