Test for `-Z borrowck=migrate`.

Note that this test is carefully crafted to *try* to not segfault
during its run.  Howver, it really is representing unsound code that
should be rejected after we manage to remove the AST-borrowck entirely
from the compiler.

src/test/ui/borrowck/borrowck-migrate-to-nll.rs

@@ -0,0 +1,32 @@
+// 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.
+
+// This is a test of the borrowck migrate mode. It leverages #27282, a
+// bug that is fixed by NLL: this code is (unsoundly) accepted by
+// AST-borrowck, but is correctly rejected by the NLL borrowck.
+//
+// Therefore, for backwards-compatiblity, under borrowck=migrate the
+// NLL checks will be emitted as *warnings*.
+
+// compile-flags: -Z borrowck=migrate
+// run-pass
+
+fn main() {
+    match Some(&4) {
+        None => {},
+        ref mut foo
+            if {
+                (|| { let bar = foo; bar.take() })();
+                false
+            } => {},
+        Some(ref _s) => println!("Note this arm is bogus; the `Some` became `None` in the guard."),
+        _ => println!("Here is some supposedly unreachable code."),
+    }
+}

src/test/ui/borrowck/borrowck-migrate-to-nll.stderr

@@ -0,0 +1,24 @@
+warning[E0507]: cannot move out of borrowed content
+  --> $DIR/borrowck-migrate-to-nll.rs:26:17
+   |
+LL |                 (|| { let bar = foo; bar.take() })();
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot move out of borrowed content
+   |
+   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
+           It represents potential unsoundness in your code.
+           This warning will become a hard error in the future.
+
+warning[E0507]: cannot move out of `foo`, as it is immutable for the pattern guard
+  --> $DIR/borrowck-migrate-to-nll.rs:26:17
+   |
+LL |                 (|| { let bar = foo; bar.take() })();
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                 |
+   |                 cannot move out of `foo`, as it is immutable for the pattern guard
+   |                 cannot move
+   |
+   = note: variables bound in patterns are immutable until the end of the pattern guard
+   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
+           It represents potential unsoundness in your code.
+           This warning will become a hard error in the future.
+