Function secp256k1_sys::secp256k1_schnorrsig_sign_custom

pub unsafe extern "C" fn secp256k1_schnorrsig_sign_custom(
    cx: *const Context,
    sig: *mut c_uchar,
    msg: *const c_uchar,
    msg_len: size_t,
    keypair: *const Keypair,
    extra_params: *const SchnorrSigExtraParams
) -> c_int

Available on non-secp256k1_fuzz only.