Proposal: user defined attributes

[F i L]

dennis luehring wrote:
> i don't get the GC relation here?
>
> all attribute values are static const - because they are 
> type-related (static) and read-only, attributes are type 
> orientated not instance orientated

C# doesn't support free-types (everything's wrapped up in 
classes) so the technique isn't going to be identical. In D, 
you'd need to be able to declare an attribute on virtually any 
entity declaration and have the compiler dish out the appropriate 
code.

Trove's suggesting that the compiler simply looks up user-defined 
functions associated with <AttributeX> and inject the code into 
the appropriate place, which I think is a good idea. The problem 
I see is how to do more advanced structural changes like C# does 
with [FieldOffset(0)]. Things like:

     @Int.Fast int index;
     assert(index.sizeof == int.sizeof) // always?

which might break the type system, so I'm not sure it's even 
possible. The compiler might have passed the point of no-return 
with such attributes.

Regardless, I think was Trove's suggesting would work well.