Enum size includes sizes of variants that can't be created

[WaffleLapkin]

I tried this code:

enum MyPhantomData<T> {
    Always,
    Never(T, core::convert::Infallible),
}

#[test]
fn size() {
    assert_eq!(core::mem::size_of::<MyPhantomData<i32>>(), 0); // Err 8 != 0
}

I expected size test to pass because there is only one value of type MyPhantomData<i32> (MyPhantomData::Always). However, it fails because the actual size of MyPhantomData<i32> is 8. It's also notable that MyPhantomData<()> is actually ZST.

This concrete example may seem strange, but this could also appear in generic code like that:

enum MaybeArrayAndT<T> {
    Nothing,
    ArrayAndT(T, [u8; 1024]),
}

fn main() {
    assert_eq!(size_of::<MaybeArrayAndT<Infallible>>(), 0); // Err: 1025 != 0
}

I'm wondering if there are already issues like that? I couldn't find any...

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Rust playground (stable 1.41.0/nightly 2020-02-09 71c7e149e42cb0fc78a8)

[hanna-kruppe]

Relevant discussion is unfortunately spread over many issues (e.g. rust-lang/unsafe-code-guidelines#79, #49298, #58374, rust-lang/unsafe-code-guidelines#216, and possibly more). I'll close this as a duplicate of #58374 to avoid even more fragmentation.