Add feature gate checks

src/test/ui/const-eval/feature-gate-const_fn_union.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.
+
+#![feature(const_fn)]
+
+fn main() {}
+
+union Foo {
+    u: u32,
+    i: i32,
+}
+
+const unsafe fn foo(u: u32) -> i32 {
+    Foo { u }.i //~ ERROR unions in const fn are unstable
+}

src/test/ui/const-eval/feature-gate-const_fn_union.stderr

@@ -0,0 +1,11 @@
+error[E0658]: unions in const fn are unstable (see issue #51909)
+  --> $DIR/feature-gate-const_fn_union.rs:21:5
+   |
+LL |     Foo { u }.i //~ ERROR unions in const fn are unstable
+   |     ^^^^^^^^^^^
+   |
+   = help: add #![feature(const_fn_union)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.