Add regression test for #18335

src/test/compile-fail/unboxed-closure-immutable-capture.rs

+// 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)]
+
+// Test that even unboxed closures that are capable of mutating their
+// environment cannot mutate captured variables that have not been
+// declared mutable (#18335)
+
+fn set(x: &mut uint) { *x = 0; }
+
+fn main() {
+    let x = 0u;
+    move |&mut:| x = 1; //~ ERROR cannot assign
+    move |&mut:| set(&mut x); //~ ERROR cannot borrow
+    move |:| x = 1; //~ ERROR cannot assign
+    move |:| set(&mut x); //~ ERROR cannot borrow
+    |&mut:| x = 1; //~ ERROR cannot assign
+    // FIXME: this should be `cannot borrow` (issue #18330)
+    |&mut:| set(&mut x); //~ ERROR cannot assign
+    |:| x = 1; //~ ERROR cannot assign
+    // FIXME: this should be `cannot borrow` (issue #18330)
+    |:| set(&mut x); //~ ERROR cannot assign
+}