all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/breaktcp_linux.go

@@ -15,7 +15,7 @@ func init() {
 
 func breakTCPConnsLinux() error {
 	var matched int
-	for fd := 0; fd < 1000; fd++ {
+	for fd := range 1000 {
 		_, err := unix.GetsockoptTCPInfo(fd, unix.IPPROTO_TCP, unix.TCP_INFO)
 		if err == nil {
 			matched++

ipn/ipnlocal/bus_test.go

@@ -36,9 +36,8 @@ func TestIsNotableNotify(t *testing.T) {
 	// We use reflect to catch fields that might be added in the future without
 	// remembering to update the [isNotableNotify] function.
 	rt := reflect.TypeFor[ipn.Notify]()
-	for i := range rt.NumField() {
+	for sf := range rt.Fields() {
 		n := &ipn.Notify{}
-		sf := rt.Field(i)
 		switch sf.Name {
 		case "_", "NetMap", "Engine", "Version":
 			// Already covered above or not applicable.
@@ -46,7 +45,7 @@ func TestIsNotableNotify(t *testing.T) {
 		case "DriveShares":
 			n.DriveShares = views.SliceOfViews[*drive.Share, drive.ShareView](make([]*drive.Share, 1))
 		default:
-			rf := reflect.ValueOf(n).Elem().Field(i)
+			rf := reflect.ValueOf(n).Elem().FieldByIndex(sf.Index)
 			switch rf.Kind() {
 			case reflect.Pointer:
 				rf.Set(reflect.New(rf.Type().Elem()))
@@ -64,7 +63,7 @@ func TestIsNotableNotify(t *testing.T) {
 			notify *ipn.Notify
 			want   bool
 		}{
-			name:   "field-" + rt.Field(i).Name,
+			name:   "field-" + sf.Name,
 			notify: n,
 			want:   true,
 		})

ipn/ipnlocal/extension_host.go

@@ -339,7 +339,7 @@ func (h *ExtensionHost) FindMatchingExtension(target any) bool {
 
 	val := reflect.ValueOf(target)
 	typ := val.Type()
-	if typ.Kind() != reflect.Ptr || val.IsNil() {
+	if typ.Kind() != reflect.Pointer || val.IsNil() {
 		panic("ipnext: target must be a non-nil pointer")
 	}
 	targetType := typ.Elem()

ipn/ipnlocal/extension_host_test.go

@@ -1010,9 +1010,8 @@ func TestNilExtensionHostMethodCall(t *testing.T) {
 	t.Parallel()
 
 	var h *ExtensionHost
-	typ := reflect.TypeOf(h)
-	for i := range typ.NumMethod() {
-		m := typ.Method(i)
+	typ := reflect.TypeFor[*ExtensionHost]()
+	for m := range typ.Methods() {
 		if strings.HasSuffix(m.Name, "ForTest") {
 			// Skip methods that are only for testing.
 			continue

ipn/ipnlocal/local.go

@@ -3687,12 +3687,7 @@ func generateInterceptTCPPortFunc(ports []uint16) func(uint16) bool {
 			f = func(p uint16) bool { return m[p] }
 		} else {
 			f = func(p uint16) bool {
-				for _, x := range ports {
-					if p == x {
-						return true
-					}
-				}
-				return false
+				return slices.Contains(ports, p)
 			}
 		}
 	}
@@ -7387,10 +7382,8 @@ var (
 // allowedAutoRoute determines if the route being added via AdvertiseRoute (the app connector featuge) should be allowed.
 func allowedAutoRoute(ipp netip.Prefix) bool {
 	// Note: blocking the addrs for globals, not solely the prefixes.
-	for _, addr := range disallowedAddrs {
-		if ipp.Addr() == addr {
-			return false
-		}
+	if slices.Contains(disallowedAddrs, ipp.Addr()) {
+		return false
 	}
 	for _, pfx := range disallowedRanges {
 		if pfx.Overlaps(ipp) {

ipn/ipnlocal/netmapcache/netmapcache_test.go

@@ -275,7 +275,7 @@ var skippedMapFields = []string{
 func checkFieldCoverage(t *testing.T, nm *netmap.NetworkMap) {
 	t.Helper()
 
-	mt := reflect.TypeOf(nm).Elem()
+	mt := reflect.TypeFor[netmap.NetworkMap]()
 	mv := reflect.ValueOf(nm).Elem()
 	for i := 0; i < mt.NumField(); i++ {
 		f := mt.Field(i)

ipn/ipnlocal/peerapi.go

@@ -103,7 +103,7 @@ func (s *peerAPIServer) listen(ip netip.Addr, tunIfIndex int) (ln net.Listener,
 	// deterministic that people will bake this into clients.
 	// We try a few times just in case something's already
 	// listening on that port (on all interfaces, probably).
-	for try := uint8(0); try < 5; try++ {
+	for try := range uint8(5) {
 		a16 := ip.As16()
 		hashData := a16[len(a16)-3:]
 		hashData[0] += try

ipn/ipnlocal/serve.go

@@ -835,8 +835,8 @@ func (b *LocalBackend) proxyHandlerForBackend(backend string) (http.Handler, err
 	targetURL, insecure := expandProxyArg(backend)
 
 	// Handle unix: scheme specially
-	if strings.HasPrefix(targetURL, "unix:") {
-		socketPath := strings.TrimPrefix(targetURL, "unix:")
+	if after, ok := strings.CutPrefix(targetURL, "unix:"); ok {
+		socketPath := after
 		if socketPath == "" {
 			return nil, fmt.Errorf("empty unix socket path")
 		}

ipn/ipnlocal/ssh.go

@@ -101,9 +101,9 @@ func (b *LocalBackend) getSSHUsernames(req *tailcfg.C2NSSHUsernamesRequest) (*ta
 				mem.HasSuffix(mem.B(line), mem.S("/false")) {
 				continue
 			}
-			colon := bytes.IndexByte(line, ':')
-			if colon != -1 {
-				add(string(line[:colon]))
+			before, _, ok := bytes.Cut(line, []byte{':'})
+			if ok {
+				add(string(before))
 			}
 		}
 	}