diff --git a/src/librustsyntax/diagnostic.rs b/src/librustsyntax/diagnostic.rs
index 184c32f2146..64904d612ce 100644
--- a/src/librustsyntax/diagnostic.rs
+++ b/src/librustsyntax/diagnostic.rs
@@ -7,6 +7,7 @@
 export span_handler, handler, mk_span_handler, mk_handler;
 export codemap_span_handler, codemap_handler;
 export ice_msg;
+export expect;
 
 type emitter = fn@(cmsp: option<(codemap::codemap, span)>,
                    msg: str, lvl: level);
@@ -253,3 +254,11 @@ fn print_macro_backtrace(cm: codemap::codemap, sp: span) {
         print_macro_backtrace(cm, ei.call_site);
     }
 }
+
+fn expect<T: copy>(diag: span_handler,
+                   opt: option<T>, msg: fn() -> str) -> T {
+    alt opt {
+       some(t) { t }
+       none { diag.handler().bug(msg()); }
+    }
+}
diff --git a/src/rustc/driver/session.rs b/src/rustc/driver/session.rs
index ea815d9e410..deecce66b30 100644
--- a/src/rustc/driver/session.rs
+++ b/src/rustc/driver/session.rs
@@ -171,10 +171,7 @@ fn basic_options() -> @options {
 
 // Seems out of place, but it uses session, so I'm putting it here
 fn expect<T: copy>(sess: session, opt: option<T>, msg: fn() -> str) -> T {
-    alt opt {
-       some(t) { t }
-       none { sess.bug(msg()); }
-    }
+    diagnostic::expect(sess.diagnostic(), opt, msg)
 }
 
 fn building_library(req_crate_type: crate_type, crate: @ast::crate,