[Proposal] Weak reference implementation for D

[Denis Shelomovskij]

--- Proposal ---

The proposal is to add weak reference functionality based on 
`unstd.memory.weakref`. It can be placed e.g. in `core.memory`.

Source code: 
https://bitbucket.org/denis-sh/unstandard/src/HEAD/unstd/memory/weakref.d
Documentation: 
http://denis-sh.bitbucket.org/unstandard/unstd.memory.weakref.html
Enhancement request: http://d.puremagic.com/issues/show_bug.cgi?id=4151

--- Reasons ---

Reasons, why we do need weak reference functionality in D (Issue 4151 [1]):

   1. It has a general use.

     I suppose no question here.


   2. It's hard to implement correctly.

     As a proof here are incorrect implementations written by 
experienced developers:
       * Weak reference functionality in `std.signal` implementation:
 
https://github.com/D-Programming-Language/phobos/blob/7134b603f8c9a2e9124247ff250c9b48ea697998/std/signals.d

       * Alex's one from MCI:
 
https://github.com/lycus/mci/blob/f9165c287f92e4ef70674828fbadb33ee3967547/src/mci/core/weak.d

       * Robert's one from his new `std.signals` implementation proposal:
 
https://github.com/phobos-x/phobosx/blob/d0cc6b45511465ef1d493b0d7226ccb990ae84e8/source/phobosx/signal.d

       * My implementation (fixed now, I hope):
 
https://bitbucket.org/denis-sh/unstandard/src/cb9a835a9ff5/unstd/memory/weakref.d

     Everybody can check his knowledge of concurrent programming and D 
GC by trying to understand what exactly every implementation does and 
where are race conditions. It's recommended to read implementations in 
the order provided here going to the next one as soon as you see why 
previous one is incorrect. For now the only [probably] fixed 
implementation is mine so one can see spoiler here:
 
https://bitbucket.org/denis-sh/unstandard/history-node/HEAD/unstd/memory/weakref.d
       (the first and the most fixing (spoiling you joy to understand 
everything yourself) commit is 6f59b33)


   3. It's hard to create a good API design for it.

     No jokes. E.g. there are two different behaviours of .NET weak 
references and even more in Java library.


   4. It is needed for correct signals implementation in D.

     The lack of correct signals implementation is one of [major?] 
disadvantages of D.
     Bug report: http://d.puremagic.com/issues/show_bug.cgi?id=9606

-- 
Денис В. Шеломовский
Denis V. Shelomovskij