There are SDL bindings and other low level libraries if you want more than a toolkit offers.
Writing a HTML/JavaScript GUI may also be an option. Linking to back-end server libraries e.g. https://rocket.rs/
To add to the list;
- webasm/asmjs are in basic working condition, would have to write a fair bit to get a working interactive canvas gui.
- OS with GUI has been written in rust https://www.redox-os.org/