Move syntax extensions and macros out of compiler

[brson]

The compiler knows a lot more about syntax extensions than it ever should have. There's a danger that our syntax extensions could become hopelessly intertwined with rustc. We need to get a real plugin mechanism working before it is too late.

Depends on #458. See alse #38.

[graydon]

Yeah. They should also shift to using the quasiquoter for definition, now that it's present.

[brson]

They are now in rustsyntax, which at least isolates them from most of the compiler. They still belong in their own little, dynamically loaded world.

[brson]

No new developments this year.

[huonw]

@brson, there has been some. I've got a proof-of-concept: #6735.

[metajack]

visiting for triage. nothing to add.

[erickt]

Nominating to close because #11151 landed. Woooooo!

[errordeveloper]

It's been a while since #11151 has been merged, is there a reason for this 2+ year old issue to stay open?

[sfackler]

The standard extensions are still embedded in libsyntax. They could be pulled out, but I'm not sure if it's worth the trouble.

[steveklabnik]

Triage: I think @eddyb was working hard on this, but nothing has come of it yet.

[eddyb]

@steveklabnik I completely forgot about it, mostly because I'm trying to use my severely limited time working on actual features.

[steveklabnik]

Hehe, no wories, I don't blame you 😄