Implement a lint mode to deal with unused 'mut' variables

src/test/compile-fail/unused-mut-variables.rs

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+// Copyright 2013 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.
+
+// Exercise the unused_mut attribute in some positive and negative cases
+
+#[allow(dead_assignment)];
+#[allow(unused_variable)];
+#[deny(unused_mut)];
+
+fn main() {
+    // negative cases
+    let mut a = 3; //~ ERROR: variable does not need to be mutable
+    let mut a = 2, b = 3; //~ ERROR: variable does not need to be mutable
+                          //~^ ERROR: variable does not need to be mutable
+    let mut a = ~[3]; //~ ERROR: variable does not need to be mutable
+
+    // positive cases
+    let mut a = 2;
+    a = 3;
+    let mut a = ~[];
+    a.push(3);
+    let mut a = ~[];
+    do callback {
+        a.push(3);
+    }
+}
+
+fn callback(f: &fn()) {}
+
+// make sure the lint attribute can be turned off
+#[allow(unused_mut)]
+fn foo(mut a: int) {
+    let mut a = 3;
+    let mut b = ~[2];
+}