Improve interoperability with http

[shouya]

I made a few changes to make reqwest types compatible with http types. This will enable a lot of interesting uses (such as a reverse proxy, example code attached in the end).

By interoperability with http I mean the following four components:

• impl TryFrom from http::Request<http::Body> to reqwest::Request (already implemented prior to this PR)
• impl TryFrom from reqwest::Response to http::Response<reqwest::Body>
• impl http::Body for reqwest::Body
• impl tower::Service<reqwest::Request, Response=reqwest::Response, Error=reqwest::Error> for reqwest::Client (also already implemented prior to this PR)

In fact, all the implementation logic is already there. I only assembled these together.

Now with all these trait implementations and conversion functions, it becomes easy to play with axum and other http-based frameworks. Here's an example reverse proxy written in axum:

use axum::Router;
use tower::{Layer, ServiceExt};

#[tokio::main(flavor = "current_thread")]
async fn main() {
  let client = reqwest::Client::new();
  let example_proxy = client
    .map_request(|r| reqwest::Request::try_from(r).unwrap())
    .map_response(|r| http::Response::try_from(r).unwrap())
    .map_err(|e| panic!("axum requires this to be infallible {:?}", e));
  let rewrite_req_url = axum::middleware::map_request(rewrite_req_url);

  let example_proxy = rewrite_req_url.layer(example_proxy);

  let app = Router::new().nest_service("/example", example_proxy);
  let service = app.into_make_service();

  axum::Server::bind(&"0.0.0.0:3000".parse().unwrap())
    .serve(service)
    .await
    .unwrap();
}

async fn rewrite_req_url<B>(mut req: http::Request<B>) -> http::Request<B> {
  let uri = format!("https://example.com{}", req.uri());
  req.headers_mut().insert(
    http::header::HOST,
    http::header::HeaderValue::from_static("example.com"),
  );
  *req.uri_mut() = uri.parse().unwrap();
  req
}

I noticed that the wrapper ImplStream is not really used anywhere, so I removed it. In my opinion, it is sensible to interpret a reqwest::Body as a Stream or a http::Body on its own. Please let me know if there are considerations where it's still necessary.

[shouya]

This PR should fix the following issues:

• Provide conversion function for reqwest::Response to http::Response<_> #1258
• Compatibility with tower and http #1370
• Add support for axum's IntoResponse #1816

Related pull requests:

• Added conversion method for reqwest::Response to http::Response<_> #1612
• Create a http::Response<hyper::Body> from a reqwest::Response #1954

[seanmonstar]

Thank you for putting this together! I do want to improve the type interoperability with http. I'm also working on a draft PR to upgrade reqwest to hyper v1, which has my current attention. I do hope to include this for the next release, though.

[seanmonstar]

Would you like to try to rebase this on to the 0.12-dev branch? I'm hope to prep a release this week. :)

[shouya]

@seanmonstar: Updated. I found that of the conversions and trait implementations I initially proposed, only the conversion from reqwest::Response to http::Response<reqwest::Body> is still needed.

As for the part for treating reqwest::Body as Stream<Item=Bytes>, I think it may be unnecessary. Given that it's easy enough to get a stream out of a body with a http_body_util::BodyStream, so I removed the relevant code. I can add it back if you think it's good to have.

Please have a look.