Apply go fix modernizations

charset.go

@@ -21,7 +21,8 @@ func (u UnknownCharsetError) Error() string {
 // IsUnknownCharset returns a boolean indicating whether the error is known to
 // report that the charset advertised by the entity is unknown.
 func IsUnknownCharset(err error) bool {
-	return errors.As(err, new(UnknownCharsetError))
+	_, ok := errors.AsType[UnknownCharsetError](err)
+	return ok
 }
 
 // CharsetReader, if non-nil, defines a function to generate charset-conversion

charset/charset.go

@@ -49,7 +49,7 @@ func charsetEncoding(charset string) (encoding.Encoding, error) {
 		enc, err = htmlindex.Get(charset)
 	}
 	if err != nil {
-		return nil, fmt.Errorf("charset %q: %v", charset, err)
+		return nil, fmt.Errorf("charset %q: %w", charset, err)
 	}
 	// See https://github.com/golang/go/issues/19421
 	if enc == nil {

charset/charset_test.go

@@ -3,7 +3,7 @@ package charset
 import (
 	"bytes"
 	"io"
-	"io/ioutil"
+
 	"strings"
 	"testing"
 )
@@ -70,7 +70,7 @@ func TestCharsetReader(t *testing.T) {
 		if test.decoded != "" {
 			if err != nil {
 				t.Errorf("Expected no error when creating reader for charset %q, but got: %v", test.charset, err)
-			} else if b, err := ioutil.ReadAll(r); err != nil {
+			} else if b, err := io.ReadAll(r); err != nil {
 				t.Errorf("Expected no error when reading charset %q, but got: %v", test.charset, err)
 			} else if s := string(b); s != test.decoded {
 				t.Errorf("Expected decoded text to be %q but got %q", test.decoded, s)

encoding.go

@@ -24,7 +24,8 @@ func (u UnknownEncodingError) Error() string {
 // IsUnknownEncoding returns a boolean indicating whether the error is known to
 // report that the encoding advertised by the entity is unknown.
 func IsUnknownEncoding(err error) bool {
-	return errors.As(err, new(UnknownEncodingError))
+	_, ok := errors.AsType[UnknownEncodingError](err)
+	return ok
 }
 
 func encodingReader(enc string, r io.Reader) (io.Reader, error) {
@@ -78,7 +79,7 @@ type whitespaceReplacingReader struct {
 func (r *whitespaceReplacingReader) Read(p []byte) (int, error) {
 	n, err := r.wrapped.Read(p)
 
-	for i := 0; i < n; i++ {
+	for i := range n {
 		if p[i] == ' ' || p[i] == '\t' {
 			p[i] = '\n'
 		}
@@ -101,8 +102,7 @@ func (w *lineWrapper) Write(b []byte) (int, error) {
 		var l []byte
 		l, b = cutLine(b, w.maxLineLen-w.curLineLen)
 
-		lf := bytes.HasSuffix(l, []byte("\n"))
-		l = bytes.TrimSuffix(l, []byte("\n"))
+		l, lf := bytes.CutSuffix(l, []byte("\n"))
 
 		n, err := w.w.Write(l)
 		if err != nil {
@@ -141,7 +141,7 @@ func (w *lineWrapper) Write(b []byte) (int, error) {
 }
 
 func cutLine(b []byte, max int) ([]byte, []byte) {
-	for i := 0; i < len(b); i++ {
+	for i := range b {
 		if b[i] == '\r' && i == max {
 			continue
 		}

encoding_test.go

@@ -3,7 +3,7 @@ package message
 import (
 	"bytes"
 	"io"
-	"io/ioutil"
+
 	"strings"
 	"testing"
 )
@@ -45,7 +45,7 @@ func TestDecode(t *testing.T) {
 		r, err := encodingReader(test.enc, strings.NewReader(test.encoded))
 		if err != nil {
 			t.Errorf("Expected no error when creating decoder for encoding %q, but got: %v", test.enc, err)
-		} else if b, err := ioutil.ReadAll(r); err != nil {
+		} else if b, err := io.ReadAll(r); err != nil {
 			t.Errorf("Expected no error when reading encoding %q, but got: %v", test.enc, err)
 		} else if s := string(b); s != test.decoded {
 			t.Errorf("Expected decoded text to be %q but got %q", test.decoded, s)

