Our MuSig API takes a &[u8] for signing, which can be any byte string of any length. We allow feeding the message into the nonce generation function for "maximal reuse prevention", but we take the message there as a Message type, which is a 32-byte array which is required to be the output of a hash function.

The Message type should only be used by ECDSA, not by anything Schnorr related.

We need to fix this before release.

Filing an issue rather than just fixing it because I have too many things going on at once in this library -- would like to get #809 in as well before the next release. (Would also like to get #806 but won't block a release on that..)