Auto merge of #21099 - sanxiyn:opt-return-ty, r=alexcrichton

This avoids having ast::Ty nodes which have no counterpart in the source.

src/test/pretty/fn-return.rs

@@ -0,0 +1,17 @@
+// 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.
+
+// pp-exact
+
+// Check that `fn f() -> () { }` does not print as `fn f() { }`.
+
+fn f() -> () { }
+
+fn main() { }