Splitting your code with crates and modules

Rust knows two types of code units: crates and modules. A crate is an external library, complete with its own Cargo.toml configuration file, dependencies, tests, and code. Modules, on the other hand, split the crate into logical parts that are only visible to the user if they import specific functions. Since the 2018 edition of Rust, the difference in using these structural encapsulations has been minimized.