The Router microservice uses the /signup route to resend a signup request to a users microservice bound to  the 127.0.0.1:8001 address. This request creates a new users with filled from UserForm, passed with a parameter wrapped with the Form type. Look at the following code:

fn signup(params: Form<UserForm>) -> FutureResponse<HttpResponse> {
    let fut = post_request("http://127.0.0.1:8001/signup", params.into_inner())
        .map(|_: ()| {
            HttpResponse::Found()
            .header(header::LOCATION, "/login.html")
            .finish()
        });
    Box::new(fut)
}

We call the post_request function that we declared before to send a POST request to a users microservice and if it returns a successful response, we return a response with a 302 status code. We create HttpResponseBuilder with the corresponding status code by the  HttpResponse::Found function call. After this, we also set the LOCATION header to redirect the user to a login form with the header method call of HttpResponseBuilder. Lastly, we call finish() to create a HttpResponse from a builder and return it as a boxed Future object.

The function has a FutureResponse return type that is implemented as follows:

type FutureResponse<I, E = Error> = Box<dyn Future<Item = I, Error = E>>;

As you can see, it's a Box with a type that implements a Future trait.

Also, the function expects Form<UserForm> as a parameter. The UserForm structs are declared as follows:

#[derive(Deserialize, Serialize)]
pub struct UserForm {
    email: String,
    password: String,
}

As you can see, it expects two parameters: email and password. Both will be extracted from the query string of a request in the format of [email protected]&password=<secret>. The Form wrapper helps to extract data from the response's body.