How to find _id in rust

use std::any::Any;
use mongodb::{bson::doc,bson::Document, options::ClientOptions,error::Result, Client};
use tokio::task;
use serde::{Deserialize, Serialize};
use futures::stream::TryStreamExt;

#[derive(Clone, Debug, Deserialize, Serialize)]
struct Item {
    index: u32,
}

#[tokio::main]
async fn main() -> mongodb::error::Result<()> {
    let mut client_options = ClientOptions::parse("mongodb://localhost:27017").await?;

    // Manually set an option
    client_options.app_name = Some("Rust Demo".to_string());

    // Get a handle to the cluster
    let client = Client::with_options(client_options)?;

    // Ping the server to see if you can connect to the cluster
    client
        .database("user")
        .run_command(doc! {"ping": 1}, None)
        .await?;


    let db = client.database("Items");

        let db_ref = db.clone();

        let handle = task::spawn(async move {
            let coll = db_ref.collection::<Item>("in_stock");
            let filter = doc! {"index": 1000};
            let mut findresult = coll.find(filter, None).await.unwrap();
            while let Some(r) = findresult.try_next().await.unwrap(){                
                println!("{:?}", r);
            }
        });

    handle.await;
    println!("successfully.");

    Ok(())
}

I want to know my _id from my collection info.
But, No _id filed in rust, nodejs easy.

let me know about how to show _id fileld?