Stage 10.8: Module Hierarchy

A module can depend on another one. In order to establish a relationship between a module and its sub-module, you must write in the parent module:
mod foo;

The declaration above will look for a file named foo.rs or foo/mod.rs and will insert its contents inside a module named foo under this scope.

Further information:

• https://doc.rust-lang.org/book/ch07-05-separating-modules-into-different-files.html
• https://doc.rust-lang.org/rust-by-example/mod/split.html