D2 weak references

[Sean Kelly]

== Quote from Jarrett Billingsley (jarrett.billingsley at gmail.com)'s article
> On Mon, Apr 20, 2009 at 5:03 PM, Sean Kelly <sean at invisibleduck.org> wrote:
> > The horrible thing about all this is it makes using a multithreaded
> > WeakRef horribly slow.  Even if the GC facility were provided for
> > what you want to do you're talking about acquiring a mutex in
> > the GC for every deref operation.  What a mess.
> Then take a hint from D2 and make them thread-local by default ;)  But
> provide a SharedWeakRef for the cases where you need it.

Right.  These problems all go away if the object is not shared.