Validity of unions with a zero-sized field

Author: gnzlbg

reference/src/SUMMARY.md

@@ -11,5 +11,7 @@
   - [Function pointers](./layout/function-pointers.md)
   - [Arrays and Slices](./layout/arrays-and-slices.md)
   - [Packed SIMD vectors](./layout/packed-simd-vectors.md)
+- [Validity](./validity.md)
+  - [Unions](./validity/unions.md)
 - [Optimizations](./optimizations.md)
   - [Return value optimization](./optimizations/return_value_optimization.md)

reference/src/validity/unions.md

@@ -0,0 +1,25 @@
+# Validity of unions
+
+**Disclaimer**: This chapter is a work-in-progress. What's contained here
+represents the consensus from issue [#73]. The statements in here are not (yet)
+"guaranteed" not to change until an RFC ratifies them.
+
+The bit `i` of an union is allowed to have value `v` _iff_ there is a variant of
+the union such that bit `i` of the variant is allowed to have value `v`. We
+assume all variants to be "filled up" to the same size with padding, which may
+have any value.
+
+## Validity of unions with zero-sized fields
+
+A union containing a zero-sized field can contain any value. An example of such
+an union is [`MaybeUninit`].
+
+<details><summary><b>Rationale</b></summary> 
+
+This follows from the definition of the values that `union` bits are allowed to
+take. The zero-sized type has size `0`, and its variant is filled up to
+the `union` size with padding. Since padding is allowed to take any value, any
+bit in the `union` is allowed to take any value.
+
+[#73]: https://github.com/rust-lang/unsafe-code-guidelines/issues/73
+[`MaybeUninit`]: https://doc.rust-lang.org/std/mem/union.MaybeUninit.html