You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: endpoint.rs
    path of file: ./repos/oauth2-rs/src
    
    the code of the file until where you have to start completion: use crate::{
    AuthType, ClientId, ClientSecret, ErrorResponse, RedirectUrl, RequestTokenError, Scope,
    CONTENT_TYPE_FORMENCODED, CONTENT_TYPE_JSON,
};

use http::header::{ACCEPT, AUTHORIZATION, CONTENT_TYPE};
use http::{HeaderValue, StatusCode};
use serde::de::DeserializeOwned;
use url::{form_urlencoded, Url};

use std::borrow::Cow;
use std::error::Error;
use std::future::Future;

/// An HTTP request.
pub type HttpRequest = http::Request<Vec<u8>>;

/// An HTTP response.
pub type HttpResponse = http::Response<Vec<u8>>;

/// An async
    
    async fn test_async_client_closure() {
        let client = new_client();

        let http_response = Response::builder()
            .status(StatusCode::OK)
            .body(
                "{\"access_token\": \"12/34\", \"token_type\": \"BEARER\"}"
                    .to_string()
                    .into_bytes(),
            )
            .unwrap();

        let token = client
            .exchange_code(AuthorizationCode::new("ccc".to_string()))
            // NB: This tests that the closure doesn't require a static lifetime.
            .request_async(&|_| async {
                Ok(clone_response(&http_response)) as Result<_, FakeError>
            })
            .await
            .unwrap();

        assert_eq!("12/34", token.access_token().secret());
    }