This blog post by Dimitri Merejkowsky clued me into the fact that I had
incorrectly ignored a compiler warning about an unused assignment:
https://dmerej.info/blog/post/awesome-rust-warnings/#the-useless-mutation

When I wrote that line, I didn't realise you could declare an
uninitialised variable, and definitely didn't get what the compiler
meant when it told me:

    warning: value assigned to `email_eid` is never read
       --> src/main.rs:102:9
        |
    102 |     let mut email_eid = String::new();
        |         ^^^^^^^^^^^^^
        |
        = note: `#[warn(unused_assignments)]` on by default
        = help: maybe it is overwritten before being read?

so I ignored the warning.

Describe the files used for authentication.

Change the file paths to use `$XDG_DATA_HOME` instead of writing the
path explicitly, as that environment variable may have been changed to
refer to a different directory.

Add description and authentication sections.