rustc: implement unstable(issue = "nnn").

This takes an issue number and points people to it in the printed error
message. This commit does not make it an error to have no `issue` field.

src/test/auxiliary/stability_attribute_issue.rs

@@ -0,0 +1,20 @@
+// 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.
+
+#![feature(staged_api)]
+#![staged_api]
+#![stable(feature = "foo", since = "1.2.0")]
+
+
+#[unstable(feature = "foo", issue = "1")]
+pub fn unstable() {}
+
+#[unstable(feature = "foo", reason = "message", issue = "2")]
+pub fn unstable_msg() {}

src/test/compile-fail/stability-attribute-issue.rs

@@ -0,0 +1,21 @@
+// Copyright 2013-2014 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.
+
+// aux-build:stability_attribute_issue.rs
+
+#![deny(deprecated)]
+
+extern crate stability_attribute_issue;
+use stability_attribute_issue::*;
+
+fn main() {
+    unstable(); //~ ERROR use of unstable library feature 'foo' (see issue #1)
+    unstable_msg(); //~ ERROR use of unstable library feature 'foo': message (see issue #2)
+}