serde_ser_comma_delim.rs

#[macro_use]
extern crate serde;
extern crate serde_json;

use serde::Serializer;

fn comma_delim<S>(
    x: &Option<Vec<String>>,
    ser: S,
) -> Result<S::Ok, S::Error>
where
    S: Serializer,
{
    match *x {
        Some(ref elems) => {
            ser.serialize_str(
                elems.join(",").as_ref(),
            )
        }
        _ => ser.serialize_none(),
    }
}

#[derive(Serialize, Debug)]
struct Foo {
  #[serde(serialize_with="comma_delim")]
  pub bar: Option<Vec<String>>
}

fn main() {
  println!("{:?}", serde_json::to_string(&Foo { bar: Some(vec!["foo".to_string(), "bar".to_string()]) }));
}