Serialization always fails with "key must be a string"

[br-olf]

Hi, I don't know if this issue belongs here or to serde itself.

I try to serialize this object

use serde;
use serde_json;
use std::collections::BTreeMap;
use std::path::PathBuf;

#[derive(Clone, Debug, serde::Serialize, serde::Deserialize)]
pub struct DedupTree {
    hash_tree: BTreeMap<[u8; 32], Vec<PathBuf>>,
    file_tree: BTreeMap<PathBuf, [u8; 32]>,
}

impl DedupTree {
    pub fn to_json(&self) -> String {
        serde_json::to_string(&self).unwrap()
    }

    pub fn from_json(json: &str) -> Result<Self, serde_json::Error> {
        serde_json::from_str(json)
    }
}

but any time I want to use to_json() I get this error:
Error("key must be a string", line: 0, column: 0)

I tried a number of different approaches to fix this by implementing serialize for [u8,32] myself but eventually I always got the same error.
As far as I see from the docs all used types (BTreeMap,[u8; 32],Vec, PathBuf) are supported by serde.

[dtolnay]

I think this is behaving correctly. JSON maps only have string keys. Map keys are not allowed to be arrays in JSON.