Allow declaring existential types inside blocks

src/test/ui/impl-trait/existential_type_in_fn_body.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.
+
+// compile-pass
+
+#![feature(existential_type)]
+
+use std::fmt::Debug;
+
+fn main() {
+    existential type Existential: Debug;
+
+    fn f() -> Existential {}
+    println!("{:?}", f());
+}