Rust rocket post json -> json example

[zeroexcuses]

1. I am trying to use rust + rocket to build a REST API that handles just POST and does JSON -> JSON

2. I have written the following code so far:

#![feature(proc_macro_hygiene, decl_macro)]

#[macro_use] extern crate rocket;

use std::path::PathBuf;

#[post("/api")]
fn root_api() -> &'static str {
    handler()
}

#[post("/api/<path..>")]
fn child_api(path: PathBuf) -> &'static str {
    handler()
}

fn handler() -> &'static str {
    "Hello, world!"
}

pub fn main() {
    rocket::ignite().mount("/", routes![root_api, child_api]).launch();
}

3. The code works fine when I run:

curl -X POST http://localhost:8000/api -d "@data.txt"

4. What I need help with now (and what I can't find docs for) is: how do I write a handler that does JSON -> JSON ? I want a handler where it uses serde to get the input field as a JSON object, then returns a JSON object.

5. Any idea where there is sample code for this? (The rocket docs seems to be focused on routing.)

Thanks!