client.rs

    pub fn fetch(self) -> impl Future<Item = Data, Error = Error> {
        let req = Request::get(self.uri)
            .body(Body::empty())
            .unwrap();

        let https = HttpsConnector::new(4).expect("tls initialization error");
        let client = Client::builder().build::<_, Body>(https);

        client
            .request(req)
            .map(|res| res.into_body())
            .and_then(|body: Body| body.concat2())
            .then(|body| {
                let body: Vec<u8> = body?;
                let json = serde_json::from_slice(&body)?;
                Ok(json)
            })
            .from_err::<Error>()

    }

/*
error[E0282]: type annotations needed
  --> src/mercury.rs:50:28
   |
50 |                 let body = body?;
   |                            ^^^^^ cannot infer type for `_`

error: aborting due to previous error
*/