Learned that mod.rs files are the old way. Rust 1.30+ has a better convention.

The Old Way (Don’t Do This)

src/
  util/
    mod.rs        # Old style
    helper.rs

The New Way

src/
  util.rs         # Module definition
  util/
    helper.rs     # Submodule

In util.rs:

mod helper;  // Load util/helper.rs

Why Change?

From the Rust docs:

Note: Prior to rustc 1.30, using mod.rs files was the way to load a module with nested children. It is encouraged to use the new naming convention as it is more consistent, and avoids having many files named mod.rs within a project.

Having 20 files all named mod.rs was confusing. The new way is clearer.

Important

You cannot have both util.rs and util/mod.rs. Rust will error. Pick one style and stick to it.

For new projects: use the new style. Cleaner and more consistent.