Properly implement labeled breaks in while conditions

src/librustc_passes/diagnostics.rs

@@ -241,6 +241,22 @@ match 5u32 {
 }
 ```
 "##,
+
+E0583: r##"
+`break` or `continue` must include a label when used in the condition of a
+`while` loop.
+
+Example of erroneous code:
+
+```compile_fail
+while break {}
+```
+
+To fix this, add a label specifying which loop is being broken out of:
+```
+`foo: while break `foo {}
+```
+"##
 }
 
 register_diagnostics! {

src/librustc_passes/loops.rs

@@ -87,11 +87,21 @@ impl<'a, 'hir> Visitor<'hir> for CheckLoopVisitor<'a, 'hir> {
                 self.with_context(Closure, |v| v.visit_nested_body(b));
             }
             hir::ExprBreak(label, ref opt_expr) => {
+                let loop_id = match label.loop_id.into() {
+                    Ok(loop_id) => loop_id,
+                    Err(hir::LoopIdError::OutsideLoopScope) => ast::DUMMY_NODE_ID,
+                    Err(hir::LoopIdError::UnlabeledCfInWhileCondition) => {
+                        self.emit_unlabled_cf_in_while_condition(e.span, "break");
+                        ast::DUMMY_NODE_ID
+                    },
+                    Err(hir::LoopIdError::UnresolvedLabel) => ast::DUMMY_NODE_ID,
+                };
+
                 if opt_expr.is_some() {
-                    let loop_kind = if label.loop_id == ast::DUMMY_NODE_ID {
+                    let loop_kind = if loop_id == ast::DUMMY_NODE_ID {
                         None
                     } else {
-                        Some(match self.hir_map.expect_expr(label.loop_id).node {
+                        Some(match self.hir_map.expect_expr(loop_id).node {
                             hir::ExprWhile(..) => LoopKind::WhileLoop,
                             hir::ExprLoop(_, _, source) => LoopKind::Loop(source),
                             ref r => span_bug!(e.span,
@@ -110,9 +120,15 @@ impl<'a, 'hir> Visitor<'hir> for CheckLoopVisitor<'a, 'hir> {
                         }
                     }
                 }
+
                 self.require_loop("break", e.span);
             }
-            hir::ExprAgain(_) => self.require_loop("continue", e.span),
+            hir::ExprAgain(label) => {
+                if let Err(hir::LoopIdError::UnlabeledCfInWhileCondition) = label.loop_id.into() {
+                    self.emit_unlabled_cf_in_while_condition(e.span, "continue");
+                }
+                self.require_loop("continue", e.span)
+            },
             _ => intravisit::walk_expr(self, e),
         }
     }
@@ -143,4 +159,12 @@ impl<'a, 'hir> CheckLoopVisitor<'a, 'hir> {
             }
         }
     }
+
+    fn emit_unlabled_cf_in_while_condition(&mut self, span: Span, cf_type: &str) {
+        struct_span_err!(self.sess, span, E0583,
+                         "`break` or `continue` with no label in the condition of a `while` loop")
+            .span_label(span,
+                        &format!("unlabeled `{}` in the condition of a `while` loop", cf_type))
+            .emit();
+    }
 }