Implement repr(transparent)

src/test/ui/feature-gate-repr_transparent.rs

@@ -0,0 +1,14 @@
+// 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.
+
+#[repr(transparent)] //~ error: the `#[repr(transparent)]` attribute is experimental
+struct Foo(u64);
+
+fn main() {}

src/test/ui/feature-gate-repr_transparent.stderr

@@ -0,0 +1,10 @@
+error[E0658]: the `#[repr(transparent)]` attribute is experimental (see issue #43036)
+  --> $DIR/feature-gate-repr_transparent.rs:11:1
+   |
+11 | #[repr(transparent)] //~ error: the `#[repr(transparent)]` attribute is experimental
+   | ^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(repr_transparent)] to the crate attributes to enable
+
+error: aborting due to previous error
+