So there're three "shape"s for a module: 1. single mode: file.rs 2. headless mode: file/ mod.rs + <submodules> 3. file.rs + file/ <submodules>.

The path attribute supports the traditional two modes(1st+2nd mode). When you specify a file using path. It acts as if that file is the mod.rs, and assumes its submodules live by the side of it. However if your project is using 2018 edition recommended mode (1st+3rd mode), that will become a mess, as the files living by the side it is its sibling modules, not submodules.