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Validity of unions with a zero-sized field #147

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Validity of unions with a zero-sized field
gnzlbg Jun 20, 2019
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Remove definition and rationale
gnzlbg Jun 20, 2019
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Update reference/src/validity/unions.md
gnzlbg Jun 20, 2019
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2 changes: 2 additions & 0 deletions reference/src/SUMMARY.md
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- [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)
25 changes: 25 additions & 0 deletions reference/src/validity/unions.md
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# 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