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>

wgengine/filter/filter_test.go

@@ -751,13 +751,13 @@ func ports(s string) PortRange {
 	}
 
 	var fs, ls string
-	i := strings.IndexByte(s, '-')
-	if i == -1 {
+	before, after, ok := strings.Cut(s, "-")
+	if !ok {
 		fs = s
 		ls = fs
 	} else {
-		fs = s[:i]
-		ls = s[i+1:]
+		fs = before
+		ls = after
 	}
 	first, err := strconv.ParseInt(fs, 10, 16)
 	if err != nil {

wgengine/magicsock/magicsock.go

@@ -1474,8 +1474,7 @@ func (c *Conn) sendUDPBatch(addr epAddr, buffs [][]byte, offset int) (sent bool,
 		err = c.pconn4.WriteWireGuardBatchTo(buffs, addr, offset)
 	}
 	if err != nil {
-		var errGSO neterror.ErrUDPGSODisabled
-		if errors.As(err, &errGSO) {
+		if errGSO, ok := errors.AsType[neterror.ErrUDPGSODisabled](err); ok {
 			c.logf("magicsock: %s", errGSO.Error())
 			err = errGSO.RetryErr
 		} else {

wgengine/magicsock/magicsock_test.go

@@ -2007,7 +2007,7 @@ func TestStressSetNetworkMap(t *testing.T) {
 
 	const iters = 1000 // approx 0.5s on an m1 mac
 	for range iters {
-		for j := 0; j < npeers; j++ {
+		for j := range npeers {
 			// Randomize which peers are present.
 			if prng.Int()&1 == 0 {
 				present[j] = !present[j]
@@ -2196,7 +2196,7 @@ func newWireguard(t *testing.T, uapi string, aips []netip.Prefix) (*device.Devic
 	if err != nil {
 		t.Fatal(err)
 	}
-	for _, line := range strings.Split(s, "\n") {
+	for line := range strings.SplitSeq(s, "\n") {
 		line = strings.TrimSpace(line)
 		if len(line) == 0 {
 			continue
@@ -4311,7 +4311,7 @@ func TestRotateDiscoKeyMultipleTimes(t *testing.T) {
 	keys := make([]key.DiscoPublic, 0, 5)
 	keys = append(keys, c.discoAtomic.Public())
 
-	for i := 0; i < 4; i++ {
+	for i := range 4 {
 		c.RotateDiscoKey()
 		newKey := c.discoAtomic.Public()
 

wgengine/pendopen.go

@@ -102,7 +102,7 @@ var (
 	canonicalIPs = sync.OnceValue(func() (checkIPFunc func(netip.Addr) bool) {
 		// https://bgp.he.net/AS41231#_prefixes
 		t := &bart.Table[bool]{}
-		for _, s := range strings.Fields(`
+		for s := range strings.FieldsSeq(`
 			91.189.89.0/24
 			91.189.91.0/24
 			91.189.92.0/24

wgengine/router/osrouter/router_linux_test.go

@@ -1073,11 +1073,9 @@ func (o *fakeOS) run(args ...string) error {
 
 	switch args[2] {
 	case "add":
-		for _, el := range *ls {
-			if el == rest {
-				o.t.Errorf("can't add %q, already present", rest)
-				return errors.New("already exists")
-			}
+		if slices.Contains(*ls, rest) {
+			o.t.Errorf("can't add %q, already present", rest)
+			return errors.New("already exists")
 		}
 		*ls = append(*ls, rest)
 		sort.Strings(*ls)

wgengine/router/osrouter/runner.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"os"
 	"os/exec"
+	"slices"
 	"strconv"
 	"strings"
 	"syscall"
@@ -42,8 +43,7 @@ func errCode(err error) int {
 	if err == nil {
 		return 0
 	}
-	var e *exec.ExitError
-	if ok := errors.As(err, &e); ok {
+	if e, ok := errors.AsType[*exec.ExitError](err); ok {
 		return e.ExitCode()
 	}
 	s := err.Error()
@@ -96,12 +96,7 @@ func newRunGroup(okCode []int, runner commandRunner) *runGroup {
 
 func (rg *runGroup) okCode(err error) bool {
 	got := errCode(err)
-	for _, want := range rg.OkCode {
-		if got == want {
-			return true
-		}
-	}
-	return false
+	return slices.Contains(rg.OkCode, got)
 }
 
 func (rg *runGroup) Output(args ...string) []byte {

wgengine/router/router_test.go

@@ -19,8 +19,8 @@ func TestConfigEqual(t *testing.T) {
 	}
 	configType := reflect.TypeFor[Config]()
 	configFields := []string{}
-	for i := range configType.NumField() {
-		configFields = append(configFields, configType.Field(i).Name)
+	for field := range configType.Fields() {
+		configFields = append(configFields, field.Name)
 	}
 	if !reflect.DeepEqual(configFields, testedFields) {
 		t.Errorf("Config.Equal check might be out of sync\nfields: %q\nhandled: %q\n",

wgengine/wgcfg/config_test.go

@@ -12,8 +12,7 @@ import (
 // that might get added in the future.
 func TestConfigEqual(t *testing.T) {
 	rt := reflect.TypeFor[Config]()
-	for i := range rt.NumField() {
-		sf := rt.Field(i)
+	for sf := range rt.Fields() {
 		switch sf.Name {
 		case "Name", "NodeID", "PrivateKey", "MTU", "Addresses", "DNS", "Peers",
 			"NetworkLogging":
@@ -28,8 +27,7 @@ func TestConfigEqual(t *testing.T) {
 // that might get added in the future.
 func TestPeerEqual(t *testing.T) {
 	rt := reflect.TypeFor[Peer]()
-	for i := range rt.NumField() {
-		sf := rt.Field(i)
+	for sf := range rt.Fields() {
 		switch sf.Name {
 		case "PublicKey", "DiscoKey", "AllowedIPs", "IsJailed",
 			"PersistentKeepalive", "V4MasqAddr", "V6MasqAddr", "WGEndpoint":