Added RFC for codegen plugins

[farcaller]

No description provided.

[lilyball]

We already have item decorators and item modifiers in the syntax extension support. With RFC 40 for attributes on statements/expressions, it seems the logical step is to propose that item decorator/modifier support be extended to statements/expressions that are tagged with the appropriate attribute.

[chris-morgan]

This seems very vague. An RFC should be something that is concrete and actionable; it seems to me that this needs discussion to nut out what you’re actually wanting—http://discuss.rust-lang.org/ is probably a better place to take it if I have judged rightly.

[lilyball]

@chris-morgan How long has that site existed? I have never heard about it before.

[thehydroimpulse]

@kballard It's only been started to get used recently. I think I first went on a couple weeks ago (not too sure), so it might of been online for longer than that.