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>

types/ipproto/ipproto_test.go

@@ -69,7 +69,7 @@ func TestProtoUnmarshalText(t *testing.T) {
 
 	for i := range 256 {
 		var p Proto
-		must.Do(p.UnmarshalText([]byte(fmt.Sprintf("%d", i))))
+		must.Do(p.UnmarshalText(fmt.Appendf(nil, "%d", i)))
 		if got, want := p, Proto(i); got != want {
 			t.Errorf("Proto(%d) = %v, want %v", i, got, want)
 		}
@@ -122,7 +122,7 @@ func TestProtoUnmarshalJSON(t *testing.T) {
 	var p Proto
 
 	for i := range 256 {
-		j := []byte(fmt.Sprintf(`%d`, i))
+		j := fmt.Appendf(nil, `%d`, i)
 		must.Do(json.Unmarshal(j, &p))
 		if got, want := p, Proto(i); got != want {
 			t.Errorf("Proto(%d) = %v, want %v", i, got, want)
@@ -130,7 +130,7 @@ func TestProtoUnmarshalJSON(t *testing.T) {
 	}
 
 	for name, wantProto := range acceptedNames {
-		must.Do(json.Unmarshal([]byte(fmt.Sprintf(`"%s"`, name)), &p))
+		must.Do(json.Unmarshal(fmt.Appendf(nil, `"%s"`, name), &p))
 		if got, want := p, wantProto; got != want {
 			t.Errorf("Proto(%q) = %v, want %v", name, got, want)
 		}

types/lazy/deferred_test.go

@@ -145,13 +145,11 @@ func TestDeferredInit(t *testing.T) {
 			// Call [DeferredInit.Do] concurrently.
 			const N = 10000
 			for range N {
-				wg.Add(1)
-				go func() {
+				wg.Go(func() {
 					gotErr := di.Do()
 					checkError(t, gotErr, nil, false)
 					checkCalls()
-					wg.Done()
-				}()
+				})
 			}
 			wg.Wait()
 		})
@@ -193,12 +191,10 @@ func TestDeferredErr(t *testing.T) {
 			var wg sync.WaitGroup
 			N := 10000
 			for range N {
-				wg.Add(1)
-				go func() {
+				wg.Go(func() {
 					gotErr := di.Do()
 					checkError(t, gotErr, tt.wantErr, false)
-					wg.Done()
-				}()
+				})
 			}
 			wg.Wait()
 		})
@@ -254,11 +250,9 @@ func TestDeferAfterDo(t *testing.T) {
 	const N = 10000
 	var wg sync.WaitGroup
 	for range N {
-		wg.Add(1)
-		go func() {
+		wg.Go(func() {
 			deferOnce()
-			wg.Done()
-		}()
+		})
 	}
 
 	if err := di.Do(); err != nil {

types/netmap/nodemut_test.go

@@ -34,7 +34,7 @@ func TestMapResponseContainsNonPatchFields(t *testing.T) {
 			return reflect.ValueOf(int64(1)).Convert(t)
 		case reflect.Slice:
 			return reflect.MakeSlice(t, 1, 1)
-		case reflect.Ptr:
+		case reflect.Pointer:
 			return reflect.New(t.Elem())
 		case reflect.Map:
 			return reflect.MakeMap(t)
@@ -43,8 +43,7 @@ func TestMapResponseContainsNonPatchFields(t *testing.T) {
 	}
 
 	rt := reflect.TypeFor[tailcfg.MapResponse]()
-	for i := range rt.NumField() {
-		f := rt.Field(i)
+	for f := range rt.Fields() {
 
 		var want bool
 		switch f.Name {

types/persist/persist_test.go

@@ -12,8 +12,8 @@ import (
 )
 
 func fieldsOf(t reflect.Type) (fields []string) {
-	for i := range t.NumField() {
-		if name := t.Field(i).Name; name != "_" {
+	for field := range t.Fields() {
+		if name := field.Name; name != "_" {
 			fields = append(fields, name)
 		}
 	}

types/views/views.go

@@ -968,8 +968,8 @@ func containsPointers(typ reflect.Type) bool {
 		if isWellKnownImmutableStruct(typ) {
 			return false
 		}
-		for i := range typ.NumField() {
-			if containsPointers(typ.Field(i).Type) {
+		for field := range typ.Fields() {
+			if containsPointers(field.Type) {
 				return true
 			}
 		}