Merge pull request rust-lang#1430 from joshlf/patch-2

type-layout.md: Warn about repr(align)/repr(packed) and field order

Author: ehuss

src/type-layout.md

@@ -524,6 +524,10 @@ assert_eq!(std::mem::size_of::<Enum16>(), 4);
 The `align` and `packed` modifiers can be used to respectively raise or lower
 the alignment of `struct`s and `union`s. `packed` may also alter the padding
 between fields (although it will not alter the padding inside of any field).
+On their own, `align` and `packed` do not provide guarantees about the order
+of fields in the layout of a struct or the layout of an enum variant, although
+they may be combined with representations (such as `C`) which do provide such
+guarantees.
 
 The alignment is specified as an integer parameter in the form of
 `#[repr(align(x))]` or `#[repr(packed(x))]`. The alignment value must be a