I've had a company actually halt the recruiting process because I wouldn't share my current salary. They asked (as they all do), and I did a standard polite "I'd rather not share my current compensation, please make an offer you feel is a good starting point." They then said, well we'd rather not make an offer without knowing this baseline. And that was that. Sure, I guess I could have just made up a fake (high) number, but I wasn't too thrilled about what I saw about the company during the interviews, so it didn't bother me at all. If a company is being shitty during the interview process, they're probably going to be shitty to work for.