Add test for duplicate features

Author: varkor

src/test/ui/feature-gate/duplicate-features.rs

@@ -0,0 +1,20 @@
+// 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.
+
+#![allow(stable_features)]
+#![deny(duplicate_features)]
+
+#![feature(rust1)]
+#![feature(rust1)] //~ ERROR duplicate `rust1` feature attribute
+
+#![feature(if_let)]
+#![feature(if_let)] //~ ERROR duplicate `if_let` feature attribute
+
+fn main() {}

src/test/ui/feature-gate/duplicate-features.stderr

@@ -0,0 +1,20 @@
+error: duplicate `if_let` feature attribute
+  --> $DIR/duplicate-features.rs:18:12
+   |
+LL | #![feature(if_let)] //~ ERROR duplicate `if_let` feature attribute
+   |            ^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/duplicate-features.rs:12:9
+   |
+LL | #![deny(duplicate_features)]
+   |         ^^^^^^^^^^^^^^^^^^
+
+error: duplicate `rust1` feature attribute
+  --> $DIR/duplicate-features.rs:15:12
+   |
+LL | #![feature(rust1)] //~ ERROR duplicate `rust1` feature attribute
+   |            ^^^^^
+
+error: aborting due to 2 previous errors
+