How to mix serde with = and serde_as rules? / u128 as Hex

[steinerkelvin]

I would like to chain something like serde with field attribute and the rule Hex from serde_with:

#[serde_as]
#[derive(Debug, Serialize, Deserialize)]
struct Repr {
  #[serde_as(as = "serde_with::hex::Hex")]
  #[serde(with = "u128_as_buffer")]
  pub meta: u128,       // metadata
  
  ...
}

/// Serializes 
mod u128_as_buffer {
  type T = u128;
  pub fn serialize<S>(v: &T, s: S) -> Result<S::Ok, S::Error> where S: Serializer {
    todo!()
  }
  pub fn deserialize<'de, D>(d: D) -> Result<T, D::Error> where D: Deserializer<'de> {
    todo!()
  }
}

Can I achieve something like this?

Also, could there be a simple way to (de)serialize a number (in this case, an u128) as a hex string?

[jonasbb]

I am unsure what exactly you think the u128_as_buffer should be doing. But I am not aware of any way of chaining multiple functions. The serde_as macro will also not let you use as and with on the same field.

In your case, your module will need to do everything, i.e., turn the u128 into a hex value and then instruct the Serializer how to serialize it. Of course it is also possible to use the SerializeAs trait from this crate, i.e., you can make a type similar to Hex which supports u128.

[steinerkelvin]

The macro serde_conv seems pretty useful for this use case (u128 <–> hex string).

[jonasbb]

I would not use serde_conv here, but really go for a full DeserializeAs implementation with Visitor. Of course, you can still macro it, if you need multiple implementation. Using a Visitor is not that complicated but you gain the benefits of using transient strings via visit_str (https://serde.rs/lifetimes.html#transient-borrowed-and-owned-data), saving some allocations, and using expecting for the error messages. You can see how DisplayFromStr is implemented.