Add feature gate tests

As part of the audit for #22820 the following feature gate tests have
been added:

* `negate_unsigned`
* `on_unimplemented`
* `optin_builtin_traits`
* `plugin`
* `rustc_attrs`
* `slice_patterns`

Author: lgrz

src/test/compile-fail/feature-gate-negate-unsigned.rs

@@ -0,0 +1,17 @@
+// 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.
+
+// Test that negating unsigned integers is gated by `negate_unsigned` feature
+// gate
+
+const MAX: usize = -1;
+//~^ ERROR unary negation of unsigned integers may be removed in the future
+
+fn main() {}

src/test/compile-fail/feature-gate-on-unimplemented.rs

@@ -0,0 +1,19 @@
+// 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.
+
+// Test that `#[rustc_on_unimplemented]` is gated by `on_unimplemented` feature
+// gate.
+
+#[rustc_on_unimplemented = "test error `{Self}` with `{Bar}`"]
+//~^ ERROR the `#[rustc_on_unimplemented]` attribute is an experimental feature
+trait Foo<Bar>
+{}
+
+fn main() {}

src/test/compile-fail/feature-gate-optin-builtin-traits.rs

@@ -0,0 +1,26 @@
+// 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.
+
+// Test that default and negative trait implementations are gated by
+// `optin_builtin_traits` feature gate
+
+struct DummyStruct;
+
+trait DummyTrait {
+    fn dummy(&self) {}
+}
+
+impl DummyTrait for .. {}
+//~^ ERROR default trait implementations are experimental and possibly buggy
+
+impl !DummyTrait for DummyStruct {}
+//~^ ERROR negative trait bounds are not yet fully implemented; use marker types for now
+
+fn main() {}

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

@@ -0,0 +1,16 @@
+// 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.
+
+// Test that `#![plugin(...)]` attribute is gated by `plugin` feature gate
+
+#![plugin(foo)]
+//~^ ERROR compiler plugins are experimental and possibly buggy
+
+fn main() {}

src/test/compile-fail/feature-gate-rustc-attrs.rs

@@ -0,0 +1,21 @@
+// 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.
+
+// ignore-tidy-linelength
+
+// Test that `#[rustc_*]` attributes are gated by `rustc_attrs` feature gate.
+
+#[rustc_variance] //~ ERROR the `#[rustc_variance]` attribute is an experimental feature
+#[rustc_error] //~ ERROR the `#[rustc_error]` attribute is an experimental feature
+#[rustc_move_fragments] //~ ERROR the `#[rustc_move_fragments]` attribute is an experimental feature
+#[rustc_foo]
+//~^ ERROR unless otherwise specified, attributes with the prefix `rustc_` are reserved for internal compiler diagnostics
+
+fn main() {}

src/test/compile-fail/feature-gate-slice-patterns.rs

@@ -0,0 +1,18 @@
+// 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.
+
+// Test that slice pattern syntax is gated by `slice_patterns` feature gate
+
+fn main() {
+    let x = [1, 2, 3, 4, 5];
+    match x {
+        [1, 2, xs..] => {} //~ ERROR slice pattern syntax is experimental
+    }
+}