type http2writePushPromise struct {
	streamID uint32   // pusher stream
	method   string   // for :method
	url      *url.URL // for :scheme, :authority, :path
	h        Header

	// Creates an ID for a pushed stream. This runs on serveG just before
	// the frame is written. The returned ID is copied to promisedID.
	allocatePromisedID func() (uint32, error)
	promisedID         uint32
}