add a UI test showing the current output from an impl trait type

Author: nikomatsakis

src/test/ui/impl-trait/universal-mismatched-type.rs

@@ -0,0 +1,19 @@
+// Copyright 2017 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.
+
+#![feature(universal_impl_trait)]
+
+use std::fmt::Debug;
+
+fn foo(x: impl Debug) -> String {
+    x
+}
+
+fn main() { }

src/test/ui/impl-trait/universal-mismatched-type.stderr

@@ -0,0 +1,13 @@
+error[E0308]: mismatched types
+  --> $DIR/universal-mismatched-type.rs:16:5
+   |
+15 | fn foo(x: impl Debug) -> String {
+   |                          ------ expected `std::string::String` because of return type
+16 |     x
+   |     ^ expected struct `std::string::String`, found type parameter
+   |
+   = note: expected type `std::string::String`
+              found type ``
+
+error: aborting due to previous error
+