std: add an external iterator for mutating vec elements

src/test/compile-fail/vec-mut-iter-borrow.rs

@@ -0,0 +1,19 @@
+// 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.
+
+use std::iterator::*;
+
+fn main() {
+    let mut xs = ~[1, 2, 3, 4];
+
+    for xs.mut_iter().advance |x| {
+        xs.push(1) //~ ERROR cannot borrow `xs` as mutable
+    }
+}