Ruling out arbitrary cost copy construction?

[dsimcha]

== Quote from Don (nospam at nospam.com)'s article
> Andrei Alexandrescu wrote:
> >> If anything, I'm inclined to say that we assume that the postblit is
> >> O(1) and
> >> let the programmer worry about any inefficiencies. We can point out
> >> that anything
> >> worse that O(1) will be a performance problem, but it seems to me that
> >> any
> >> attempt to either accomodate arbitrary cost postblit constructors or
> >> to try and
> >> use any kind of scheme which forces programmers to write postblits in
> >> a certain
> >> way is too complicated and doomed to failure. And even if it works, it
> >> will be
> >> highly annoying to deal with.
> >
> > It sure is annoying, but it does work.
> >
> > Don, can you estimate how difficult it would be to convert BigInt to a
> > refcounted implementation?
> At the moment, I think it's impossible.
> Has anyone succesfully implemented refcounting in D? As long as bug 3516
> (Destructor not called on temporaries) remains open, it doesn't seem to
> be possible.
> Is that the only blocker, or are there others?

Can someone please clarify something for me?  I thought BigInt was already COW
(though I guess COW w/o ref counting is still pretty inefficient).