Add an additional field while serializing

Question

In Serde serializers, how to add an additional field:

#[derive(Serialize)]
struct X {
  a: u32,
  b: u32,
  c: u32,
}

I want to add to JSON serialization field d with value "qwe". How without writing completely a serializer for X from scratch?

Answer 1

Assuming that you don't mind that the field d exists, but you just don't want it taking space in your struct, you can make it zero-sized and use the serialize_with attribute to emit the desired data when serializing:

#[derive(Serialize)]
struct X {
  a: u32,
  b: u32,
  c: u32,
  #[serde(serialize_with = "emit_qwe")]
  d: (),
}

fn emit_qwe<S: Serializer>(_: &(), s: S) -> Result<S::Ok, S::Error> {
    s.serialize_str("qwe")
}

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