class ActionDispatch::Request

Early Hints headers are included by default if supported.
If you are using +javascript_include_tag+ or +stylesheet_link_tag+ the

send_early_hints("Link" => "; rel=preload; as=style\n; rel=preload")

The +send_early_hints+ method accepts a hash of links as follows:

If the env contains +rack.early_hints+ then the server accepts HTTP2 push for Link headers.

making preparations for processing the final response.
Early Hints is an HTTP/2 status code that indicates hints to help a client start

def send_early_hints(links)
  return unless env["rack.early_hints"]
  env["rack.early_hints"].call(links)
end