rename variables, struct fields and functions to include an "x" when they can panic for handling errors

and document the convention in develop.txt.
spurred by running errcheck again (it has been a while). it still has too many
false to enable by default.

message/compose.go

@@ -44,20 +44,20 @@ func NewComposer(w io.Writer, maxSize int64, smtputf8 bool) *Composer {
 
 // Write implements io.Writer, but calls panic (that is handled higher up) on
 // i/o errors.
-func (c *Composer) Write(buf []byte) (int, error) {
-	if c.maxSize > 0 && c.Size+int64(len(buf)) > c.maxSize {
-		c.Checkf(ErrMessageSize, "writing message")
+func (xc *Composer) Write(buf []byte) (int, error) {
+	if xc.maxSize > 0 && xc.Size+int64(len(buf)) > xc.maxSize {
+		xc.Checkf(ErrMessageSize, "writing message")
 	}
-	n, err := c.bw.Write(buf)
+	n, err := xc.bw.Write(buf)
 	if n > 0 {
-		c.Size += int64(n)
+		xc.Size += int64(n)
 	}
-	c.Checkf(err, "write")
+	xc.Checkf(err, "write")
 	return n, nil
 }
 
 // Checkf checks err, panicing with sentinel error value.
-func (c *Composer) Checkf(err error, format string, args ...any) {
+func (xc *Composer) Checkf(err error, format string, args ...any) {
 	if err != nil {
 		// We expose the original error too, needed at least for ErrMessageSize.
 		panic(fmt.Errorf("%w: %w: %v", ErrCompose, err, fmt.Sprintf(format, args...)))
@@ -65,14 +65,14 @@ func (c *Composer) Checkf(err error, format string, args ...any) {
 }
 
 // Flush writes any buffered output.
-func (c *Composer) Flush() {
-	err := c.bw.Flush()
-	c.Checkf(err, "flush")
+func (xc *Composer) Flush() {
+	err := xc.bw.Flush()
+	xc.Checkf(err, "flush")
 }
 
 // Header writes a message header.
-func (c *Composer) Header(k, v string) {
-	fmt.Fprintf(c, "%s: %s\r\n", k, v)
+func (xc *Composer) Header(k, v string) {
+	fmt.Fprintf(xc, "%s: %s\r\n", k, v)
 }
 
 // NameAddress holds both an address display name, and an SMTP path address.
@@ -82,7 +82,7 @@ type NameAddress struct {
 }
 
 // HeaderAddrs writes a message header with addresses.
-func (c *Composer) HeaderAddrs(k string, l []NameAddress) {
+func (xc *Composer) HeaderAddrs(k string, l []NameAddress) {
 	if len(l) == 0 {
 		return
 	}
@@ -93,7 +93,7 @@ func (c *Composer) HeaderAddrs(k string, l []NameAddress) {
 			v += ","
 			linelen++
 		}
-		addr := mail.Address{Name: a.DisplayName, Address: a.Address.Pack(c.SMTPUTF8)}
+		addr := mail.Address{Name: a.DisplayName, Address: a.Address.Pack(xc.SMTPUTF8)}
 		s := addr.String()
 		if v != "" && linelen+1+len(s) > 77 {
 			v += "\r\n\t"
@@ -105,16 +105,16 @@ func (c *Composer) HeaderAddrs(k string, l []NameAddress) {
 		v += s
 		linelen += len(s)
 	}
-	fmt.Fprintf(c, "%s: %s\r\n", k, v)
+	fmt.Fprintf(xc, "%s: %s\r\n", k, v)
 }
 
 // Subject writes a subject message header.
-func (c *Composer) Subject(subject string) {
+func (xc *Composer) Subject(subject string) {
 	var subjectValue string
 	subjectLineLen := len("Subject: ")
 	subjectWord := false
 	for i, word := range strings.Split(subject, " ") {
-		if !c.SMTPUTF8 && !isASCII(word) {
+		if !xc.SMTPUTF8 && !isASCII(word) {
 			word = mime.QEncoding.Encode("utf-8", word)
 		}
 		if i > 0 {
@@ -129,19 +129,19 @@ func (c *Composer) Subject(subject string) {
 		subjectLineLen += len(word)
 		subjectWord = true
 	}
-	c.Header("Subject", subjectValue)
+	xc.Header("Subject", subjectValue)
 }
 
 // Line writes an empty line.
-func (c *Composer) Line() {
-	_, _ = c.Write([]byte("\r\n"))
+func (xc *Composer) Line() {
+	_, _ = xc.Write([]byte("\r\n"))
 }
 
 // TextPart prepares a text part to be added. Text should contain lines terminated
 // with newlines (lf), which are replaced with crlf. The returned text may be
 // quotedprintable, if needed. The returned ct and cte headers are for use with
 // Content-Type and Content-Transfer-Encoding headers.
-func (c *Composer) TextPart(subtype, text string) (textBody []byte, ct, cte string) {
+func (xc *Composer) TextPart(subtype, text string) (textBody []byte, ct, cte string) {
 	if !strings.HasSuffix(text, "\n") {
 		text += "\n"
 	}
@@ -153,10 +153,10 @@ func (c *Composer) TextPart(subtype, text string) (textBody []byte, ct, cte stri
 	if NeedsQuotedPrintable(text) {
 		var sb strings.Builder
 		_, err := io.Copy(quotedprintable.NewWriter(&sb), strings.NewReader(text))
-		c.Checkf(err, "converting text to quoted printable")
+		xc.Checkf(err, "converting text to quoted printable")
 		text = sb.String()
 		cte = "quoted-printable"
-	} else if c.Has8bit || charset == "utf-8" {
+	} else if xc.Has8bit || charset == "utf-8" {
 		cte = "8bit"
 	} else {
 		cte = "7bit"