Suppress error messages about function types whose result is ty_err

Ideally we would suppress error messages involving any types that
contain ty_err, but that's awkward to do right now.

Author: catamorphism

src/librustc/middle/typeck/infer/mod.rs

@@ -737,10 +737,19 @@ impl infer_ctxt {
     fn type_error_message(sp: span, mk_msg: fn(~str) -> ~str,
                           actual_ty: ty::t, err: Option<&ty::type_err>) {
         let actual_ty = self.resolve_type_vars_if_possible(actual_ty);
+        let mut actual_sty = ty::get(copy actual_ty);
 
         // Don't report an error if actual type is ty_err.
-        match ty::get(actual_ty).sty {
+        match actual_sty.sty {
             ty::ty_err => return,
+            // Should really not report an error if the type
+            // has ty_err anywhere as a component, but that's
+            // annoying since we haven't written a visitor for
+            // ty::t yet
+            ty::ty_fn(ref fty) => match ty::get(fty.sig.output).sty {
+                ty::ty_err => return,
+                _ => ()
+            },
             _           => ()
         }
         let error_str = err.map_default(~"", |t_err|