require that there NOT be the non_exhaustive attribute on the ZST

Author: Lokathor

text/0000-result_ffi_guarantees.md

@@ -28,7 +28,10 @@ I'm not sure how to write a "guide" portion of this that's any simpler than the
 When either of these two `core` types:
 
 * `Option<T>`
-* `Result<T, E>` where either `T` or `E` are a zero-sized type with alignment 1 (a "1-ZST") and either no fields (eg: `()` or `struct Foo;`) or with `repr(transparent)` if there are fields.
+* `Result<T, E>` where either `T` or `E`:
+  * Are a zero-sized type with alignment 1 (a "1-ZST").
+  * Either have no fields (eg: `()` or `struct Foo;`) or have `repr(transparent)` if there are fields.
+  * Do not have the `#[non_exhaustive]` attribute.
 
 Is combined with a non-zero or non-null type (see the chart), the combination has the same layout (size and alignment) and the same ABI as the primitive form of the data.