check bytes of repr(C) enums

[dvc94ch]

any way to do it? I guess repr(C) is not defined on enums, although I'd hope it would generate something like this:

#[repr(C)]
struct {
    discriminant: u8, 
    variant: #[repr(C)] union,
}

[djkoloski]

repr(C) is actually well-defined for enums, and it lays them out as you described. Is the issue that only repr(Int) is supported? That has a slightly more efficient layout while still being C-compatible.

[dvc94ch]

I'm trying to send archived data structures over a network and use check_archive_root to verify the bytes. I didn't realize that I only need to #[archive_attr(derive(CheckBytes))] and that that always seems to work.

[djkoloski]

Awesome! I'll add an enum example to the book to make it clearer.

[dvc94ch]

so reopening. it's still a problem when using archive(as = "Self"). although it can be worked around by not using that.

[djkoloski]

When using #[archive(as = "Self")], you may need to use #[derive(CheckBytes)] on the enum itself instead of in an #[archive_attr(...)]. I think that as long as you have the enum set to a compatible repr, it should work. If it doesn't, I can take a closer look at it.

[dvc94ch]

Well it's set to repr(C) but it claims it isn't

[djkoloski]

Hmm, this might be really dumb but do you have the #[repr(C)] before the #[derive(CheckBytes)]? The ordering can matter for determining whether some attributes get picked up.

[dvc94ch]

Maybe I'm doing something wrong...

use bytecheck::CheckBytes;

#[derive(CheckBytes)]
#[repr(C)]
pub enum AnEnum {
    Var(u8),
    Other(u16),
}

fn main() {
    println!("Hello, world!");
}

error: archive self enums must be repr(C) or repr(Int)
 --> src/main.rs:4:8
  |
4 | #[repr(C)]
  |        ^

error: aborting due to previous error

[djkoloski]

Okay, I have located the issue. Thanks for your patience.

That error is outdated and inaccurate. Right now bytecheck only has support for deriving validation on repr(Int) enums, but repr(C) enums could feasibly be supported. That error message will be updated as soon as possible and I'll file an issue for adding repr(C) enum support.

[dvc94ch]

if that's the case I'm a bit confused why this works:

use bytecheck::CheckBytes;
use rkyv::Archive;

#[derive(Archive)]
#[archive_attr(derive(CheckBytes))]
#[repr(C)]
pub enum AnEnum {
    Var(u8),
    Other(u16),
}

fn main() {
    println!("Hello, world!");
}

is ArchiveAnEnum always a repr(Int) enum?

[djkoloski]

Yes, rkyv will always generate archived enums that are repr(Int) even when the original enum is repr(C).

[djkoloski]

As an update, I'm no longer planning on adding this myself since I have no need for repr(C) enums. I would not turn down a PR though!