Crates and Modules
When a project starts getting large, it's considered a good software engineering practice to split it up into a bunch of smaller pieces, and then fit them together. It's also important to have a well-defined interface, so that some of your functionality is private, and some is public.
https://www.cs.brandeis.edu/~cs146a/rust/doc-02-21-2015/book/crates-and-modules.html
