incr.comp.: Add regression test for detecting feature gate changes.

Author: michaelwoerister

src/test/incremental/feature_gate.rs

@@ -0,0 +1,23 @@
+// 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.
+
+// This test makes sure that we detect changed feature gates.
+
+// revisions:rpass1 cfail2
+// compile-flags: -Z query-dep-graph
+
+#![feature(rustc_attrs)]
+#![cfg_attr(rpass1, feature(nll))]
+
+fn main() {
+    let mut v = vec![1];
+    v.push(v[0]);
+    //[cfail2]~^ ERROR cannot borrow
+}