auto merge of #5966 : alexcrichton/rust/issue-3083, r=graydon

Closes #3083.

This takes a similar approach to #5797 where a set is present on the `tcx` of used mutable definitions. Everything is by default warned about, and analyses must explicitly add mutable definitions to this set so they're not warned about.

Most of this was pretty straightforward, although there was one caveat that I ran into when implementing it. Apparently when the old modes are used (or maybe `legacy_modes`, I'm not sure) some different code paths are taken to cause spurious warnings to be issued which shouldn't be issued. I'm not really sure how modes even worked, so I was having a lot of trouble tracking this down. I figured that because they're a legacy thing that I'd just de-mode the compiler so that the warnings wouldn't be a problem anymore (or at least for the compiler).

Other than that, the entire compiler compiles without warnings of unused mutable variables. To prevent bad warnings, #5965 should be landed (which in turn is waiting on #5963) before landing this. I figured I'd stick it out for review anyway though.

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

@@ -0,0 +1,42 @@
+// 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];
+}