Function heif_have_encoder_for_format 

pub unsafe extern "C" fn heif_have_encoder_for_format(
    format: heif_compression_format,
) -> c_int

Quick check whether there is an enoder available for the given format. Note that the encoder may be limited to a certain subset of features (e.g. only 8 bit, only lossy). You will have to query the specific capabilities further.