New chapter on procedural macros

[japaric]

Our macro_rules! chapter covers macros by example, which is one flavour of macros.

We need a new chapter that covers procedural macros, which work at the AST level.

For some ideas check the #[quickcheck] attribute used by the quickcheck library, which expands into a test harness and a quickcheck call: #[test] + quickcheck(property). I think this is one of the simplest uses of procedural macros.

For more examples in the wild, the rustc --pretty expanded command can be used to reveal the magic behind the #[deriving(...)] attribute, which is also a procedural macro. This will show the expansion of the macro, but the definiton of these macros can be found under libsyntax/ext/deriving

[mitchmindtree]

+1 would love to see a nice concise example for procedural macros 👍

[steveklabnik]

Given that procedural macros aren't going to be stable for a long time, while I agree this would be something to add, it isn't really relevant until then. Therefore, I'm giving this a close for now.