Using serde_with::rust::sets_last_value_wins with serde_as notation

[seowalex]

I would like to use serde_with::rust::sets_last_value_wins with the serde_as notation, but that does not seem to be possible. Am I missing something?

[jonasbb]

Hi, thanks for raising the issue. That is indeed currently not possible, as this module and the related ones were never updated. I don't think there is any technical challenge here, just a bit of busy work.
This part of the guide contains the relevant part for converting. https://docs.rs/serde_with/latest/serde_with/guide/serde_as/index.html#using-serde_as-on-types-without-serializeas-and-serialize-implementations
Additionally, the documentation would need to be updated too.

If you need this urgently, you can also create a conversion type in your codebase. You can follow the guide and replace MODULE with serde_with::rust::sets_last_value_wins.

[seowalex]

I am interested in implementing this, but what would the syntax look like in the serde_as notation?

[jonasbb]

You need a new type and implement the traits for it (see link).

pub struct SetLastValueWins<V>(PhantomData<V>);

You should then be able to use it like any other serde_as attribute:

#[serde_as(as = "SetLastValueWins<_>")]
BTreeSet<u32>,

[seowalex]

pub(crate) struct SetLastValueWins<T>(PhantomData<T>);

impl<T, U> SerializeAs<IndexSet<T>> for SetLastValueWins<U>
where
    T: Eq + Hash + Serialize,
{
    fn serialize_as<S>(value: &IndexSet<T>, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        sets_last_value_wins::serialize(value, serializer)
    }
}

impl<'de, T, U> DeserializeAs<'de, IndexSet<T>> for SetLastValueWins<U>
where
    T: Eq + Hash + Deserialize<'de>,
{
    fn deserialize_as<D>(deserializer: D) -> Result<IndexSet<T>, D::Error>
    where
        D: Deserializer<'de>,
    {
        sets_last_value_wins::deserialize(deserializer)
    }
}

With my implementation above, it seems like it does not use the serde_conv defined adapters. For example, when I try to use the above code with #[serde_as(as = "Option<SetLastValueWins<CustomSerdeConv>>")], I get an error complaining about an invalid type as I am trying to deserialise a string into a struct.

[jonasbb]

In this case, you will need to adapt the existing code a bit. The values need to be transformed, since you cannot directly serialize a SerializeAs. You might want to look at the documentation of the traits: https://docs.rs/serde_with/2.1.0/serde_with/trait.SerializeAs.html#differences-to-serialize especially how the SerializeAsWrap is working.