Turn unsafe_no_drop_flag back into a gated-feature.

src/libsyntax/feature_gate.rs

@@ -243,7 +243,9 @@ pub static KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType)] = &[
     ("static_assert", Whitelisted),
     ("no_debug", Whitelisted),
     ("omit_gdb_pretty_printer_section", Whitelisted),
-    ("unsafe_no_drop_flag", Whitelisted),
+    ("unsafe_no_drop_flag", Gated("unsafe_no_drop_flag",
+                                  "unsafe_no_drop_flag has unstable semantics \
+                                   and may be removed in the future")),
 
     // used in resolve
     ("prelude_import", Whitelisted),

src/test/compile-fail/unsafe_no_drop_flag-gate.rs

@@ -0,0 +1,23 @@
+// 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.
+
+pub struct T;
+
+#[unsafe_no_drop_flag]
+//~^ ERROR unsafe_no_drop_flag has unstable semantics and may be removed
+pub struct S {
+    pub x: T,
+}
+
+impl Drop for S {
+    fn drop(&mut self) {}
+}
+
+pub fn main() {}