Auto merge of #71 - dtolnay:cfg, r=erickt

Reimplement derive to work with cfg_attr

Fixes serde-rs/serde#148.

The implementation expands `cfg_attr` and `derive` in-line during decorator expansion rather than as pre/post-expansion passes. I think this is a simpler and more correct design, but at the expense of being more invasive into the libsyntax code.

syntex/src/lib.rs

@@ -1,7 +1,5 @@
 extern crate syntex_syntax;
 
-mod squash_derive;
-
 use std::fs::File;
 use std::io::{self, Write};
 use std::path::Path;
@@ -175,7 +173,6 @@ impl Registry {
         let ecx = ExtCtxt::new(&sess, cfg, ecfg, &mut gated_cfgs, &mut macro_loader);
 
         let (krate, _) = expand::expand_crate(ecx, self.syntax_exts, krate);
-        let krate = squash_derive::squash_derive(krate);
 
         let krate = self.post_expansion_passes.iter()
             .fold(krate, |krate, f| (f)(krate));

syntex_syntax/src/ext/base.rs

@@ -469,8 +469,6 @@ fn initial_syntax_expander_table<'feat>(ecfg: &expand::ExpansionConfig<'feat>)
     syntax_expanders.insert(intern("macro_rules"), MacroRulesTT);
     */
 
-    ext::deriving::register_all(&mut syntax_expanders);
-
     if ecfg.enable_quotes() {
         // Quasi-quoting expanders
         syntax_expanders.insert(intern("quote_tokens"),