Rustdoc: size layout should show "uninhabited" for empty enums, not 0

[jyn514]

Originally posted by @jyn514 in #86263 (comment)

[jyn514]

cc @camelid @pickfire

[camelid]

I'm ambivalent, I don't think showing zero is necessarily wrong, especially since that's what's reported by layout_of.

[asquared31415]

In my opinion, I think that while size 0 is not wrong, that showing that it's uninhabited is more right, or at least more clear and makes the distinction.

[pickfire]

I don't understand uninhabited when I first see it. It is a hard word compared to 0. I think 0 is easier. Why not both? What if we show 0 (uninhabited)?

Also, what if the enum is not empty but have a single/multiple ZST like phantom type?

[GuillaumeGomez]

I think displaying 0 is better as well, but we can do as @pickfire suggested and simply display both.

[jyn514]

0 is wrong. 0 means "this is a zero sized type", like () or NoneError; it can be created. Uninhabited types cannot be created, they are things like ! and enum Void {}. IMO posting 0 there is actively misleading. See also https://internals.rust-lang.org/t/size-of-uninhabited-types/7651.

I don't understand uninhabited when I first see it.

Ok, then we can link to some docs. I didn't see anything in the nomicon but I'm sure they'd take a PR adding it to "exotically sized types".

[jyn514]

I'm ambivalent, I don't think showing zero is necessarily wrong, especially since that's what's reported by layout_of.

This is a bug imo, it allows transmuting from ZSTs to uninhabited types which is UB.

[GuillaumeGomez]

Even if they cannot be created, their size remain 0 as they don't "exist", however since they cannot be constructed, it makes sense to mention they're unhabited types. Well, I don't have a strong conviction on this in any case. I think we all agree for the mention of "unhabited" though.

[pickfire]

Ok, then we can link to some docs. I didn't see anything in the nomicon but I'm sure they'd take a PR adding it to "exotically sized types".

Yeah, that solved the issue of 0 which is wrong. I do think it is better to link it to https://doc.rust-lang.org/stable/reference/glossary.html?highlight=uninhabited#uninhabited, but I think it's wrong to put that in "size" such that it should be in "type" instead, because it doesn't have a size to begin with.

Well, there are parts where rust sacrificed correctness for ease of understanding, one example is mut which means uniqueness not exactly mutable all the time.

[nerandell]

@rustbot claim

[scottmcm]

Note that uninhabited is orthogonal to size. You can have an uninhabited type of any size.

So I think showing uninhabitedness is good, but in addition to the size+align, not instead.

[camelid]

Note that uninhabited is orthogonal to size. You can have an uninhabited type of any size.

Hmm, I think that layout_of will report that the size is always 0 for an uninhabited type though, unless that's changed recently.

[scottmcm]

That would surprise me, since that'd be different from what size_of and align_of do: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e1ad034d9812b5356493e4faefe4be61