syntax: Switch field privacy as necessary

src/libsyntax/lib.rs

@@ -30,6 +30,8 @@ This API is completely unstable and subject to change.
            quote)]
 #![allow(deprecated)]
 
+#![allow(visible_private_types)] // NOTE: remove after a stage0 snap
+
 extern crate serialize;
 extern crate term;
 extern crate collections;