Add a sanity test for nesting other items inside the existential type

src/test/run-pass/impl-trait/nesting.rs

@@ -0,0 +1,22 @@
+// Copyright 2018 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.
+
+fn foo<T>(t: T) -> impl Into<[T; { const FOO: usize = 1; FOO }]> {
+    [t]
+}
+
+fn bar() -> impl Into<[u8; { const FOO: usize = 1; FOO }]> {
+    [99]
+}
+
+fn main() {
+    println!("{:?}", foo(42).into());
+    println!("{:?}", bar().into());
+}