Always eat up mut when parsing general args, fixes #7483

src/libsyntax/parse/parser.rs

@@ -1074,10 +1074,9 @@ impl Parser {
     // This version of parse arg doesn't necessarily require
     // identifier names.
     pub fn parse_arg_general(&self, require_name: bool) -> arg {
-        let mut is_mutbl = false;
+        let mut is_mutbl = self.eat_keyword(keywords::Mut);
         let pat = if require_name || self.is_named_argument() {
             self.parse_arg_mode();
-            is_mutbl = self.eat_keyword(keywords::Mut);
             let pat = self.parse_pat();
 
             if is_mutbl && !ast_util::pat_is_ident(pat) {

src/test/run-pass/traits-default-method-mut.rs

@@ -0,0 +1,17 @@
+// Copyright 2013 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(default_methods)];
+
+trait Foo {
+    fn foo(&self, mut v: int) { v = 1; }
+}
+
+fn main() {}