Guarantee that some structs are zero sized

Author: gnzlbg

reference/src/layout/structs-and-tuples.md

@@ -122,6 +122,12 @@ compiler will not reorder it, to allow for the possibility of
 unsizing. E.g., `struct Foo { x: u16, y: u32 }` and `struct Foo<T> {
 x: u16, y: T }` where `T = u32` are not guaranteed to be identical.
 
+#### Zero-sized structs
+
+Structs with default layout are zero-sized, if they contain no fields of
+non-zero size. That is, either the type has no fields, or all of its fields have
+zero size.
+
 #### Unresolved questions
 
 During the course of the discussion in [#11] and [#12], various
@@ -131,15 +137,6 @@ issue has been opened for further discussion on the repository. This
 section documents the questions and gives a few light details, but the
 reader is referred to the issues for further discussion.
 
-**Zero-sized structs ([#37]).** If you have a struct which --
-transitively -- contains no data of non-zero size, then the size of
-that struct will be zero as well. These zero-sized structs appear
-frequently as exceptions in other layout considerations (e.g.,
-single-field structs). An example of such a struct is
-`std::marker::PhantomData`.
-
-[#37]: https://github.com/rust-rfcs/unsafe-code-guidelines/issues/37
-
 **Single-field structs ([#34]).** If you have a struct with single field
 (`struct Foo { x: T }`), should we guarantee that the memory layout of
 `Foo` is identical to the memory layout of `T` (note that ABI details