RFC: Implement a #[derive] or #[auto] meta-syntax-extension

[nikomatsakis]

Right now to use deriving/auto you have to add individual annotations on types:

#[deriving_eq]
#[deriving_ord]
#[auto_encode]
#[auto_decode]
struct Foo { }

This is tedious and an eye-sore. However, it has the advantage that if some third-party X comes along and implement a trait Bar, they can implement a syntax extension #[deriving_X] that fits right in. Therefore, I propose we have a "meta" syntax extension called #[derive] or #[auto] (I will use auto in the examples that follow) that would expand the following declaration:

#[auto(Eq,Ord,Bar)]
struct Foo {}

into:

#[auto_Eq]
#[auto_Ord]
#[auto_Bar]
struct Foo {}

This makes deriving convenient to use while preserving the ability for third-parties to seamlessly integrate.

As for what to call it: auto seems like a clearer name for those not familiar with Haskell, but derive or deriving has precedent. I don't really care.

[bstrie]

More consistent, more intuitive, and prettier. +1

[catamorphism]

+1. I prefer deriving to auto -- auto has too many meanings.

[graydon]

Pull #5320 handles this, or at least moves in that direction.

[graydon]

landed