diff --git a/src/rustc/driver/driver.rs b/src/rustc/driver/driver.rs
index 93ab61e245a..ebdc8bc848c 100644
--- a/src/rustc/driver/driver.rs
+++ b/src/rustc/driver/driver.rs
@@ -173,7 +173,7 @@ fn compile_upto(sess: session, cfg: ast::crate_cfg,
         front::test::modify_for_testing(sess, crate));
 
     crate = time(time_passes, ~"expansion", ||
-        syntax::ext::expand::expand_crate(sess.parse_sess, sess.opts.cfg,
+        syntax::ext::expand::expand_crate(sess.parse_sess, cfg,
                                           crate));
 
     if upto == cu_expand { return {crate: crate, tcx: None}; }
diff --git a/src/rustc/driver/session.rs b/src/rustc/driver/session.rs
index 01dcd6930b3..8c085e6391d 100644
--- a/src/rustc/driver/session.rs
+++ b/src/rustc/driver/session.rs
@@ -144,6 +144,10 @@ type options =
      addl_lib_search_paths: ~[Path],
      maybe_sysroot: Option<Path>,
      target_triple: ~str,
+     // User-specified cfg meta items. The compiler itself will add additional
+     // items to the crate config, and during parsing the entire crate config
+     // will be added to the crate AST node.  This should not be used for
+     // anything except building the full crate config prior to parsing.
      cfg: ast::crate_cfg,
      binary: ~str,
      test: bool,