the minimal test for two-phase borrows: the core example from niko's …

…blog post on it.

src/test/run-pass/borrowck/two-phase-baseline.rs

@@ -0,0 +1,21 @@
+// Copyright 2017 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.
+
+// revisions: lxl nll
+//[lxl]compile-flags: -Z borrowck=mir -Z two-phase-borrows
+//[nll]compile-flags: -Z borrowck=mir -Z two-phase-borrows -Z nll
+
+// This is the "goto example" for why we want two phase borrows.
+
+fn main() {
+    let mut v = vec![0, 1, 2];
+    v.push(v.len());
+    assert_eq!(v, [0, 1, 2, 3]);
+}