Fix illegal instruction error when breaking within condition of labelled while loop

src/librustc_passes/diagnostics.rs

@@ -218,12 +218,16 @@ E0590: r##"
 Example of erroneous code:
 
 ```compile_fail
-while break {}
+loop {
+    while break {}
+}
 ```
 
 To fix this, add a label specifying which loop is being broken out of:
 ```
-'foo: while break 'foo {}
+'foo: loop {
+    while break 'foo {}
+}
 ```
 "##,
 

src/librustc_passes/loops.rs

@@ -215,6 +215,7 @@ impl<'a, 'hir> CheckLoopVisitor<'a, 'hir> {
         }
         return false;
     }
+
     fn emit_unlabled_cf_in_while_condition(&mut self, span: Span, cf_type: &str) {
         struct_span_err!(self.sess, span, E0590,
                          "`break` or `continue` with no label in the condition of a `while` loop")

src/librustc_resolve/diagnostics.rs

@@ -1097,7 +1097,7 @@ loop {
 }
 ```
 
-Please verify you spelt or declare the label correctly. Example:
+Please verify you spelt and declared the label correctly. Example:
 
 ```
 'a: loop {

src/librustc_resolve/lib.rs

@@ -2112,11 +2112,9 @@ impl<'a> Resolver<'a> {
         }
     }
 
-    /// Searches the current set of local scopes for labels. Returns the first non-None label that
-    /// is returned by the given predicate function
-    ///
-    /// Stops after meeting a closure.
-    fn search_label<P, R>(&self, mut ident: Ident, pred: P) -> Option<R>
+    /// Searches the current set of local scopes for labels. Returns the first non-`None` label
+    /// that is returned by the given predicate function. Stops after meeting a closure.
+    fn search_label<P, R: std::fmt::Debug>(&self, mut ident: Ident, pred: P) -> Option<R>
         where P: Fn(&Rib, Ident) -> Option<R>
     {
         for rib in self.label_ribs.iter().rev() {
@@ -2130,7 +2128,7 @@ impl<'a> Resolver<'a> {
                     }
                 }
                 _ => {
-                    // Do not resolve labels across function boundary
+                    // Do not resolve labels across function boundary.
                     return None;
                 }
             }
@@ -3731,8 +3729,8 @@ impl<'a> Resolver<'a> {
                 match self.search_label(label.ident, |rib, id| rib.bindings.get(&id).cloned()) {
                     None => {
                         // Search again for close matches...
-                        // Picks the first label that is "close enough", which is not necessarily
-                        // the closest match
+                        // Picks the first label that is "close enough", which is not
+                        // necessarily the closest match.
                         let close_match = self.search_label(label.ident, |rib, ident| {
                             let names = rib.bindings.iter().map(|(id, _)| &id.name);
                             find_best_match_for_name(names, &*ident.name.as_str(), None)
@@ -3775,15 +3773,15 @@ impl<'a> Resolver<'a> {
             ExprKind::Loop(ref block, label) => self.resolve_labeled_block(label, expr.id, &block),
 
             ExprKind::While(ref subexpression, ref block, label) => {
+                self.visit_expr(subexpression);
                 self.with_resolved_label(label, expr.id, |this| {
-                    this.visit_expr(subexpression);
                     this.visit_block(block);
                 });
             }
 
             ExprKind::WhileLet(ref pats, ref subexpression, ref block, label) => {
+                self.visit_expr(subexpression);
                 self.with_resolved_label(label, expr.id, |this| {
-                    this.visit_expr(subexpression);
                     this.ribs[ValueNS].push(Rib::new(NormalRibKind));
                     let mut bindings_list = FxHashMap();
                     for pat in pats {

src/test/ui/while-label-break-condition.rs

@@ -0,0 +1,16 @@
+// Copyright 2018 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.
+
+#![allow(unreachable_code)]
+
+fn main() {
+    'a: while break 'a {} //~ ERROR: use of undeclared label
+    'b: while break 'b {}; //~ ERROR: use of undeclared label
+}

src/test/ui/while-label-break-condition.stderr

@@ -0,0 +1,15 @@
+error[E0426]: use of undeclared label `'a`
+  --> $DIR/while-label-break-condition.rs:14:21
+   |
+LL |     'a: while break 'a {} //~ ERROR: use of undeclared label
+   |                     ^^ undeclared label `'a`
+
+error[E0426]: use of undeclared label `'b`
+  --> $DIR/while-label-break-condition.rs:15:21
+   |
+LL |     'b: while break 'b {}; //~ ERROR: use of undeclared label
+   |                     ^^ undeclared label `'b`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0426`.