Prior to Rust 1.30, in order to have modules nested in subdirectories, they needed to be named some_module_name/mod.rs. This lead to a lot of files being named mod.rs though, which could make grepping difficult. Nowadays, the modules can be (and are encouraged to be) named <modulename>.rs and submodules named <modulename>/<submodulename>.rs, which is much easier to search for.

See https://doc.rust-lang.org/reference/items/modules.html#module-source-filenames for more info.