Rust: Serialize & Deserialize partially known / dynamic JSON. Using structs (type safe).

json.rs

// Add to your Cargo.toml:
// [dependencies]
// serde = { version = "1.0.193", features = ["derive"] }
// serde_json = "1.0.108"

use serde::{Deserialize, Serialize};
use serde_json::{Map, Value};

#[derive(Serialize, Deserialize)]
struct TopStruct {
    #[serde(rename(serialize = "topKnownKey", deserialize = "topKnownKey"))]
    top_known_key: i32,
    #[serde(rename(serialize = "nestedKnownArray", deserialize = "nestedKnownArray"))]
    nested_known_array: Vec<NestedStruct>,
    #[serde(skip_serializing, skip_deserializing)]
    excluded_known_key: i32, // Field sample for internal needs, excluded from JSON processing.
    #[serde(flatten)]
    unknown: Map<String, Value>, // All unknown fields go here.
}

#[derive(Serialize, Deserialize)]
struct NestedStruct {
    #[serde(rename(serialize = "nestedKnownKey", deserialize = "nestedKnownKey"))]
    nested_known_key: i32,
    #[serde(skip_serializing, skip_deserializing)]
    excluded_known_key: i32, // Field sample for internal needs, excluded from JSON processing.
    #[serde(flatten)]
    unknown: Map<String, Value>, // All unknown fields go here.
}

fn main() {
    let input = r#"
	{
		"topKnownKey": 1,
		"topUnknownKey": 2,
		"nestedKnownArray": [
		  {
			"nestedKnownKey": 3,
			"nestedUnknownKey": 4
		  },
		  {
			"nestedKnownKey": 5,
			"nestedUnknownKey": 6
		  }
		],
		"nestedUnknownArray": [
		  {
			"nestedKnownKey": 7,
			"nestedUnknownKey": 8
		  }
		]
	  }
	"#;

    // Decode.
    let mut ts: TopStruct = match serde_json::from_str(input) {
        Ok(ts) => ts,
        Err(err) => panic!("Error decoding struct: {:?}", err),
    };

    // Modify.
    ts.top_known_key = 100;
    ts.excluded_known_key = 200;
    ts.nested_known_array[0].nested_known_key = 300;
    ts.nested_known_array[0].excluded_known_key = 400;

    // Encode.
    let out: String = match serde_json::to_string_pretty(&ts) {
        Ok(s) => s,
        Err(err) => panic!("Error encoding struct: {:?}", err),
    };

    // Print output.
    println!("{}", out);
}