dns/dnsmessage: avoid use of "strings" and "math" in dns/dnsmessage

dns/dnsmessage is used by the net package, which does not have
strings and math in its permitted dependencies.

Change-Id: I2b59887c59891dc61e49dd6430f3a72486ddd66a
Reviewed-on: https://go-review.googlesource.com/c/net/+/723902
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>

Author: neild

dns/dnsmessage/svcb.go

@@ -5,9 +5,7 @@
 package dnsmessage
 
 import (
-	"math"
 	"slices"
-	"strings"
 )
 
 // An SVCBResource is an SVCB Resource record.
@@ -23,19 +21,18 @@ func (r *SVCBResource) realType() Type {
 
 // GoString implements fmt.GoStringer.GoString.
 func (r *SVCBResource) GoString() string {
-	var b strings.Builder
-	b.WriteString("dnsmessage.SVCBResource{")
-	b.WriteString("Priority: " + printUint16(r.Priority) + ", ")
-	b.WriteString("Target: " + r.Target.GoString() + ", ")
-	b.WriteString("Params: []dnsmessage.SVCParam{")
+	b := []byte("dnsmessage.SVCBResource{" +
+		"Priority: " + printUint16(r.Priority) + ", " +
+		"Target: " + r.Target.GoString() + ", " +
+		"Params: []dnsmessage.SVCParam{")
 	if len(r.Params) > 0 {
-		b.WriteString(r.Params[0].GoString())
+		b = append(b, r.Params[0].GoString()...)
 		for _, p := range r.Params[1:] {
-			b.WriteString(", " + p.GoString())
+			b = append(b, ", "+p.GoString()...)
 		}
 	}
-	b.WriteString("}}")
-	return b.String()
+	b = append(b, "}}"...)
+	return string(b)
 }
 
 // An HTTPSResource is an HTTPS Resource record.
@@ -176,7 +173,7 @@ func (r *SVCBResource) pack(msg []byte, _ map[string]uint16, _ int) ([]byte, err
 		if i > 0 && param.Key <= previousKey {
 			return oldMsg, &nestedError{"SVCBResource.Params", errParamOutOfOrder}
 		}
-		if len(param.Value) > math.MaxUint16 {
+		if len(param.Value) > (1<<16)-1 {
 			return oldMsg, &nestedError{"SVCBResource.Params", errTooLongSVCBValue}
 		}
 		msg = packUint16(msg, uint16(param.Key))