So you should be able to keep it and use FromVec, right? If not then FromVec hasn't really achieved its goals.

I don't think we should be so aggressively opinionated about the use of ~[T] - I don't think we should use it, but I don't think we need to make life unnecessarily difficult for others to use it.

Once we have DST, we will want to support [T] so that it can be used in data structures, that will naturally allow for Box<[T]>. In the meantime, I don't want to take away functionality and then return it, that is unnecessary churn and can cause pain for developers. I believe that compatibility with the standard libs will be enough to motivate people to use Vec<T> rather than ~[T], we don't need to cripple ~[T] to make that happen.