I am trying to use the custom implementation in Serialize trait of Serde. Here is the code (playground):

use serde::ser::{Serialize, Serializer, SerializeStruct};

struct Data {
    key: String,    // or &str
    value: i32,
}

impl Serialize for Data {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        let mut state = serializer.serialize_struct("Data", 1)?;

        // let key = (&self.key).to_string();
        // state.serialize_field(&*key, &self.value)?;  //shows lifetime error
      
        state.serialize_field("key", &self.value)?;
        state.end()
    }
}

fn main() {
    let data = Data {
        key: String::from("age"),
        value: 21,
    };

    let json = serde_json::to_string_pretty(&data).unwrap();

    println!("the JSON is: {}", json)
}

by default, serde serialize Data as:

{
  "key": "age",
  "value": 21
}

but, i want this:

{
  "age": 21,
}

This question is also similar, but I need more info in this context.

CodePudding user response：

Your Data struct is essentially a singleton map. You should serialize it as such:

impl Serialize for Data {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        use serde::ser::SerializeMap;
        let mut map = serializer.serialize_map(Some(1))?;
        map.serialize_entry(&self.key, &self.value)?;
        map.end()
    }
}

Playground