Add lint checks for unused loop labels

src/test/ui/lint/unused_loop_label.rs

@@ -0,0 +1,55 @@
+// Copyright 2017 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.
+
+// The output should warn when a loop label is not used. However, it
+// should also deal with the edge cases where a label is shadowed,
+// within nested loops
+
+// compile-pass
+// compile-flags: -W unused_loop_label
+
+fn main() {
+    'unused_while_label: while 0 == 0 {
+        //~^ WARN unused loop label
+    }
+
+    let opt = Some(0);
+    'unused_while_let_label: while let Some(_) = opt {
+        //~^ WARN unused loop label
+    }
+
+    'unused_for_label: for _ in 0..10 {
+        //~^ WARN unused loop label
+    }
+
+    'used_loop_label: loop {
+        break 'used_loop_label;
+    }
+
+    'used_loop_label_outer: loop {
+        'used_loop_label_inner: loop {
+            break 'used_loop_label_inner;
+        }
+        break 'used_loop_label_outer;
+    }
+
+    'unused_loop_label_outer: loop {
+        'unused_loop_label_inner: loop {
+            //~^ WARN unused loop label
+            break 'unused_loop_label_outer;
+        }
+    }
+
+    // This is diverging, so put it at the end so we don't get unreachable_code errors everywhere
+    // else
+    'unused_loop_label: loop {
+        //~^ WARN unused loop label
+    }
+}

src/test/ui/lint/unused_loop_label.stderr

@@ -0,0 +1,32 @@
+warning: unused loop label
+  --> $DIR/unused_loop_label.rs:19:5
+   |
+LL |     'unused_while_label: while 0 == 0 {
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: requested on the command line with `-W unused-loop-label`
+
+warning: unused loop label
+  --> $DIR/unused_loop_label.rs:24:5
+   |
+LL |     'unused_while_let_label: while let Some(_) = opt {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: unused loop label
+  --> $DIR/unused_loop_label.rs:28:5
+   |
+LL |     'unused_for_label: for _ in 0..10 {
+   |     ^^^^^^^^^^^^^^^^^
+
+warning: unused loop label
+  --> $DIR/unused_loop_label.rs:44:9
+   |
+LL |         'unused_loop_label_inner: loop {
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^
+
+warning: unused loop label
+  --> $DIR/unused_loop_label.rs:52:5
+   |
+LL |     'unused_loop_label: loop {
+   |     ^^^^^^^^^^^^^^^^^^
+