If you use biodegradable litter then it's perfectly fine to use the toilet waste and used litter from your rabbit as a composting material. Unlike cat and dogs, a rabbit's diets are mostly fiber and vegetables, so their poops are perfectly fine for composting.
Even if it has some pee mixed in with it this will be fine. You can check if it's biodegradable by asking at your pet store or check the packaging. We recommend the paper-based litter rather than a wood-based litter.