Unions with ZSTs are no longer usable

[mjbshaw]

It looks like this was broken in the release of 1.32.0 (and is still broken, even on nightly) (checked on https://rust.godbolt.org/). The following example program:

union Uninit {
    _never_use: *const u8,
    uninit: (),
}

const UNINIT: Uninit = Uninit { uninit: () };

fn main() {
    let _ = UNINIT; // This line isn't actually necessary.
}

Fails to compile:

$ rustc --edition=2018 t.rs
error[E0080]: it is undefined behavior to use this value
 --> t.rs:6:1
  |
6 | const UNINIT: Uninit = Uninit { uninit: () };
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempted to read undefined bytes
  |
  = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior

error: aborting due to previous error

For more information about this error, try `rustc --explain E0080`.

Interestingly, making the union #[repr(C)] fixes it.

My Rust version:

$ rustc --version
rustc 1.35.0-nightly (e68bf8ae1 2019-03-11)

I know union field accesses aren't stable, but I don't think this qualifies as a field access. And yes, I'm aware of MaybeUninit.

cc @RalfJung

[RalfJung]

Interesting. Looks like a validation issue?

Cc @oli-obk