Added test for immutable unique closure upvar mutation.

Author: davidtwco

src/test/compile-fail/issue-46023.rs

@@ -0,0 +1,22 @@
+// Copyright 2016 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.
+
+// revisions: ast mir
+//[mir]compile-flags: -Z emit-end-regions -Z borrowck=mir
+
+fn main() {
+    let x = 0;
+
+    (move || {
+        x = 1;
+        //[mir]~^ ERROR cannot assign to immutable item `x` [E0594]
+        //[ast]~^^ ERROR cannot assign to captured outer variable in an `FnMut` closure [E0594]
+    })()
+}