Empty repr(Rust) enums are ZSTs

[zachs18]

Guarantee that enums with Rust representation with no variants are ZSTs.

Followup to #2262 , (which guaranteed the same for enums with Rust representation with a single variant whose fields are all ZSTs).

[zachs18]

@rustbot ready

Followup to #2262 to guarantee that zero-variant repr(Rust) enums are zero-sized. Rebased since #2262 merged, using similar phrasing as in that PR.

[rustbot]

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[zachs18]

cc @traviscross As per #2262 (comment) , here's a followup PR to extend the ZST guarantee to zero-variant repr(Rust) enums.

[scottmcm]

This seems obvious to me: of course an uninhabitable type with no variants and no fields needn't take up any space, so might as well be guaranteed a ZST. Especially since this doesn't say anything about alignment it seems fine.

@rfcbot merge lang

(Since 1-ZSTs are extra meaningful in various ways it'd probably also be fine to make these 1-ZSTs specifically, not just ZSTs, but let's accept the proposal here for just "size ≡ 0" for now since we have it in front of us.)

[rust-rfcbot]

@scottmcm has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[traviscross]

@rfcbot reviewed

[joshtriplett]

These types are uninhabited, but for the purposes of layout, yes, they should not affect the size (or alignment) of whatever they're put inside.

@rfcbot reviewed

[rust-rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[nikomatsakis]

@rfcbot reviewed

[steffahn]

I would appreciate if there’s some (at least minor) indication about this case being an uninhabited type, too; because those feel different from the more typical case of a “ZST”. Also, looking up at the previous section about uninhabited types, which lists ! and variant-less enums, it feels inconsistent not to list ! here as well? Or is the size of ! not guaranteed?

Maybe the (or part of the) callout needed would just be in that previous section about uninhabited types, noting the fact that types can be uninhabited and zero-sized at the same time. (And that uninhabited types often - but not always - are zero-sized.)

[zachs18]

I would appreciate if there’s some (at least minor) indication about this case being an uninhabited type, too

I added a note to layout.repr.rust.enum-empty-zst with a link to the "uninhabited" glossary section.

it feels inconsistent not to list ! here as well? Or is the size of ! not guaranteed?

I think ! should be listed in the primitives section, not the repr(Rust) section. I'll make a separate PR for that, since this one is already in FCP.

Maybe the (or part of the) callout needed would just be in that previous section about uninhabited types, noting the fact that types can be uninhabited and zero-sized at the same time. (And that uninhabited types often - but not always - are zero-sized.)

I'll do that in the ! PR

[zachs18]

Hmm, maybe the ! layout guarantees should be in or after #2283 ?

I'll put the note that that types can be uninhabited and zero-sized at the same time in this PR then.

[rust-rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

[DanielEScherzer]

@rustbot label -S-waiting-on-author
Seems to have been present from the start when this was a draft PR, no longer waiting

[traviscross]

Thanks @zachs18.