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.
A union containing a zero-sized field can contain any value. An example of such
an union is MaybeUninit.
Rationale
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.