syntax: Conditionally deriving(Hash) with Writers

If #[feature(default_type_parameters)] is enabled for a crate, then
deriving(Hash) will expand with Hash<W: Writer> instead of Hash<SipState> so
more hash algorithms can be used.

src/libsyntax/ext/base.rs

@@ -283,7 +283,7 @@ pub struct ExtCtxt<'a> {
     parse_sess: @parse::ParseSess,
     cfg: ast::CrateConfig,
     backtrace: Option<@ExpnInfo>,
-    loader: &'a mut CrateLoader,
+    ecfg: expand::ExpansionConfig<'a>,
 
     mod_path: Vec<ast::Ident> ,
     trace_mac: bool
@@ -291,13 +291,13 @@ pub struct ExtCtxt<'a> {
 
 impl<'a> ExtCtxt<'a> {
     pub fn new<'a>(parse_sess: @parse::ParseSess, cfg: ast::CrateConfig,
-               loader: &'a mut CrateLoader) -> ExtCtxt<'a> {
+                   ecfg: expand::ExpansionConfig<'a>) -> ExtCtxt<'a> {
         ExtCtxt {
             parse_sess: parse_sess,
             cfg: cfg,
             backtrace: None,
-            loader: loader,
             mod_path: Vec::new(),
+            ecfg: ecfg,
             trace_mac: false
         }
     }