var (
	errProxyOff       = notExistErrorf("module lookup disabled by GOPROXY=off")
	errNoproxy  error = notExistErrorf("disabled by GOPRIVATE/GONOPROXY")
	errUseProxy error = notExistErrorf("path does not match GOPRIVATE/GONOPROXY")
)

func lookupDirect(ctx context.Context, path string) (Repo, error) {
	security := web.SecureOnly

	if module.MatchPrefixPatterns(cfg.GOINSECURE, path) {

	// equivalent to os.ErrNotFound, which is true for 404 and 410 responses.
	fallBackOnError bool
}

func proxyList() ([]proxySpec, error) {
	proxyOnce.Do(func() {
		if cfg.GONOPROXY != "" && cfg.GOPROXY != "direct" {
			proxyOnce.list = append(proxyOnce.list, proxySpec{url: "noproxy"})
		}

		goproxy := cfg.GOPROXY
		for goproxy != "" {
			var url string
			fallBackOnError := false

	GOPROXY, GOPROXYChanged     = EnvOrAndChanged("GOPROXY", "")
	GOSUMDB, GOSUMDBChanged     = EnvOrAndChanged("GOSUMDB", "")
	GOPRIVATE                   = Getenv("GOPRIVATE")
	GONOPROXY, GONOPROXYChanged = EnvOrAndChanged("GONOPROXY", GOPRIVATE)
	GONOSUMDB, GONOSUMDBChanged = EnvOrAndChanged("GONOSUMDB", GOPRIVATE)
	GOINSECURE                  = Getenv("GOINSECURE")
	GOVCS                       = Getenv("GOVCS")
)

		{Name: "GOHOSTOS", Value: runtime.GOOS},
		{Name: "GOINSECURE", Value: cfg.GOINSECURE},
		{Name: "GOMODCACHE", Value: cfg.GOMODCACHE, Changed: cfg.GOMODCACHEChanged},
		{Name: "GONOPROXY", Value: cfg.GONOPROXY, Changed: cfg.GONOPROXYChanged},
		{Name: "GONOSUMDB", Value: cfg.GONOSUMDB, Changed: cfg.GONOSUMDBChanged},
		{Name: "GOOS", Value: cfg.Goos, Changed: cfg.Goos != runtime.GOOS},

		"TESTGO_VCSTEST_TLS_HOST=" + httpsURL.Host,
		"TESTGO_VCSTEST_CERT=" + srvCertFile,
		"TESTGONETWORK=panic", // cleared by the [net] condition
		"GOSUMDB=" + testSumDBVerifierKey,
		"GONOPROXY=",
		"GONOSUMDB=",
		"GOVCS=*:all",
		"devnull=" + os.DevNull,
		"goversion=" + gover.Local(),
		"CMDGO_TEST_RUN_MAIN=true",
		"HGRCPATH=",
		"GOTOOLCHAIN=auto",
		"newline=\n",
	}

matching either pattern, including git.corp.example.com/xyzzy, rsc.io/private,
and rsc.io/private/quux.

For fine-grained control over module download and validation, the GONOPROXY
and GONOSUMDB environment variables accept the same kind of glob list
and override GOPRIVATE for the specific decision of whether to use the proxy
and checksum database, respectively.

	tmp := t.TempDir()
	if err := os.WriteFile(filepath.Join(tmp, "go.mod"), []byte("module m"), 0666); err != nil {
		t.Fatal(err)
	}
	t.Setenv("GO111MODULE", "on")
	t.Setenv("GOPROXY", "off")
	t.Setenv("GONOPROXY", "none")

	ctxt := Default
	ctxt.Dir = tmp

	pkgPath := "example.com/hello"
	_, err := ctxt.Import(pkgPath, tmp, FindOnly)
	if err == nil {
		t.Fatal("unexpected success")
	}

		Controls where various files are stored. See: 'go help gopath'.
	GOPROXY
		URL of Go module proxy. See https://golang.org/ref/mod#environment-variables
		and https://golang.org/ref/mod#module-proxy for details.
	GOPRIVATE, GONOPROXY, GONOSUMDB
		Comma-separated list of glob patterns (in the syntax of Go's path.Match)
		of module path prefixes that should always be fetched directly
		or that should not be compared against the checksum database.