auto merge of #16940 : treeman/rust/fail-non-utf8, r=pnkfelix

Closes #16877.

src/test/run-fail/fail-non-utf8.rs

@@ -0,0 +1,26 @@
+
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Previously failed formating invalid utf8.
+// cc #16877
+
+// error-pattern:failed at 'hello<6C>'
+
+struct Foo;
+impl std::fmt::Show for Foo {
+    fn fmt(&self, fmtr:&mut std::fmt::Formatter) -> std::fmt::Result {
+        // Purge invalid utf8: 0xff
+        fmtr.write(&[104, 101, 108, 108, 111, 0xff])
+    }
+}
+fn main() {
+    fail!("{}", Foo)
+}