Unit test for the new implicit borrow and deref within the

guard expressions of matches (activated only when using
new NLL mode).

Review feedback: removed 27282 from filename. (The test still
references it in a relevant comment in the file itself so that seemed
like a reasonable compromise.)

src/test/compile-fail/nll/match-guards-always-borrow.rs

@@ -0,0 +1,61 @@
+// Copyright 2012-2014 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: ast mir
+//[mir] compile-flags: -Z borrowck=mir
+
+#![feature(rustc_attrs)]
+
+// Here is arielb1's basic example from rust-lang/rust#27282
+// that AST borrowck is flummoxed by:
+
+fn should_reject_destructive_mutate_in_guard() {
+    match Some(&4) {
+        None => {},
+        ref mut foo if {
+            (|| { let bar = foo; bar.take() })();
+            //[mir]~^ ERROR cannot move out of borrowed content [E0507]
+            false } => { },
+        Some(s) => std::process::exit(*s),
+    }
+}
+
+// Here below is a case that needs to keep working: we only use the
+// binding via immutable-borrow in the guard, and we mutate in the arm
+// body.
+fn allow_mutate_in_arm_body() {
+    match Some(&4) {
+        None => {},
+        ref mut foo if foo.is_some() && false => { foo.take(); () }
+        Some(s) => std::process::exit(*s),
+    }
+}
+
+// Here below is a case that needs to keep working: we only use the
+// binding via immutable-borrow in the guard, and we move into the arm
+// body.
+fn allow_move_into_arm_body() {
+    match Some(&4) {
+        None => {},
+        mut foo if foo.is_some() && false => { foo.take(); () }
+        Some(s) => std::process::exit(*s),
+    }
+}
+
+// Since this is a compile-fail test that is explicitly encoding the
+// different behavior of AST- vs MIR-borrowck where AST-borrowck does
+// not error, we need to use rustc_error to placate the test harness
+// that wants *some* error to occur.
+#[rustc_error]
+fn main() { //[ast]~ ERROR compilation successful
+    should_reject_destructive_mutate_in_guard();
+    allow_mutate_in_arm_body();
+    allow_move_into_arm_body();
+}