Add a test for feature attribute consistency

Note that this doesn't test consistency of some properties, like `reason`.

Author: varkor

src/test/ui/feature-gate/stability-attribute-consistency.rs

@@ -0,0 +1,26 @@
+// Copyright 2018 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.
+
+#![stable(feature = "stable_test_feature", since = "1.0.0")]
+
+#![feature(staged_api)]
+
+#[stable(feature = "foo", since = "1.0.0")]
+fn foo_stable_1_0_0() {}
+
+#[stable(feature = "foo", since = "1.29.0")]
+//~^ ERROR feature `foo` is declared stable since 1.29.0
+fn foo_stable_1_29_0() {}
+
+#[unstable(feature = "foo", issue = "0")]
+//~^ ERROR feature `foo` is declared unstable
+fn foo_unstable() {}
+
+fn main() {}

src/test/ui/feature-gate/stability-attribute-consistency.stderr

@@ -0,0 +1,15 @@
+error[E0711]: feature `foo` is declared stable since 1.29.0, but was previously declared stable since 1.0.0
+  --> $DIR/stability-attribute-consistency.rs:18:1
+   |
+LL | #[stable(feature = "foo", since = "1.29.0")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0711]: feature `foo` is declared unstable, but was previously declared stable
+  --> $DIR/stability-attribute-consistency.rs:22:1
+   |
+LL | #[unstable(feature = "foo", issue = "0")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0711`.