fix #[derive] implementation for repr(packed) structs

Fix the derive implementation for repr(packed) structs to move the
fields out instead of calling functions on references to each subfield.

That's it, `#[derive(PartialEq)]` on a packed struct now does:
```Rust
fn eq(&self, other: &Self) {
    let field_0 = self.0;
    let other_field_0 = other.0;
    &field_0 == &other_field_0
}
```

Instead of
```Rust
fn eq(&self, other: &Self) {
    let ref field_0 = self.0;
    let ref other_field_0 = other.0;
    &*field_0 == &*other_field_0
}
```

Taking (unaligned) references to each subfield is undefined, unsound and
is an error with MIR effectck, so it had to be prevented. This causes
a borrowck error when a `repr(packed)` struct has a non-Copy field (and
therefore is a [breaking-change]), but I don't see a sound way to avoid
that error.

src/test/compile-fail/deriving-with-repr-packed-not-copy.rs

@@ -0,0 +1,28 @@
+// 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.
+
+// check that derive on a packed struct with non-Copy fields
+// correctly. This can't be made to work perfectly because
+// we can't just use the field from the struct as it might
+// not be aligned.
+
+#[derive(PartialEq)]
+struct Y(usize);
+
+#[derive(PartialEq)]
+//~^ ERROR cannot move out of borrowed
+//~| ERROR cannot move out of borrowed
+//~| ERROR cannot move out of borrowed
+//~| ERROR cannot move out of borrowed
+#[repr(packed)]
+struct X(Y);
+
+fn main() {
+}

src/test/run-pass/deriving-with-repr-packed.rs

@@ -0,0 +1,45 @@
+// 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.
+
+// check that derive on a packed struct does not call field
+// methods with a misaligned field.
+
+use std::mem;
+
+#[derive(Copy, Clone)]
+struct Aligned(usize);
+
+#[inline(never)]
+fn check_align(ptr: *const Aligned) {
+    assert_eq!(ptr as usize % mem::align_of::<Aligned>(),
+               0);
+}
+
+impl PartialEq for Aligned {
+    fn eq(&self, other: &Self) -> bool {
+        check_align(self);
+        check_align(other);
+        self.0 == other.0
+    }
+}
+
+#[repr(packed)]
+#[derive(PartialEq)]
+struct Packed(Aligned, Aligned);
+
+#[derive(PartialEq)]
+#[repr(C)]
+struct Dealigned<T>(u8, T);
+
+fn main() {
+    let d1 = Dealigned(0, Packed(Aligned(1), Aligned(2)));
+    let ck = d1 == d1;
+    assert!(ck);
+}