How do I use serde to serialize a HashMap with non-serializable keys?

Question

I have a HashMap as a value in a struct that I'm manually serializing:

pub struct GitInfo { pub branches: HashMap<Oid, Branch>, }

Branch is something I've defined, but Oid is an external type I don't own, with a to_string() method I'd be happy to use...

I've read How do I use Serde to serialize a HashMap with structs as keys to JSON? but it refers to keys the author defined - I can't implement Serialize for Oid as it's not in my crate. And I can't implement Serialize for HashMap<Oid, Branch> for similar reasons.

Is there some way around this? I could build a wrapper struct around HashMap<Oid, Branch> but that seems like overkill.

It was suggested that I look at How to transform fields during serialization using Serde? or How can I implement Serialize using an existing Display trait implementation? - both seem to boil down to using serialize_with - which I could possibly do, but I'd have to use the derive(Serialize) macro, when I was planning to serialize GitInfo manually (see below). Or I could use a wrapper object.

If those are the only options, I could live with it, but it seems a bit surprising that there isn't a simpler way - is there some way to call a function like the serialize_with macro uses, but from within my GitInfo serialization?

Something like:

impl Serialize for GitInfo { fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error> where S: Serializer, { let mut state = serializer.serialize_struct("GitInfo", 2)?; state.serialize_field("other bits", &self.other_bits)?; state.serialize_field("branches", /* something */)?; state.end() } }

Why are you using serialize_field instead of serialize_map? — Shepmaster
– Shepmaster, Commented Oct 9, 2019 at 19:11
I'm serializing a struct, GitInfo, with a field which is a HashMap. So I'm using serialize_struct which returns a SerializeStruct which has no serialize_map method. If there is some way to call serialize_map to make a new serialized version of the map, and then insert the results as a parameter to serialize_field, that'd be awesome - but I haven't found a way to do that yet. — Korny
– Korny, Commented Oct 9, 2019 at 20:42

Anders Evensen · Accepted Answer · 2023-10-24 22:49:02Z

You don't need to build a wrapper struct around HashMap<Oid, Branch> when storing it within your GitInfo struct, but you will need to use a wrapper struct when serializing the HashMap in a custom manner. You can define the new wrapper struct within your Serialize implementation to indicate that it should only be used for serialization.

 fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error> where S: Serializer, { struct Branches<'a>(&'a HashMap<Oid, Branch>); impl Serialize for Branches<'_> { fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error> where S: Serializer, { let mut map = serializer.serialize_map(Some(self.0.len()))?; for (oid, branch) in self.0 { map.serialize_entry(&oid.to_string(), &branch)?; } map.end() } } let mut state = serializer.serialize_struct("GitInfo", 2)?; state.serialize_field("branches", &Branches(&self.branches))?; state.serialize_field("other bits", &self.other_bits)?; state.end() } }

Here is a playground showing a basic implementation of this solution.

Note that the code generated by serialize_with follows this same pattern: it defines a newtype wrapper struct along with a Serialize implementation, and within that trait implementation it calls the function you provide.

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