Unions with uninhabited variants are considered uninhabited

[gereeter]

Uninhabited variants should ideally be irrelevant to the representation, just like in enums.

As a test, this prints out 0 instead of 8:

#![feature(never_type)]
use std::mem;

union Foo {
    a: u64,
    b: !
}

fn main() {
    println!("{}", mem::size_of::<Foo>());
}

This happens because rustc considers unions more like structs than enums, and the test for uninhabitedness sees a single variant with two fields, one of which is uninhabited.

I have a fix for this, but it's slightly easier to bundle it with some other changes I'm working on.

[durka]

This becomes a problem when making an array of unions:

#![allow(unused)]
use std::mem;

#[derive(Copy, Clone)]
enum Never {}

union Foo {
    a: u64,
    b: Never
}

fn main() {
    println!("{}", mem::size_of::<Foo>());
    
    let f = [Foo { a: 42 }, Foo { a: 10 }];
    println!("{:?}", unsafe { f[0].a });
}

The above code represents a regression (prints 42 on stable, garbage on beta/nightly).

[gereeter]

I didn't realize it was a regression to beta - in that case I'll put up an independent fix so that it can be backported if desired.

[gereeter]

Although I can't trigger it on the playground, you code is actually triggering an ICE for me:

error: internal compiler error: /checkout/src/librustc_trans/mir/place.rs:418: using operand local _19 as place

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.24.0-nightly (dc39c3169 2017-12-17) running on x86_64-unknown-linux-gnu

EDIT: I bisected the ICE to somewhere between "rust 1.23.0-nightly (e97ba83 2017-11-25)" and "rust 1.23.0-nightly (827cb0d 2017-11-26)".

[arielb1]

cc @eddyb

[gereeter]

I put up a PR to fix this, but I'm still confused about the ICE (which I moved to #46855).