make accessing packed fields a future-compat warning

src/test/compile-fail/issue-27060.rs

@@ -20,6 +20,7 @@ pub struct JustArray {
     array: [u32]
 }
 
+#[deny(safe_packed_borrows)]
 fn main() {
     let good = Good {
         data: &0,
@@ -33,7 +34,9 @@ fn main() {
     }
 
     let _ = &good.data; //~ ERROR borrow of packed field requires unsafe
+                        //~| hard error
     let _ = &good.data2[0]; //~ ERROR borrow of packed field requires unsafe
+                            //~| hard error
     let _ = &*good.data; // ok, behind a pointer
     let _ = &good.aligned; // ok, has align 1
     let _ = &good.aligned[2]; // ok, has align 1

src/test/run-pass/issue-27060.rs

@@ -0,0 +1,42 @@
+// 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.
+
+#[repr(packed)]
+pub struct Good {
+    data: &'static u32,
+    data2: [&'static u32; 2],
+    aligned: [u8; 32],
+}
+
+#[repr(packed)]
+pub struct JustArray {
+    array: [u32]
+}
+
+// kill this test when that turns to a hard error
+#[allow(safe_packed_borrows)]
+fn main() {
+    let good = Good {
+        data: &0,
+        data2: [&0, &0],
+        aligned: [0; 32]
+    };
+
+    unsafe {
+        let _ = &good.data; // ok
+        let _ = &good.data2[0]; // ok
+    }
+
+    let _ = &good.data;
+    let _ = &good.data2[0];
+    let _ = &*good.data; // ok, behind a pointer
+    let _ = &good.aligned; // ok, has align 1
+    let _ = &good.aligned[2]; // ok, has align 1
+}