We are moving away from &[mut T] types towards mutable reference slots, like &mut ~[T]. This works great for functions like vec::push<T>(v: &mut ~[T], initval: T), which modify the length of the vector, but I don't believe we have a clear strategy to upgrading vec::raw::memcpy, which can be used to update a subset of a vector. I think we might need two separate types to capture this functionality.