Function module_dir_of 

fn module_dir_of(path: &str) -> String

The directory a file’s own submodules live in.

lib.rs and mod.rs are their containing directory; foo.rs is foo/. This is what a relative pub use path is resolved against, and getting it wrong is the latent bug §1.3.100 recorded: pub use gp::TvGp; inside crates/atune/src/pb2/mod.rs means crate::pb2::gp::TvGp, not crate::gp.