// from io.Copy.
type writerOnly struct {
	io.Writer
}

// ReadFrom is here to optimize copying from an [*os.File] regular file
// to a [*net.TCPConn] with sendfile, or from a supported src type such
// as a *net.TCPConn on Linux with splice.
func (w *response) ReadFrom(src io.Reader) (n int64, err error) {
	buf := getCopyBuf()
	defer putCopyBuf(buf)