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>

tstest/deptest/deptest.go

@@ -124,7 +124,7 @@ func ImportAliasCheck(t testing.TB, relDir string) {
 	}
 	badRx := regexp.MustCompile(`^([^:]+:\d+):\s+"golang\.org/x/exp/(slices|maps)"`)
 	if s := strings.TrimSpace(string(matches)); s != "" {
-		for _, line := range strings.Split(s, "\n") {
+		for line := range strings.SplitSeq(s, "\n") {
 			if m := badRx.FindStringSubmatch(line); m != nil {
 				t.Errorf("%s: the x/exp/%s package should be imported as x%s", m[1], m[2], m[2])
 			}

tstest/integration/integration_test.go

@@ -1673,7 +1673,7 @@ func TestNetstackTCPLoopback(t *testing.T) {
 		defer lis.Close()
 
 		writeFn := func(conn net.Conn) error {
-			for i := 0; i < writeBufIterations; i++ {
+			for range writeBufIterations {
 				toWrite := make([]byte, writeBufSize)
 				var wrote int
 				for {

tstest/integration/nat/nat_test.go

@@ -317,9 +317,7 @@ func (nt *natTest) runTest(addNode ...addNodeFunc) pingRoute {
 	}
 	defer srv.Close()
 
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
+	wg.Go(func() {
 		for {
 			c, err := srv.Accept()
 			if err != nil {
@@ -327,7 +325,7 @@ func (nt *natTest) runTest(addNode ...addNodeFunc) pingRoute {
 			}
 			go nt.vnet.ServeUnixConn(c.(*net.UnixConn), vnet.ProtocolQEMU)
 		}
-	}()
+	})
 
 	for i, node := range nodes {
 		disk := fmt.Sprintf("%s/node-%d.qcow2", nt.tempDir, i)
@@ -391,7 +389,6 @@ func (nt *natTest) runTest(addNode ...addNodeFunc) pingRoute {
 
 	var eg errgroup.Group
 	for i, c := range clients {
-		i, c := i, c
 		eg.Go(func() error {
 			node := nodes[i]
 			t.Logf("%v calling Status...", node)

tstest/integration/vms/vms_test.go

@@ -364,7 +364,7 @@ func (h *Harness) testDistro(t *testing.T, d Distro, ipm ipMapping) {
 		// starts with testcontrol sometimes there can be up to a few seconds where
 		// tailscaled is in an unknown state on these virtual machines. This exponential
 		// delay loop should delay long enough for tailscaled to be ready.
-		for count := 0; count < 10; count++ {
+		for range 10 {
 			sess := getSession(t, cli)
 
 			outp, err = sess.CombinedOutput("tailscale status")

tstest/natlab/natlab.go

@@ -18,6 +18,7 @@ import (
 	"net"
 	"net/netip"
 	"os"
+	"slices"
 	"sort"
 	"strconv"
 	"sync"
@@ -247,12 +248,7 @@ func (f *Interface) String() string {
 
 // Contains reports whether f contains ip as an IP.
 func (f *Interface) Contains(ip netip.Addr) bool {
-	for _, v := range f.ips {
-		if ip == v {
-			return true
-		}
-	}
-	return false
+	return slices.Contains(f.ips, ip)
 }
 
 type routeEntry struct {
@@ -348,10 +344,8 @@ func (m *Machine) isLocalIP(ip netip.Addr) bool {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	for _, intf := range m.interfaces {
-		for _, iip := range intf.ips {
-			if ip == iip {
-				return true
-			}
+		if slices.Contains(intf.ips, ip) {
+			return true
 		}
 	}
 	return false
@@ -565,7 +559,7 @@ func (m *Machine) interfaceForIP(ip netip.Addr) (*Interface, error) {
 func (m *Machine) pickEphemPort() (port uint16, err error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
-	for tries := 0; tries < 500; tries++ {
+	for range 500 {
 		port := uint16(rand.IntN(32<<10) + 32<<10)
 		if !m.portInUseLocked(port) {
 			return port, nil

tstest/natlab/vnet/vnet.go

@@ -1917,7 +1917,7 @@ func (n *network) doPortMap(src netip.Addr, dstLANPort, wantExtPort uint16, sec
 		}
 	}
 
-	for try := 0; try < 20_000; try++ {
+	for range 20_000 {
 		if wanAP.Port() > 0 && !n.natTable.IsPublicPortUsed(wanAP) {
 			mak.Set(&n.portMap, wanAP, portMapping{
 				dst:    dst,

tstest/resource_test.go

@@ -245,7 +245,7 @@ func TestParseGoroutines(t *testing.T) {
 					t.Errorf("sort field has different number of words: got %d, want %d", len(sorted), len(original))
 					continue
 				}
-				for i := 0; i < len(original); i++ {
+				for i := range original {
 					if original[i] != sorted[len(sorted)-1-i] {
 						t.Errorf("sort field word mismatch at position %d: got %q, want %q", i, sorted[len(sorted)-1-i], original[i])
 					}

tstest/typewalk/typewalk.go

@@ -54,14 +54,13 @@ func MatchingPaths(rt reflect.Type, match func(reflect.Type) bool) iter.Seq[Path
 				return
 			}
 			switch t.Kind() {
-			case reflect.Ptr, reflect.Slice, reflect.Array:
+			case reflect.Pointer, reflect.Slice, reflect.Array:
 				walk(t.Elem(), func(root reflect.Value) reflect.Value {
 					v := getV(root)
 					return v.Elem()
 				})
 			case reflect.Struct:
-				for i := range t.NumField() {
-					sf := t.Field(i)
+				for sf := range t.Fields() {
 					fieldName := sf.Name
 					if fieldName == "_" {
 						continue