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

You can use serde_as from the serde_with crate to encode the HashMap as a sequence of key-value pairs:

use serde_with::serde_as; // 1.5.1

#[serde_as]
#[derive(Serialize, Deserialize, Debug)]
struct Bar {
    #[serde_as(as = "Vec<(_, _)>")]
    x: HashMap<Foo, f64>,
}

Which will serialize to (and deserialize from) this:

{
  "x":[
    [{"x": 0}, 0.0],
    [{"x": 1}, 0.0],
    [{"x": 2}, 0.0]
  ]
}

There is likely some overhead from converting the HashMap to Vec, but this can be very convenient.

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)

I've found the bulletproof solution 😃

• Extra dependencies not required
• Compatible with HashMap, BTreeMap and other iterable types
• Works with flexbuffers

The following code converts a field (map) to the intermediate Vec representation:

pub mod vectorize {
    use serde::{Deserialize, Deserializer, Serialize, Serializer};
    use std::iter::FromIterator;

    pub fn serialize<'a, T, K, V, S>(target: T, ser: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
        T: IntoIterator<Item = (&'a K, &'a V)>,
        K: Serialize + 'a,
        V: Serialize + 'a,
    {
        let container: Vec<_> = target.into_iter().collect();
        serde::Serialize::serialize(&container, ser)
    }

    pub fn deserialize<'de, T, K, V, D>(des: D) -> Result<T, D::Error>
    where
        D: Deserializer<'de>,
        T: FromIterator<(K, V)>,
        K: Deserialize<'de>,
        V: Deserialize<'de>,
    {
        let container: Vec<_> = serde::Deserialize::deserialize(des)?;
        Ok(T::from_iter(container.into_iter()))
    }
}

To use it just add the module's name as an attribute:

#[derive(Debug, Serialize, Deserialize)]
struct MyComplexType {
    #[serde(with = "vectorize")]
    map: HashMap<MyKey, String>,
}

The remained part if you want to check it locally:

use anyhow::Error;
use serde::{Deserialize, Serialize};
use std::collections::HashMap;

#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord, Hash)]
struct MyKey {
    one: String,
    two: u16,
    more: Vec<u8>,
}

#[derive(Debug, Serialize, Deserialize)]
struct MyComplexType {
    #[serde(with = "vectorize")]
    map: HashMap<MyKey, String>,
}

fn main() -> Result<(), Error> {
    let key = MyKey {
        one: "1".into(),
        two: 2,
        more: vec![1, 2, 3],
    };
    let mut map = HashMap::new();
    map.insert(key.clone(), "value".into());
    let instance = MyComplexType { map };
    let serialized = serde_json::to_string(&instance)?;
    println!("JSON: {}", serialized);
    let deserialized: MyComplexType = serde_json::from_str(&serialized)?;
    let expected_value = "value".to_string();
    assert_eq!(deserialized.map.get(&key), Some(&expected_value));
    Ok(())
}

And on the Rust playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=bf1773b6e501a0ea255ccdf8ce37e74d