entity_test.go

@@ -5,7 +5,6 @@ import (
 	"errors"
 	"golang.org/x/text/encoding/ianaindex"
 	"io"
-	"io/ioutil"
 	"math"
 	"reflect"
 	"strings"
@@ -27,7 +26,7 @@ func TestNewEntity(t *testing.T) {
 	e := testMakeEntity()
 
 	expected := "cc sava"
-	if b, err := ioutil.ReadAll(e.Body); err != nil {
+	if b, err := io.ReadAll(e.Body); err != nil {
 		t.Error("Expected no error while reading entity body, got", err)
 	} else if s := string(b); s != expected {
 		t.Errorf("Expected %q as entity body but got %q", expected, s)
@@ -129,7 +128,7 @@ func testMultipart(t *testing.T, e *Entity) {
 		if mediaType := p.Header.Get("Content-Type"); mediaType != expectedType {
 			t.Errorf("Expected part Content-Type to be %q, got %q", expectedType, mediaType)
 		}
-		if b, err := ioutil.ReadAll(p.Body); err != nil {
+		if b, err := io.ReadAll(p.Body); err != nil {
 			t.Error("Expected no error while reading part body, got", err)
 		} else if s := string(b); s != expectedBody {
 			t.Errorf("Expected %q as part body but got %q", expectedBody, s)
@@ -150,7 +149,7 @@ func TestNewMultipart(t *testing.T) {
 func TestNewMultipart_read(t *testing.T) {
 	e := testMakeMultipart()
 
-	if b, err := ioutil.ReadAll(e.Body); err != nil {
+	if b, err := io.ReadAll(e.Body); err != nil {
 		t.Error("Expected no error while reading multipart body, got", err)
 	} else if s := string(b); s != testMultipartBody {
 		t.Errorf("Expected %q as multipart body but got %q", testMultipartBody, s)
@@ -172,9 +171,9 @@ func TestRead_single(t *testing.T) {
 		t.Fatalf("Read() = %v", err)
 	}
 
-	b, err := ioutil.ReadAll(e.Body)
+	b, err := io.ReadAll(e.Body)
 	if err != nil {
-		t.Fatalf("ioutil.ReadAll() = %v", err)
+		t.Fatalf("io.ReadAll() = %v", err)
 	}
 
 	expected := "Message body"
@@ -475,11 +474,11 @@ func TestNew_unknownTransferEncoding(t *testing.T) {
 	if !IsUnknownEncoding(err) {
 		t.Fatal("New(unknown transfer encoding): expected an error that verifies IsUnknownEncoding")
 	}
-	if !errors.As(err, &UnknownEncodingError{}) {
-		t.Fatal("New(unknown transfer encoding): expected an error that verifies errors.As(err, &EncodingError{})")
+	if _, ok := errors.AsType[UnknownEncodingError](err); !ok {
+		t.Fatal("New(unknown transfer encoding): expected an error that verifies errors.AsType[UnknownEncodingError]")
 	}
 
-	if b, err := ioutil.ReadAll(e.Body); err != nil {
+	if b, err := io.ReadAll(e.Body); err != nil {
 		t.Error("Expected no error while reading entity body, got", err)
 	} else if s := string(b); s != expected {
 		t.Errorf("Expected %q as entity body but got %q", expected, s)
@@ -501,7 +500,7 @@ func TestNew_unknownCharset(t *testing.T) {
 		t.Fatal("New(unknown charset): expected an error that verifies IsUnknownCharset")
 	}
 
