Cancel an error before it panics

Fixes #30715

Author: nrc

src/libsyntax/parse/parser.rs

@@ -2793,8 +2793,15 @@ impl<'a> Parser<'a> {
                     // We have 2 alternatives here: `x..y` and `x..` The other two variants are
                     // handled with `parse_prefix_range_expr` call above.
                     let rhs = if self.is_at_start_of_range_notation_rhs() {
-                        self.parse_assoc_expr_with(op.precedence() + 1,
-                                                   LhsExpr::NotYetParsed).ok()
+                        let rhs = self.parse_assoc_expr_with(op.precedence() + 1,
+                                                             LhsExpr::NotYetParsed);
+                        match rhs {
+                            Ok(e) => Some(e),
+                            Err(mut e) => {
+                                e.cancel();
+                                None
+                            }
+                        }
                     } else {
                         None
                     };

src/test/compile-fail/issue-30715.rs

@@ -0,0 +1,26 @@
+// Copyright 2016 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.
+
+macro_rules! parallel {
+    (
+        for $id:ident in $iter:expr {
+            $( $inner:expr; )*
+        }
+    ) => {};
+}
+
+
+fn main() {
+    parallel! {
+        for i in 0..n {
+            x += i; //~ ERROR no rules expected the token `+=`
+        }
+    }
+}