Ruling out arbitrary cost copy construction?

[Don]

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?