Verifying solutions in Rust

use serde::Deserialize;
use reqwest::{Error, Client, header::ACCEPT, header::CONTENT_TYPE};
use std::collections::HashMap;

#[derive(Deserialize, Debug)]
struct BotpoisonResponse {
    ok: bool,
    // Absent when ok is true, so this has to be optional
    message: Option<String>,
}

#[tokio::main]
async fn main() -> Result<(), Error> {
    let request_url = "https://api.botpoison.com/verify";

    let secret_key = "sk_xxxxxxxx";
    let solution = "..."; // Extract the solution from the request body (_botpoison)

    let payload: HashMap<&str, &str> = [("secretKey", secret_key), ("solution", solution)].iter().cloned().collect();

    let response: BotpoisonResponse = Client::new().post(request_url).header(ACCEPT, "application/json")
        .header(CONTENT_TYPE, "application/json").json(&payload).send().await?.json().await?;

    if response.ok {
        println!("Not spam: allow request");
    } else {
        println!("Spam: reject request");
    }

    Ok(())
}