Infer type ! for a loop that can only break out of other loops

Closes #23451.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/librustc_typeck/check/mod.rs

@@ -5239,7 +5239,7 @@ pub fn may_break(cx: &ty::ctxt, id: ast::NodeId, b: &ast::Block) -> bool {
     // inside the loop?
     (loop_query(&*b, |e| {
         match *e {
-            ast::ExprBreak(_) => true,
+            ast::ExprBreak(None) => true,
             _ => false
         }
     })) ||

src/test/compile-fail/loop-labeled-break-value.rs

@@ -0,0 +1,21 @@
+// Copyright 2015 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() {
+    loop {
+        let _: i32 = loop { break }; //~ ERROR mismatched types
+    }
+    loop {
+        let _: i32 = 'inner: loop { break 'inner }; //~ ERROR mismatched types
+    }
+    loop {
+        let _: i32 = 'inner: loop { loop { break 'inner } }; //~ ERROR mismatched types
+    }
+}

src/test/run-pass/loop-labeled-break-value.rs

@@ -0,0 +1,18 @@
+// Copyright 2015 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() {
+    'outer: loop {
+        let _: i32 = loop { break 'outer };
+    }
+    'outer: loop {
+        let _: i32 = loop { loop { break 'outer } };
+    }
+}