return nil, fmt.Errorf("invalid proxy address %q: %v", proxy, err)
	}
	return proxyURL, nil
}

// useProxy reports whether requests to addr should use a proxy,
// according to the NO_PROXY or no_proxy environment variable.
// addr is always a canonicalAddr with a host and port.
func (cfg *config) useProxy(addr string) bool {
	if len(addr) == 0 {
		return true
	}
	host, port, err := net.SplitHostPort(addr)