1. This API is not documented as supporting both JSON and CBOR, so I assumed it was just CBOR (https://docs.rs/cddl/latest/cddl/ast/struct.CDDL.html#method.validate). Assuming is bad, but also CBOR is the default for what CDDL does.
2. A bunch of the logic in there is something I'm going to have reproduce, since I need to only do CBOR and I'm keeping around a CDDL instance.
3. I discovered this because it guessed wrong on a document because every once in a while the CBOR is valid UTF8, coincidentally.

Perhaps there should be CDDL::validate_cbor() and validate_json()?

I am not sure an auto-guessing method is super-useful, but if it continues to exit the fact it's guessing should be documented, as well as the heuristic it uses.

If you can indicate your preferred next step, I can submit an appropriate PR.