User Defined Attributes (UDA) in Phobos/druntime

[Jacob Carlborg]

This was brought up in the thread reviewing std.serialization:

http://forum.dlang.org/thread/adyanbsdsxsfdpvoozne@forum.dlang.org

Currently we haven't started to use UDA's in Phobos or druntime yet (as 
far as I know). I would like to start a discussion about putting out 
some guidelines for using UDA's in Phobos/druntime.

I've created a small module that handles UDA's:

https://github.com/jacob-carlborg/orange/blob/master/orange/core/Attribute.d

The idea is to have a kind of meta UDA called "attribute". This 
attribute needs to be attached to all structs, enums and so on that is 
to be used as an attribute.

The module provides a template (getAttributes) to get, by default, only 
those values attached to a given symbol that is an attribute. That is, 
marked with the "attribute" attribute.

So my suggestion for guidelines are:

* Only types with the "attribute" UDA is to be used as attributes
* In general, don't use primitive values as a UDA

Don't use this

@(3) int a;

Use this:

@attribute struct foo { int b; }

@foo(3) int a;

* A user defined type marked with "attribute" should not be used for 
something else than an UDA

* All attributes use camel case names

If we agree on this and that we need a module like the one above I think 
it should be added to druntime. The reason for that is that we most 
likely want to use UDA's in druntime and not only in Phobos. Example, if 
we ever get std.serialization into Phobos we would want to mark Thread 
and similar structs/classes as "nonSerialized".

Thoughts?

-- 
/Jacob Carlborg