Structs with no fields or all-ZST fields are ZSTs

[DanielEScherzer]

Guarantee that structs with Rust representation and with no fields, or where all fields are ZSTs, are themselves ZSTs.

Guarantee that enums with Rust representation with a single struct-like variant with no fields, or where all fields are ZSTs, are themselves ZSTs.

[traviscross]

cc @RalfJung @rust-lang/opsem

[theemathas]

Seems bizarre to guarantee this for struct-like variants but not unit-like variants or tuple-like variants.

View changes since the review

[RalfJung]

That seems reasonable to me (specifically for repr(Rust); not necessarily for other reprs). Someone should check whether it is honored by -Zrandomize-layout.

[traviscross]

We talked about this in the lang call today. Let's propose to do this with the correction that, for enums, we mean those repr(Rust) enums without a primitive representation specified and that have a single field-struct-like variant, a single unit-struct-like variant, or a single tuple-struct-like variant where the struct-like thing has no fields or where all of the fields are zero-sized.

@rfcbot fcp merge lang

[rust-rfcbot]

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

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

Concerns:

• check-randomize-layout (Structs with no fields or all-ZST fields are ZSTs #2262 (comment))

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!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[traviscross]

@rfcbot concern check-randomize-layout

[tmandry]

This seems stronger than what we guarantee for non-ZST structs, which require #[repr(transparent)] to have the same representation as the inner type. In the lang meeting, @steffahn pointed out that the non-ZST case is harder because certain types get passed in registers, etc.

I don't see a reason not to make this guarantee, and it seems like a useful one, so let's do it.

@rfcbot reviewed