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>

tsweb/tsweb.go

@@ -13,6 +13,7 @@ import (
 	"expvar"
 	"fmt"
 	"io"
+	"maps"
 	"net"
 	"net/http"
 	"net/netip"
@@ -734,8 +735,8 @@ func (h errorHandler) handleError(w http.ResponseWriter, r *http.Request, lw *lo
 
 	// Extract a presentable, loggable error.
 	var hOK bool
-	var hErr HTTPError
-	if errors.As(err, &hErr) {
+	hErr, hAsOK := errors.AsType[HTTPError](err)
+	if hAsOK {
 		hOK = true
 		if hErr.Code == 0 {
 			lw.logf("[unexpected] HTTPError %v did not contain an HTTP status code, sending internal server error", hErr)
@@ -854,9 +855,7 @@ func WriteHTTPError(w http.ResponseWriter, r *http.Request, e HTTPError) {
 	h.Set("X-Content-Type-Options", "nosniff")
 
 	// Custom headers from the error.
-	for k, vs := range e.Header {
-		h[k] = vs
-	}
+	maps.Copy(h, e.Header)
 
 	// Write the msg back to the user.
 	w.WriteHeader(e.Code)

tsweb/varz/varz.go

@@ -93,8 +93,8 @@ func prometheusMetric(prefix string, key string) (string, string, string) {
 		typ = "histogram"
 		key = strings.TrimPrefix(key, histogramPrefix)
 	}
-	if strings.HasPrefix(key, labelMapPrefix) {
-		key = strings.TrimPrefix(key, labelMapPrefix)
+	if after, ok := strings.CutPrefix(key, labelMapPrefix); ok {
+		key = after
 		if a, b, ok := strings.Cut(key, "_"); ok {
 			label, key = a, b
 		}
@@ -154,7 +154,7 @@ func writePromExpVar(w io.Writer, prefix string, kv expvar.KeyValue) {
 	case PrometheusMetricsReflectRooter:
 		root := v.PrometheusMetricsReflectRoot()
 		rv := reflect.ValueOf(root)
-		if rv.Type().Kind() == reflect.Ptr {
+		if rv.Type().Kind() == reflect.Pointer {
 			if rv.IsNil() {
 				return
 			}
@@ -419,8 +419,7 @@ func structTypeSortedFields(t reflect.Type) []sortedStructField {
 		return v.([]sortedStructField)
 	}
 	fields := make([]sortedStructField, 0, t.NumField())
-	for i, n := 0, t.NumField(); i < n; i++ {
-		sf := t.Field(i)
+	for sf := range t.Fields() {
 		name := sf.Name
 		if v := sf.Tag.Get("json"); v != "" {
 			v, _, _ = strings.Cut(v, ",")
@@ -433,7 +432,7 @@ func structTypeSortedFields(t reflect.Type) []sortedStructField {
 			}
 		}
 		fields = append(fields, sortedStructField{
-			Index:           i,
+			Index:           sf.Index[0],
 			Name:            name,
 			SortName:        removeTypePrefixes(name),
 			MetricType:      sf.Tag.Get("metrictype"),
@@ -467,7 +466,7 @@ func foreachExportedStructField(rv reflect.Value, f func(fieldOrJSONName, metric
 		sf := ssf.StructFieldType
 		if ssf.MetricType != "" || sf.Type.Kind() == reflect.Struct {
 			f(ssf.Name, ssf.MetricType, rv.Field(ssf.Index))
-		} else if sf.Type.Kind() == reflect.Ptr && sf.Type.Elem().Kind() == reflect.Struct {
+		} else if sf.Type.Kind() == reflect.Pointer && sf.Type.Elem().Kind() == reflect.Struct {
 			fv := rv.Field(ssf.Index)
 			if !fv.IsNil() {
 				f(ssf.Name, ssf.MetricType, fv.Elem())