Parse, feature-gate, and validate the #[marker] attribute

src/test/ui/feature-gates/feature-gate-marker_trait_attr.rs

@@ -0,0 +1,19 @@
+// 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.
+
+use std::fmt::{Debug, Display};
+
+#[marker] trait ExplicitMarker {}
+//~^ ERROR marker traits is an experimental feature (see issue #29864)
+
+impl<T: Display> ExplicitMarker for T {}
+impl<T: Debug> ExplicitMarker for T {}
+
+fn main() {}

src/test/ui/feature-gates/feature-gate-marker_trait_attr.stderr

@@ -0,0 +1,11 @@
+error[E0658]: marker traits is an experimental feature (see issue #29864)
+  --> $DIR/feature-gate-marker_trait_attr.rs:13:1
+   |
+LL | #[marker] trait ExplicitMarker {}
+   | ^^^^^^^^^
+   |
+   = help: add #![feature(marker_trait_attr)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.