split_at_mut doesn't allow you to read through aliased "active" &mut pointers either (I would assume that restrict pointers that cannot be either read from or written to don't count).
Hm, in that case you don't need to appeal to split_at_mut--reborrowing is sufficient to demonstrate that point. I think what czwarich is saying is more subtle--that restrict allows "multiple readers, exclusive writer", which means that both shared pointers without UnsafeCell and mutable references in Rust are restrict in this sense, while &mut is more restrictive.
