Code shared from the Rust Playground

playground.rs

#![feature(extern_prelude)]

extern crate serde; // 1.0.68
#[macro_use]
extern crate serde_derive; // 1.0.68

use std::collections::HashMap;
use serde::*;
use std::fmt::{Display, Formatter};
use serde::ser::{Serialize, Serializer, SerializeSeq, SerializeMap};

#[derive(Serialize, Deserialize, Debug, PartialEq, Eq, Hash)]
struct Foo{x: u64}
#[derive(Deserialize, Debug)]
struct Bar {
    x: HashMap<Foo, f64>,
}

impl Display for Foo {
    fn fmt(&self, f: &mut Formatter) -> std::fmt::Result {
        write!(f, "{}", self.x)
    }
}

impl Serialize for Bar {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        let mut seq = serializer.serialize_map(Some(self.x.len()))?;
        for (k, v) in &self.x {
            seq.serialize_entry(&k.to_string(), &v)?;
        }
        seq.end()
    }
}
    
fn main() {
    let mut p = Bar{x: HashMap::new()};
    p.x.insert(Foo{x:0}, 0.0);
    let serialized = serde_json::to_string(&p).unwrap();
    println!("{:?}", serialized)
}