How do I use Serde to serialize a HashMap with structs as keys to JSON?

Question

I want to serialize a HashMap with structs as keys:

use serde::{Deserialize, Serialize}; // 1.0.68
use std::collections::HashMap;

fn main() {
    #[derive(Serialize, Deserialize, Debug, PartialEq, Eq, Hash)]
    struct Foo {
        x: u64,
    }

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

    let mut p = Bar { x: HashMap::new() };
    p.x.insert(Foo { x: 0 }, 0.0);
    let serialized = serde_json::to_string(&p).unwrap();
}

This code compiles, but when I run it I get an error:

Error("key must be a string", line: 0, column: 0)'

I changed the code:

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

let mut p = Bar { x: HashMap::new() };
p.x.insert(0, 0.0);
let serialized = serde_json::to_string(&p).unwrap();

The key in the HashMap is now a u64 instead of a string. Why does the first code give an error?

Answer 1

According to JSONs specification, JSON keys must be strings. serde_json uses fmt::Display in here, for some non-string keys, to allow serialization of wider range of HashMaps. That's why HashMap<u64, f64> works as well as HashMap<String, f64> would. However, not all types are covered (Foo's case here).

That's why we need to provide our own Serialize implementation:

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 map = serializer.serialize_map(Some(self.x.len()))?;
        for (k, v) in &self.x {
            map.serialize_entry(&k.to_string(), &v)?;
        }
        map.end()
    }
}

(playground)