Apply go fix to simplify using modern features

This patch contains the result of running `go fix ./...`, which applies
simplifications that make use of modern language features.

There are no logic or behaviour changes.

internal/smtpsrv/conn.go

@@ -519,7 +519,7 @@ func (c *Conn) checkSPF(addr string) (spf.Result, error) {
 		defer spfTr.Finish()
 		res, err := spf.CheckHostWithSender(
 			tcp.IP, envelope.DomainOf(addr), addr,
-			spf.WithTraceFunc(func(f string, a ...interface{}) {
+			spf.WithTraceFunc(func(f string, a ...any) {
 				spfTr.Debugf(f, a...)
 			}))
 
@@ -872,7 +872,7 @@ func checkData(data []byte) error {
 // as post-data hooks), so this function sanitizes the value to make it
 // shell-safe.
 func sanitizeEHLODomain(s string) string {
-	n := ""
+	var n strings.Builder
 	for _, c := range s {
 		// Allow a-zA-Z0-9 and []-.:
 		// That's enough for all domains, IPv4 and IPv6 literals, and also
@@ -884,11 +884,11 @@ func sanitizeEHLODomain(s string) string {
 			c >= '0' && c <= '9',
 			c == '-', c == '.',
 			c == '[', c == ']', c == ':':
-			n += string(c)
+			n.WriteString(string(c))
 		}
 	}
 
-	return n
+	return n.String()
 }
 
 // runPostDataHook and return the new headers to add, and on error a boolean
@@ -909,7 +909,7 @@ func (c *Conn) runPostDataHook(data []byte) ([]byte, bool, error) {
 
 	// Prepare the environment, copying some common variables so the hook has
 	// something reasonable, and then setting the specific ones for this case.
-	for _, v := range strings.Fields("USER PWD SHELL PATH") {
+	for v := range strings.FieldsSeq("USER PWD SHELL PATH") {
 		cmd.Env = append(cmd.Env, v+"="+os.Getenv(v))
 	}
 	cmd.Env = append(cmd.Env, "REMOTE_ADDR="+c.remoteAddr.String())
@@ -1269,7 +1269,7 @@ func (c *Conn) writeResponse(code int, msg string) error {
 	return writeResponse(c.writer, code, msg)
 }
 
-func (c *Conn) printfLine(format string, args ...interface{}) {
+func (c *Conn) printfLine(format string, args ...any) {
 	fmt.Fprintf(c.writer, format+"\r\n", args...)
 	c.writer.Flush()
 }

internal/smtpsrv/server_test.go

@@ -307,7 +307,7 @@ func TestTooManyRecipients(t *testing.T) {
 		t.Fatalf("Mail: %v", err)
 	}
 
-	for i := 0; i < 101; i++ {
+	for i := range 101 {
 		if err := c.Rcpt(fmt.Sprintf("to%d@somewhere", i)); err != nil {
 			t.Fatalf("Rcpt: %v", err)
 		}
@@ -363,7 +363,7 @@ func sendLargeEmail(tb testing.TB, c *smtp.Client, sizeMiB int) error {
 	}
 
 	// Write the 1 MiB string sizeMiB times.
-	for i := 0; i < sizeMiB; i++ {
+	for range sizeMiB {
 		if _, err := w.Write([]byte(str1MiB)); err != nil {
 			tb.Fatalf("Data write: %v", err)
 		}
@@ -615,7 +615,7 @@ func realMain(m *testing.M) int {
 
 	// Create a 1MiB string, which the large message tests use.
 	buf := make([]byte, 1024*1024)
-	for i := 0; i < len(buf); i++ {
+	for i := range buf {
 		buf[i] = 'a'
 	}
 	str1MiB = string(buf)