For example, for a python submodule defined in a separate rust module, the following fails to compile with an error saying that no rules expected the ::.

use pyo3::prelude::*;
use pyo3::{wrap_pymodule};

mod a;

#[pymodule]
fn mymodule(_py: Python, module: &PyModule) -> PyResult<()> {
	module.add_wrapped(wrap_pymodule!(a::submodule))?;

	Ok(())
}

There was a tiny bit of discussion about this over on the matrix channel.