rustc: represent the discriminant as a field for Layout::{Raw,StructWrapped}NullablePointer.

src/test/run-pass/packed-struct-optimized-enum.rs

@@ -0,0 +1,25 @@
+// 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)]
+#[derive(Copy, Clone)]
+struct Packed<T>(T);
+
+fn main() {
+    let one = (Some(Packed((&(), 0))), true);
+    let two = [one, one];
+    let stride = (&two[1] as *const _ as usize) - (&two[0] as *const _ as usize);
+
+    // This can fail if rustc and LLVM disagree on the size of a type.
+    // In this case, `Option<Packed<(&(), u32)>>` was erronously not
+    // marked as packed despite needing alignment `1` and containing
+    // its `&()` discriminant, which has alignment larger than `1`.
+    assert_eq!(stride, std::mem::size_of_val(&one));
+}