Auto merge of #37780 - GuillaumeGomez:E0571, r=nikomatsakis

Add checkup for return statement outside of a function

Fixes #37778.

r? @eddyb (don't know who is in charge. Don't hesitate to set another one)

cc @jonathandturner

src/test/compile-fail/E0572.rs

@@ -0,0 +1,13 @@
+// Copyright 2016 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.
+
+const FOO: u32 = return 0; //~ ERROR E0572
+
+fn main() {}