rkyv-with

Third-party derive macro for rkyv's {Archive/Serialize/Deserialize}With traits.

The main use-case for this derive is to be able to use rkyv on remote types that don't implement Archive, Serialize, and Deserialize themselves. This provides a somewhat similar workaround to rusts orphan rule as serde's remote support.

Macros

The ArchiveWith derive macro implements both the ArchiveWith and the SerializeWith traits. For the DeserializeWith trait, use the DeserializeWith derive macro.

The #[archive_with(...)] attribute helps to fine-tune the implementations.

• archive_with(from(TypeName)) indicates what the original type is. This attribute is required to be specified at the top level of the type definition. Multiple comma-separated types are allowed, i.e. from(Type1, Type2). The attribute can also be used on fields.
• archive_with(via(TypeWrapper)) provides a way to convert the type of a field into something else e.g. the unarchivable type contains a PathBuf field and in the archivable counterpart it can be a String by specifying via(AsString)
• archive_with(getter = "method_name") must be used in case the unarchivable type includes private fields. The only argument to the specified method must be a reference to the unarchivable type.
• archive_with(getter_owned) can be used in case the getter takes an owned instance instead of a reference e.g. if Copy is implemented and the getter takes ownership of self.

Applying the macros

// Both trait and macro must be imported
use rkyv::with::ArchiveWith;
use rkyv_with::ArchiveWith;

// This could come from some dependency or otherwise remote module.
// Importantly, this does not implement the rkyv traits
struct UnarchivableInner {
    a: u32,
    b: Vec<u8>,
    unimportant: String,
}

#[derive(rkyv::Archive, ArchiveWith)]
#[archive_with(from(UnarchivableInner))] // must be specified
struct ArchivableInner {
    // fields must have the same name
    a: u32,
    #[with(rkyv::with::CopyOptimize)] // archive wrappers work as usual
    b: Vec<u8>,
    // not all fields must be included but if fields
    // are omitted, `DeserializeWith` can not be derived
}

struct UnarchivableOuter {
    archivable_field: i32,
    inner: UnarchivableInner,
    opt: Option<UnarchivableInner>,
    buf: std::path::PathBuf,
}

#[derive(rkyv::Archive, ArchiveWith)]
#[archive_with(from(UnarchivableOuter))]
struct ArchivableOuter {
    // if the field's type is archivable, no annotation is required
    archivable_field: i32,
    // otherwise one must specify the original type through `archive_with(from(...))`
    #[archive_with(from(UnarchivableInner))]
    inner: ArchivableInner,
    // and if the unarchivable type is wrapped with a Vec or Option,
    // it's necessary to specify both the original type, as well as the
    // archivable type wrapped in rkyv's Map wrapper
    #[archive_with(
        from(Option<UnarchivableInner>),
        via(rkyv::with::Map<ArchivableInner>),
    )]
    opt: Option<ArchivableInner>,
    // if the field has a different type, a wrapper that converts
    // the original into the new type must be specified
    #[archive_with(from(std::path::PathBuf), via(rkyv::with::AsString))]
    buf: String,
}

Using the resulting implementations

use rkyv::with::{ArchiveWith, DeserializeWith, With};
use rkyv::{Archive, Deserialize, Infallible, Serialize};
use rkyv_with::{ArchiveWith, DeserializeWith};

struct Unarchivable {
    a: String,
}

// Can be serialized as usual and
// also serves as serialization wrapper
#[derive(Archive, ArchiveWith, Deserialize, DeserializeWith)]
#[archive_with(from(Unarchivable))]
struct ArchivesTheUnarchivable {
    a: String,
}

// Can be serialized as usual
#[derive(Archive, Serialize)]
struct Container {
    a: i32,
    #[with(ArchivesTheUnarchivable)]
    b: Unarchivable,
}

let unarchivable = Unarchivable { a: String::new() };

// Serialize the instance
let wrapper = With::<_, ArchivesTheUnarchivable>::cast(&unarchivable);
let bytes = rkyv::to_bytes::<_, 32>(wrapper).unwrap();

let archived = unsafe { rkyv::archived_root::<ArchivesTheUnarchivable>(&bytes) };

// Can go back to the original type
let deserialized_unarchivable: Unarchivable =
    ArchivesTheUnarchivable::deserialize_with(archived, &mut Infallible).unwrap();

// Or stick to the wrapper
let deserialized_wrapper: ArchivesTheUnarchivable =
    archived.deserialize(&mut Infallible).unwrap();

Private fields

If the current crate is unable to access fields of a remote type directly due to them being private, deriving the traits requires another way of accessing those fields, namely by manually specifying getter functions.

use rkyv::with::ArchiveWith;
use rkyv::Archive;
use rkyv_with::ArchiveWith;

// Imagine again that this is a remote module that you have no way of modifying
mod remote {
    #[derive(Clone)]
    pub struct Remote {
        pub public_field: u32,
        private_field: u32,
    }

    impl Remote {
        // By default it will be assumed that the function will take a reference
        pub fn get_private_field(&self) -> u32 {
            self.private_field
        }

        // If it takes ownership instead, the type will need to be cloned internally
        pub fn into_private_field(self) -> u32 {
            self.private_field
        }

        // Some publicly available way of creating the type in case you need to deserialize
        pub fn new(public_field: u32, private_field: u32) -> Self {
            Self { public_field, private_field }
        }
    }
}

#[derive(Archive, ArchiveWith)]
#[archive_with(from(remote::Remote))]
struct NativeByReference {
    public_field: u32,
    // Specifying the path to the getter function which only takes a reference
    #[archive_with(getter = "remote::Remote::get_private_field")]
    private_field: u32,
}

#[derive(Archive, ArchiveWith)]
#[archive_with(from(remote::Remote))]
struct NativeByValue {
    public_field: u32,
    // If the function takes ownership, be sure to also specify `getter_owned`
    #[archive_with(getter = "remote::Remote::into_private_field", getter_owned)]
    private_field: u32,
}

// Since creating instances of a type that has private fields cannot be done in a general way,
// `DeserializeWith` cannot be derived for such types and instead has to be implemented manually.
// An implementation for the example above could look as follows

use rkyv::with::DeserializeWith;
use rkyv::{Archived, Fallible};

impl<D: Fallible> DeserializeWith<Archived<NativeByValue>, remote::Remote, D> for NativeByValue {
    fn deserialize_with(
        archived: &Archived<NativeByValue>,
        _deserializer: &mut D,
    ) -> Result<remote::Remote, <D as Fallible>::Error> {
        // Use whichever method is available to create an instance
        Ok(remote::Remote::new(archived.public_field, archived.private_field))
    }
}