add tracking issue and feature-gate and run-make tests

Author: Jorge Aparicio

src/libsyntax/feature_gate.rs

@@ -342,7 +342,7 @@ declare_features! (
     (active, abi_x86_interrupt, "1.17.0", Some(40180)),
 
     // Used to preserve symbols
-    (active, used, "1.17.0", None),
+    (active, used, "1.17.0", Some(40289)),
 );
 
 declare_features! (

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

@@ -0,0 +1,15 @@
+// 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.
+
+#[used]
+fn foo() {}
+//~^^ ERROR the `#[used]` attribute is an experimental feature
+
+fn main() {}

src/test/run-make/used/Makefile

@@ -0,0 +1,12 @@
+-include ../tools.mk
+
+ifdef IS_WINDOWS
+# Do nothing on MSVC.
+all:
+	exit 0
+else
+all:
+	$(RUSTC) -C opt-level=3 --emit=obj used.rs
+	nm -C used.o | grep FOO
+	nm -C used.o | grep -v BAR
+endif

src/test/run-make/used/used.rs

@@ -0,0 +1,17 @@
+// 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.
+
+#![crate_type = "lib"]
+#![feature(used)]
+
+#[used]
+static FOO: u32 = 0;
+
+static BAR: u32 = 0;