rust crate not installed with `pip install sphinxcontrib-rust`

On pypi it says

There are two components that are required for this to work

1. The ``sphinx-rustdocgen`` Rust crate for extracting the docs.
2. The ``sphinxcontrib_rust`` Python package, which is a Sphinx extension.

Both components are installed when installing the Python package with

.. code-block::

   pip install sphinxcontrib-rust

But as far as I can see the crate is not installed.

As a workaround I have to do cargo install sphinx-rustdocgen

Am I doing something wrong?