Expand docs on rustc_abi::Variants

[ada4a]

This information is scattered throughout the comments at the use sites. I thought I'd consolidate it.

cc @RalfJung since you seem to be the one who wrote most of these comments

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[RalfJung]

Sounds mostly right. :)

View changes since this review

[ada4a]

Pulled out the enum/coroutine explanation to the top, to avoid repeating it in each variant, with a FIXME for coroutines

[RalfJung]

Pulled out the enum/coroutine explanation to the top, to avoid repeating it in each variant, with a FIXME for coroutines

I have to admit I'm not a fan of that approach.

I would simply define Single as all inhabited types that are not Multiple, that also avoids the redundancy. ;)

[ada4a]

Pulled out the enum/coroutine explanation to the top, to avoid repeating it in each variant, with a FIXME for coroutines

I have to admit I'm not a fan of that approach.

I would simply define Single as all inhabited types that are not Multiple, that also avoids the redundancy. ;)

Hm, let me try that

[ada4a]

Hm, one downside of that is that the explanation of Single.index becomes a bit awkward: currently, I "attach" it to the two described cases, but with the change you propose, those would no longer be present

[RalfJung]

@oli-obk you might know about this?

AFAIK they are always Multiple.

View changes since the review

[RalfJung]

Yeah looks like even the simplest async block has 3 states
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=38679eb35c0c5ac1ab8d7e3b2aa07ee1

[ada4a]

I tried looking for the code that does uninhabited variant pruning for enums (to see if it applies to coroutines, too), but wasn't able to find it... So I guess I'll go with coroutines always being Multiple

[RalfJung]

I think it is this function

rust/compiler/rustc_abi/src/layout.rs

Line 341 in 6da3077

pub fn layout_of_struct_or_enum<

That is only called for ADTs.

[RalfJung]

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