Guarantee the layout of structs with a single non-zero-sized field

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

@@ -141,6 +141,25 @@ struct Zst2(Zst1, Zst0);
 # }
 ```
 
+#### Default layout of structs where only a single field is not a 1-ZST
+
+The default layout of structs where only a single field is not a 1-ZST is the
+same as the layout of that non-1-ZST field.
+
+For example, the layout of:
+
+```rust
+struct SomeStruct(i32, ());
+```
+
+is the same as the layout of `i32`, but the layout of:
+
+```rust
+#[repr(align(16))] struct Zst;
+struct SomeOtherStruct(i32, Zst);
+```
+is **unspecified**, since `Zst` is not a [1-ZST].
+
 #### Unresolved questions
 
 During the course of the discussion in [#11] and [#12], various
@@ -150,14 +169,14 @@ 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.
 
-**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
-around function calls may still draw a distinction, which is why
-`#[repr(transparent)]` is needed). What about zero-sized types like
-`PhantomData`?
+**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`.
 
-[#34]: https://github.com/rust-rfcs/unsafe-code-guidelines/issues/34
+[#37]: https://github.com/rust-rfcs/unsafe-code-guidelines/issues/37
 
 **Homogeneous structs ([#36]).** If you have homogeneous structs, where all
 the `N` fields are of a single type `T`, can we guarantee a mapping to
@@ -400,3 +419,5 @@ proposal (and -- further -- it does not match our existing behavior):
   thread](https://github.com/rust-rfcs/unsafe-code-guidelines/pull/31#discussion_r224955817)).
 - Many people would prefer the name ordering to be chosen for
   "readability" and not optimal layout.
+
+[1-ZST]: ../glossary.md#zero-sized-type--zst