I started off avoiding mod.rs because it's the old way and I prefer having the module name as the filename. However, if the module needs a folder for submodules anyway, then there's a reason to tuck it away as mod.rs, especially if not doing so leaves lots of duplicate names (a.rs, b.rs, c.rs, ..., a/, b/, c/).

But then I don't really like to have much else in mod.rs other than mod declarations and pub use. Maybe a utility fn or a not-unwieldly implementation of struct "Foo", the module's namesake.