Add compile fail test for SIMD

Author: danobi

src/test/compile-fail/feature-gate-simd.rs

@@ -0,0 +1,23 @@
+// Copyright 2017 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.
+
+
+// pretty-expanded FIXME #23616
+
+#[repr(simd)]
+struct RGBA {
+    r: f32,
+    g: f32,
+    b: f32,
+    a: f32
+}
+//~^^^^^^ ERROR SIMD types are experimental and possibly buggy (see issue #27731)
+
+pub fn main() {}

src/tools/tidy/src/features.rs

@@ -167,7 +167,6 @@ pub fn check(path: &Path, bad: &mut bool) {
 
     // FIXME get this whitelist empty.
     let whitelist = vec![
-        "simd",
         "stmt_expr_attributes",
         "cfg_target_thread_local", "unwind_attributes",
     ];