Refine strategy surrounding writing headers to a response

[reem]

Right now writing to the headers field after the headers have been written leads to those changes being silently ignored.

Additionally, there is no public way to know if the headers have been written.

Together, these form a rather lethal combination.

[seanmonstar]

As alluded to in the code comments, I'd love to come up with a solution that tracks this statically. Such that once headers have been written, the status code and headers are no longer mutable, and the compiler will error if tried to mutate.

I've considering something like:

struct Request {
  pub headers: Headers,
  pub status: StatusCode,
}

struct InFlightRequest {
  headers: HeadersView, // can look at headers, but no set/remove methods.
  // something similar with `status`. or maybe it just becomes a getter function
}

// ..
fn handle(req: Request, res: Response) {
  req.headers.set(Foo);
  let body = req.start();
  body.headers.set(Bar); // No method `set`
}