Uninhabited `repr(uX)` enum shouldn't have size zero

[theemathas]

I tried this code:

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

#[repr(u32)]
enum MyEnum {
    A(Never),
    B(Never),
}

#[repr(C)]
union MyEnumRepr {
    A: MyVariantA,
    B: MyVariantB,
}

#[repr(u32)]
#[derive(Copy, Clone)]
enum MyEnumDiscriminant { A, B }

#[repr(C)]
#[derive(Clone, Copy)]
struct MyVariantA(MyEnumDiscriminant, Never);

#[repr(C)]
#[derive(Clone, Copy)]
struct MyVariantB(MyEnumDiscriminant, Never);

fn main() {
    println!("{}", size_of::<MyEnum>());
    println!("{}", size_of::<MyEnumRepr>());
}

I expected MyEnum and MyEnumRepr to have the same size, since this is the desugaring given as an example in the reference.

Instead, MyEnum has size 0, while MyEnumRepr has size 4.

Discovered by masterjason9637 on the rust community discord.

Meta

Reproducible on the playground with version 1.92.0-nightly (2025-09-23 975e6c8fec280816d24f)

[zachs18]

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