[swift-evolution] associated objects

[Jay Abbott]

I have implemented Associated Objects (and values) in pure swift here:
https://github.com/j-h-a/AssociatedObjects

The README is very short and straight-forward so I won't re-describe it
here.

The implementation isn't great, but it's a small and simple proof of
concept. I have further extended this (not in GH, but very simple and happy
to share if anyone cares) to add dynamic methods using closures onto
individual object instances. Useful for user interactions, or adding
'actions' to objects.

I'd like to propose that this or something similar be added to the standard
library. It could potentially even be added to AnyObject so that developers
can use it without having to declare an extension for whichever classes
they want to use it on.

As mentioned, this can easily be extended (either in the standard library
or by developers) to add closures dynamically to any object, or to any
class, which could serve as a useful way for those not concerned with type
safety and the like to get some dynamic behaviour in there in the shorter
term. With a little language support it could even be used to implement
stored properties in extensions very easily.

A better implementation would need some language changes - specifically
deinit hooks (has that ever been discussed?) because of the way weak
references are lazily zeroed. Another implementation improvement might
lazily add the dictionary to each object instead of storing it globally -
not sure if this would have ABI implications.

Interested in feedback and thoughts :)
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