Add tests.

src/test/compile-fail/cannot-mutate-captured-non-mut-var.rs

@@ -0,0 +1,15 @@
+// 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.
+
+fn main() {
+    let x = 1i;
+    proc() { x = 2; };
+    //~^ ERROR: cannot assign to immutable captured outer variable in a proc `x`
+}

src/test/compile-fail/unused-mut-warning-captured-var.rs

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+// 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.
+
+#![forbid(unused_mut)]
+
+fn main() {
+    let mut x = 1i;
+    //~^ ERROR: variable does not need to be mutable
+    proc() { println!("{}", x); };
+}

src/test/run-pass/issue-11958.rs

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+// 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.
+
+#![forbid(warnings)]
+
+// Pretty printing tests complain about `use std::predule::*`
+#![allow(unused_imports)]
+
+// We shouldn't need to rebind a moved upvar as mut if it's already
+// marked as mut
+
+pub fn main() {
+    let mut x = 1i;
+    proc() { x = 2; };
+}