It may not do what you want; you can still
impl A for () {}
impl B for () {}
But then you can't actually use the impls apparently (and bad things would probably happen if you found a way to). I.e. this feature is still wonky. (This probably deserves a dedicated issue.)
I haven't had time to play much, but here's some more reading for you. Also, historically, the ability to express mutually exclusive traits has led to putting things on hold until the feature of mutually exclusive traits can receive "more in-depth consideration".