auto merge of #7371 : alexcrichton/rust/trying, r=cmr

This is an attempt at a smaller request than #7113, it's just the first two commits

src/test/compile-fail/lint-change-warnings.rs

@@ -0,0 +1,30 @@
+// 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.
+
+#[deny(warnings)];
+
+fn main() {
+    while true {} //~ ERROR: infinite
+}
+
+#[allow(warnings)]
+fn foo() {
+    while true {}
+}
+
+#[warn(warnings)]
+fn bar() {
+    while true {} //~ WARNING: infinite
+}
+
+#[forbid(warnings)]
+fn baz() {
+    while true {} //~ ERROR: warnings
+}