Rollup merge of #21504 - blackbeam:has_test_signature_fix, r=alexcrichton

Fix for `error: functions used as tests must have signature fn() -> ()` and `error: functions used as benches must have signature `fn(&mut Bencher) -> ()` in case of explicit return type declaration.

src/test/run-pass/test-fn-signature-verification-for-explicit-return-type.rs

@@ -0,0 +1,19 @@
+// 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.
+
+// compile-flags: --test
+extern crate test;
+
+#[bench]
+fn bench_explicit_return_type(_: &mut ::test::Bencher) -> () {}
+
+#[test]
+fn test_explicit_return_type() -> () {}
+