Modules Inline

• A sub-module can be directly inlined within a module's code

• One very common use for inline modules is creating unit tests

// This macro removes this inline module when Rust 
// is not in test mode.
#[cfg(test)]
mod tests {
    // Notice that we don't immediately get access to the 
    // parent module. We must be explicit.
    use super::*;

    // ... tests go here ...
}