Ensure fail expression is walked, and make fail with a non-string expression fail typechecking.

src/comp/middle/trans.rs

@@ -6375,9 +6375,9 @@ fn trans_fail_expr(&@block_ctxt cx, &option::t[common::span] sp_opt,
                                         [C_int(0), C_int(abi::vec_elt_data)]);
                 ret trans_fail_value(cx, sp_opt, elt);
             } else {
-                cx.fcx.lcx.ccx.sess.span_fatal(expr.span,
-                                               "fail called with unsupported \
-                                               type " + ty_to_str(tcx, e_ty));
+                cx.fcx.lcx.ccx.sess.span_bug(expr.span,
+                                             "fail called with unsupported \
+                                              type " + ty_to_str(tcx, e_ty));
             }
         }
         case (_) {

src/comp/middle/typeck.rs

@@ -1625,7 +1625,12 @@ fn check_expr(&@fn_ctxt fcx, &@ast::expr expr) {
         case (ast::expr_fail(?expr_opt)) {
             alt (expr_opt) {
                 case (none) { /* do nothing */ }
-                case (some(?e)) { check_expr(fcx, e); }
+                case (some(?e)) {
+                    check_expr(fcx, e);
+                    auto tcx = fcx.ccx.tcx;
+                    auto ety = expr_ty(tcx, e);
+                    demand::simple(fcx, e.span, ty::mk_str(tcx), ety);
+                }
             }
             write::bot_ty(fcx.ccx.tcx, id);
         }

src/comp/middle/visit.rs

@@ -359,7 +359,7 @@ fn visit_expr[E](&@expr ex, &E e, &vt[E] v) {
         case (expr_ext(_, _, _, ?expansion)) {
             vt(v).visit_expr(expansion, e, v);
         }
-        case (expr_fail(_)) { }
+        case (expr_fail(?eo)) { visit_expr_opt(eo, e, v); }
         case (expr_break) { }
         case (expr_cont) { }
         case (expr_ret(?eo)) { visit_expr_opt(eo, e, v); }

src/test/compile-fail/fail-expr.rs

@@ -0,0 +1,5 @@
+// error-pattern:mismatched types
+
+fn main() {
+  fail 5;
+}