Improve rust test coverage.

[rsyi]

The rust side of things is embarrassingly uncovered by tests. :)

This will be a standing issue, until we get to the point where our test coverage matches Python.

[RiccardoTonini]

Hi all,

I'm happy to spend a little bit of time on this task. I might need to have a brief chat, so that we're on the same page. I'll ping you on your Slack channel, after having a better look at the code.

Thanks

[rsyi]

Love it @RiccardoTonini . :) Go for it, and I'd be happy to have a quick chat.