postcard-schema: maps with non-string keys don't work in cases that are supported by serde_json

[Calsign]

I'm attempting to switch a bunch of types from serializing with serde_json to postcard, with postcard-schema to support inspecting messages serialized with older schema versions.

serde_json doesn't support maps with non-string keys, and since postcard-dyn uses serde_json::Value it has this limitation as well. But serde_json does support non-string keys as long as those keys are primitive types which are trivially convertible to strings. Consider this example:

use std::collections::HashMap;

use postcard_schema::{Schema, schema::owned::OwnedNamedType};
use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize, Schema)]
struct Foobar {
    map: HashMap<u32, u32>,
}

fn main() {
    let foobar = Foobar {
        map: HashMap::from([(1, 1)]),
    };

    // this works
    let _json_encoded = serde_json::to_vec(&foobar).unwrap();

    let postcard_encoded = postcard::to_stdvec(&foobar).unwrap();
    let schema = OwnedNamedType::from(Foobar::SCHEMA);

    // this fails
    let _json_dyn = postcard_dyn::from_slice_dyn(&schema, &postcard_encoded).unwrap();
}

The serde_json path works but the postcard-dyn path doesn't. Output:

thread 'main' panicked at src/bin/ex1.rs:23:78:
called `Result::unwrap()` on an `Err` value: ShouldSupportButDont

serde_json implements serialization for all primitive types by converting them to strings, for example: https://github.com/serde-rs/json/blob/ce410dd77926181445321ce178fbc492a44328aa/src/ser.rs#L970

This is where postcard-dyn fails:

postcard/source/postcard-dyn/src/ser.rs

Line 242 in 07b436d

return Err(Error::ShouldSupportButDont);

Obviously full support for arbitrary non-string map keys would be ideal, but a smaller step would be for postcard-schema/postcard-dyn to not have this regression compared to serde_json. I think postcard-dyn can implement roughly equivalent functionality, but I'm not sure how difficult this is to do.

(Long-term it seems like the best solution would be to stop using serde_json::Value and use a forked version of that type with support for non-string map keys, and possibly other changes like bytearrays. I'm surprised I haven't seen this discussed elsewhere in this repo. That would add a lot of code surface to postcard-dyn so I understand not being eager to do that.)