If you remove stuff that uses UnsafeCell, and remove syscalls, you have a purely functional programming language. You don't really have mutability unless you'r

If you remove stuff that uses UnsafeCell, and remove syscalls, you have a purely functional programming language. You don't really have mutability unless you're using something like RefCell or Mutex. You can program your way around any use of mutable references, or recast their use in a form that avoids them.