Derive Macro Versionize 

#[derive(Versionize)]
{
    // Attributes available to this derive:
    #[versionize]
}

This derives the Versionize and Unversionize trait for the target type.

This macro has a mandatory attribute parameter, which is the name of the versioned enum for this type. This enum can be anywhere in the code but should be in scope.

Example:

// The structure that should be versioned, as defined in your code
#[derive(Versionize)]
// We have to link to the enum type that will holds all the versions of this
// type. This can also be written `#[versionize(dispatch = MyStructVersions)]`.
#[versionize(MyStructVersions)]
struct MyStruct<T> {
    attr: T,
    builtin: u32,
}

// To avoid polluting your code, the old versions can be defined in another module/file, along with
// the dispatch enum
#[derive(Version)] // Used to mark an old version of the type
struct MyStructV0 {
    builtin: u32,
}

// The Upgrade trait tells how to go from the first version to the last. During unversioning, the
// upgrade method will be called on the deserialized value enough times to go to the last variant.
impl<T: Default> Upgrade<MyStruct<T>> for MyStructV0 {
    type Error = Infallible;

    fn upgrade(self) -> Result<MyStruct<T>, Self::Error> {
        Ok(MyStruct {
            attr: T::default(),
            builtin: self.builtin,
        })
    }
}

// This is the dispatch enum, that holds one variant for each version of your type.
#[derive(VersionsDispatch)]
// This enum is not directly used but serves as a template to generate a new enum that will be
// serialized. This allows recursive versioning.
#[allow(unused)]
enum MyStructVersions<T> {
    V0(MyStructV0),
    V1(MyStruct<T>),
}