add new test for dyn<Trait + '_> used in a struct

`'_` is illegal in structs; this currently gets a duplicate error

Author: nikomatsakis

src/test/ui/underscore-lifetime/dyn-trait-underscore-in-struct.rs

@@ -0,0 +1,26 @@
+// Copyright 2015 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.
+
+// Check that the `'_` in `dyn Trait + '_` acts like ordinary elision,
+// and not like an object lifetime default.
+//
+// cc #48468
+
+#![feature(dyn_trait)]
+#![feature(underscore_lifetimes)]
+
+use std::fmt::Debug;
+
+struct Foo {
+    x: Box<dyn Debug + '_>, //~ ERROR missing lifetime specifier
+    //~^ ERROR E0228
+}
+
+fn main() { }

src/test/ui/underscore-lifetime/dyn-trait-underscore-in-struct.stderr

@@ -0,0 +1,16 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/dyn-trait-underscore-in-struct.rs:22:24
+   |
+LL |     x: Box<dyn Debug + '_>, //~ ERROR missing lifetime specifier
+   |                        ^^ expected lifetime parameter
+
+error[E0228]: the lifetime bound for this object type cannot be deduced from context; please supply an explicit bound
+  --> $DIR/dyn-trait-underscore-in-struct.rs:22:12
+   |
+LL |     x: Box<dyn Debug + '_>, //~ ERROR missing lifetime specifier
+   |            ^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0106, E0228.
+For more information about an error, try `rustc --explain E0106`.