Unit tests

A good software needs tests to ensure that it works in most cases. So, we will add tests to our FTP server by starting to write unit tests for the FTP codec.

Unit tests verify only a unit of the program, which may be a function. They are different from the integration tests, which we will see later, that test the software as a whole.

Let's go in the codec module and add a new inner module to it:

#[cfg(test)]
mod tests {
}

We are again using the #[cfg] attribute; this time, it only compiles the following module when running the tests. This is to avoid adding useless code in the final binary.

In this new module, we will add a few import statements that we will need later when writing the tests:

#[cfg(test)]
mod tests {
    use std::path::PathBuf;

    use ftp::ResultCode;
    use super::{Answer, BytesMut, Command, Decoder, Encoder, FtpCodec};
}

As you can see, we use super to access some types from the parent module (codec): this is very frequent for unit tests because we usually test...