How to use a custom serde deserializer for chrono timestamps?

While @edwardw's answer is technically correct it IMHO contains too much boilerplate.

NaiveDataTime implements FromStr which means you can write a reusable generic deserializer function.

A convoluted example - did add the age field (u8) represented as string in the JSON. Just to demonstrate that you can use it for anything that implements FromStr.

use std::fmt::Display;
use std::str::FromStr;

use chrono::NaiveDateTime;
use serde::{de, Deserialize, Deserializer};

#[derive(Deserialize, Debug)]
struct MyJson {
    name: String,
    #[serde(deserialize_with = "deserialize_from_str")]
    timestamp: NaiveDateTime,
    #[serde(deserialize_with = "deserialize_from_str")]
    age: u8,
}

// You can use this deserializer for any type that implements FromStr
// and the FromStr::Err implements Display
fn deserialize_from_str<'de, S, D>(deserializer: D) -> Result<S, D::Error>
where
    S: FromStr,      // Required for S::from_str...
    S::Err: Display, // Required for .map_err(de::Error::custom)
    D: Deserializer<'de>,
{
    let s: String = Deserialize::deserialize(deserializer)?;
    S::from_str(&s).map_err(de::Error::custom)
}

fn main() {
    let result: MyJson = serde_json::from_str(
        r#"{"name": "asdf", "timestamp": "2019-08-15T17:41:18.106108", "age": "11"}"#,
    )
    .unwrap();
    println!("{:?}", result);
}

It's even easier if you want to specify format (use NaiveDateTime::parse_from_str):

use chrono::NaiveDateTime;
use serde::{de, Deserialize, Deserializer};

#[derive(Deserialize, Debug)]
struct MyJson {
    name: String,
    #[serde(deserialize_with = "naive_date_time_from_str")]
    timestamp: NaiveDateTime,
}

fn naive_date_time_from_str<'de, D>(deserializer: D) -> Result<NaiveDateTime, D::Error>
where
    D: Deserializer<'de>,
{
    let s: String = Deserialize::deserialize(deserializer)?;
    NaiveDateTime::parse_from_str(&s, "%Y-%m-%dT%H:%M:%S.%f").map_err(de::Error::custom)
}

fn main() {
    let result: MyJson =
        serde_json::from_str(r#"{"name": "asdf", "timestamp": "2019-08-15T17:41:18.106108"}"#)
            .unwrap();
    println!("{:?}", result);
}

#[serde(deserialize_with = "path")] documentation:

Deserialize this field using a function that is different from its implementation of Deserialize. The given function must be callable as fn<'de, D>(D) -> Result<T, D::Error> where D: Deserializer<'de>, although it may also be generic over T. Fields used with deserialize_with are not required to implement Deserialize.

This is rather involved, but the following works:

use chrono::NaiveDateTime;
use serde::de;
use serde::Deserialize;
use std::fmt;

struct NaiveDateTimeVisitor;

impl<'de> de::Visitor<'de> for NaiveDateTimeVisitor {
    type Value = NaiveDateTime;

    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        write!(formatter, "a string represents chrono::NaiveDateTime")
    }

    fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
    where
        E: de::Error,
    {
        match NaiveDateTime::parse_from_str(s, "%Y-%m-%dT%H:%M:%S.%f") {
            Ok(t) => Ok(t),
            Err(_) => Err(de::Error::invalid_value(de::Unexpected::Str(s), &self)),
        }
    }
}

fn from_timestamp<'de, D>(d: D) -> Result<NaiveDateTime, D::Error>
where
    D: de::Deserializer<'de>,
{
    d.deserialize_str(NaiveDateTimeVisitor)
}

#[derive(Deserialize, Debug)]
struct MyJson {
    name: String,
    #[serde(deserialize_with = "from_timestamp")]
    timestamp: NaiveDateTime,
}

fn main() {
    let result: MyJson =
        serde_json::from_str(r#"{"name": "asdf", "timestamp": "2019-08-15T17:41:18.106108"}"#)
            .unwrap();
    println!("{:?}", result);
}