[Feature Request] Copy custom archived multibyte discriminants for enums with custom repr

[novacrazy]

I'm using rkyv in an RPC system where I have a Procedure enum that's basically:

#[repr(u16)]
enum Procedure {
    DoThingA(Box<ThingAData>) = 100,
    DoThingB(Box<ThingBData>) = 200,
    // many more...
}

where commands are grouped together and have custom discriminant values. My idea was to have the discriminants be constant, but allow room in between for more to be added in a way that is meaningful to humans. By keeping them constant in such a simple structure, it would be easy for CheckBytes to validate it even if more are added in the future, helping with stability when I update distributed systems gradually in a production environment.

Unfortunately, unless I'm missing something, rkyv currently generates new linear u8 discriminants for the Tag and Archived values/discriminants, due to its own compatibility concerns regarding Endianness and alignment.

To solve the Endian issue, I came up with the idea to double the width of my discriminants and mirror their bytes so the value is the same between Big/Little endian, and thereby making the Tag/Archived enum #[repr(u32)] (and maybe #[repr(u32, align(4))]), and immune to that cross-platform difference.

I implemented this manually Here, but would like a way to enable copying the raw multibyte discriminants into the Tag/Archived enum from the derive macros, as that would make my own code much more maintainable.

I could turn the enum in that referenced code into something that only generates:

#[repr(u32, align(4))]
#[derive(rkyv::Archive, rkyv::Serialize, rkyv::Deserialize)]
enum Procedure {
    DoThingA(Box<ThingAData>) = mirror_tag(100),
    DoThingB(Box<ThingBData>) = mirror_tag(200),
    // many more...
}

and rkyv could copy that repr information and custom discriminant expressions onto the Tag/Archived enum.

This could either be done via an attribute such as #[unsafe_archive_repr(u32, align(4))] (as suggested in Discord), or a variation of that which will assert at compile time if the discriminants are cross-Endian-safe, likely via checking of the Tag variants are mirrored.

So in essence, I'd like for rkyv to provide a way to specify the repr of the resulting Archived enum/tags, and copy the expressions that generate the discriminant values from the originating struct, such that I can create RPC enums that can maintain backwards compatibility in an elegant way.