Custom date deserialize but with Option<Datetime<Utc>>

[yageek]

I found the example about this exanple about custom date formatting.

I want to apply exactly the same but with an Option<DateTime<Utc>>.

I found this stackoverflow thread and try to combine one deserialize_with and default attributes but no success:

fn datefmt<'de, D>(deserializer: D) -> Result<DateTime<Utc>, D::Error>
where
  D: Deserializer<'de>,
{
  let s = String::deserialize(deserializer)?;
  Utc
    .datetime_from_str(&s, FORMAT)
    .map_err(serde::de::Error::custom)
}

struct MyStruct {

  #[serde(default)]
  #[serde(deserialize_with = "datefmt")]
  expiration_date: Option<DateTime<Utc>>,
}

But I have the following error:

#[derive(Deserialize, Debug)]
   |          -----------
   |          |
   |          expected enum `std::option::Option`, found struct `chrono::datetime::DateTime`
   |          match arm with an incompatible type
   |          in this macro invocation
   |
   = note: expected type `std::option::Option<chrono::datetime::DateTime<_>>`
              found type `chrono::datetime::DateTime<_>`

How to handle the case of Option<T> where T has custom serialise/deserialize helpers ?

[dtolnay]

This is not well supported right now (but tracked in #723). You need to provide a separate deserialize_with function that produces Option<T>. I would write this as:

use chrono::{DateTime, TimeZone, Utc};
use serde::{Deserialize, Deserializer};

const FORMAT: &str = "%Y-%m-%d %H:%M:%S";

fn datefmt<'de, D>(deserializer: D) -> Result<DateTime<Utc>, D::Error>
where
    D: Deserializer<'de>,
{
    let s = String::deserialize(deserializer)?;
    Utc.datetime_from_str(&s, FORMAT)
        .map_err(serde::de::Error::custom)
}

fn option_datefmt<'de, D>(deserializer: D) -> Result<Option<DateTime<Utc>>, D::Error>
where
    D: Deserializer<'de>,
{
    #[derive(Deserialize)]
    struct Wrapper(#[serde(deserialize_with = "datefmt")] DateTime<Utc>);

    let v = Option::deserialize(deserializer)?;
    Ok(v.map(|Wrapper(a)| a))
}

#[derive(Deserialize, Debug)]
struct MyStruct {
    #[serde(default, deserialize_with = "option_datefmt")]
    expiration_date: Option<DateTime<Utc>>,
}

fn main() {
    let j = r#" {"expiration_date": null} "#;
    println!("{:?}", serde_json::from_str::<MyStruct>(j).unwrap());

    let j = r#" {"expiration_date": "2017-02-16 21:54:30"} "#;
    println!("{:?}", serde_json::from_str::<MyStruct>(j).unwrap());
}

[yageek]

Thanks for the complete and clear response :)