app.c

#include <stdio.h>
#include <stdlib.h>

struct UploadTask {
  uint8_t tag;
  const char* uuid;
  const char* url;
  const char* body;
};

struct Wait {
  uint8_t tag;
};

struct Done {
  uint8_t tag;
};

union PingUploadTask {
  struct UploadTask U;
  struct Wait W;
  struct Done D;
};

union PingUploadTask union_get();

int main(int argc, char** argv)
{
  union PingUploadTask task = union_get();
  printf("tag: %d\n", task.U.tag);
  if (task.U.tag == 0) {
    printf("uuid: %s\n", task.U.uuid);
    printf("url: %s\n", task.U.url);
    printf("body: %s\n", task.U.body);
  }

  return 0;
}

lib.rs

use std::os::raw::c_char;
use std::ffi::CString;

#[repr(u8)]
pub enum PingUploadTask {
    /// A PingRequest popped from the front of the queue.
    /// See [`PingRequest`](struct.PingRequest.html) for more information.
    Upload { uuid: *const c_char, url: *const c_char, body: *const c_char },
    /// A flag signaling that the pending pings directories are not done being processed,
    /// thus the requester should wait and come back later.
    Wait,
    /// A flag signaling that the pending pings queue is empty and requester is done.
    Done,
}

#[no_mangle]
extern "C" fn union_get() -> PingUploadTask {
    let uuid = CString::new("c0ffee").unwrap();
    let url = CString::new("https://incoming.telemetry.mozilla.org/").unwrap();
    let body = CString::new("{}").unwrap();
    PingUploadTask::Upload {
        uuid: uuid.into_raw(),
        url: url.into_raw(),
        body: body.into_raw(),
    }
}