Add regression test for rust-lang#18238 and rust-lang#18336

Author: bkoropoff

src/test/run-pass/unboxed-closures-move-mutable.rs

@@ -0,0 +1,28 @@
+// Copyright 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.
+
+#![feature(unboxed_closures)]
+#![deny(unused_mut)]
+
+// Test that mutating a mutable upvar in a capture-by-value unboxed
+// closure does not ice (issue #18238) and marks the upvar as used
+// mutably so we do not get a spurious warning about it not needing to
+// be declared mutable (issue #18336).
+
+fn main() {
+    {
+        let mut x = 0u;
+        move |&mut:| x += 1;
+    }
+    {
+        let mut x = 0u;
+        move |:| x += 1;
+    }
+}