-	if b, err := ioutil.ReadAll(e.Body); err != nil {
+	if b, err := io.ReadAll(e.Body); err != nil {
 		t.Error("Expected no error while reading entity body, got", err)
 	} else if s := string(b); s != expected {
 		t.Errorf("Expected %q as entity body but got %q", expected, s)
@@ -537,7 +536,7 @@ func TestNew_paddedBase64(t *testing.T) {
 		t.Fatal("New(padded Base64): expected no error, got", err)
 	}
 
-	if b, err := ioutil.ReadAll(e.Body); err != nil {
+	if b, err := io.ReadAll(e.Body); err != nil {
 		t.Error("Expected no error while reading entity body, got", err)
 	} else if s := string(b); s != expected {
 		t.Errorf("Expected %q as entity body but got %q", expected, s)
@@ -565,7 +564,7 @@ func walkCollect(e *Entity) ([]testWalkPart, error) {
 	err := e.Walk(func(path []int, part *Entity, err error) error {
 		var body string
 		if part.MultipartReader() == nil {
-			b, err := ioutil.ReadAll(part.Body)
+			b, err := io.ReadAll(part.Body)
 			if err != nil {
 				return err
 			}

mail/header.go

@@ -70,7 +70,7 @@ func (p *headerParser) consumeComment() (string, bool) {
 	// '(' already consumed.
 	depth := 1
 
-	var comment string
+	var comment strings.Builder
 	for {
 		if p.empty() || depth == 0 {
 			break
@@ -85,13 +85,13 @@ func (p *headerParser) consumeComment() (string, bool) {
 		}
 
 		if depth > 0 {
-			comment += p.s[:1]
+			comment.WriteString(p.s[:1])
 		}
 
 		p.s = p.s[1:]
 	}
 
-	return comment, depth == 0
+	return comment.String(), depth == 0
 }
 
 func (p *headerParser) parseAtomText(dot bool) (string, error) {

mail/reader_test.go

@@ -2,7 +2,6 @@ package mail_test
 
 import (
 	"io"
-	"io/ioutil"
 	"log"
 	"strings"
 	"testing"
@@ -31,7 +30,7 @@ func ExampleReader() {
 
 		switch h := p.Header.(type) {
 		case *mail.InlineHeader:
-			b, _ := ioutil.ReadAll(p.Body)
+			b, _ := io.ReadAll(p.Body)
 			log.Printf("Got text: %v\n", string(b))
 		case *mail.AttachmentHeader:
 			filename, _ := h.Filename()
@@ -92,7 +91,7 @@ func testReader(t *testing.T, r io.Reader) {
 			expectedBody = "I'm Mitsuha."
 		}
 
-		if b, err := ioutil.ReadAll(p.Body); err != nil {
+		if b, err := io.ReadAll(p.Body); err != nil {
 			t.Error("Expected no error while reading part body, but got:", err)
 		} else if string(b) != expectedBody {
 			t.Errorf("Expected part body to be:\n%v\nbut got:\n%v", expectedBody, string(b))
@@ -131,7 +130,7 @@ func TestReader_nonMultipart(t *testing.T) {
 	}
 
 	expectedBody := "Who are you?"
-	if b, err := ioutil.ReadAll(p.Body); err != nil {
+	if b, err := io.ReadAll(p.Body); err != nil {
 		t.Error("Expected no error while reading part body, but got:", err)
 	} else if string(b) != expectedBody {
 		t.Errorf("Expected part body to be:\n%v\nbut got:\n%v", expectedBody, string(b))
@@ -185,7 +184,7 @@ func TestReader_nested(t *testing.T) {
 			}
 
 			expectedBody := "I forgot."
-			if b, err := ioutil.ReadAll(p.Body); err != nil {
+			if b, err := io.ReadAll(p.Body); err != nil {
 				t.Error("Expected no error while reading part body, but got:", err)
 			} else if string(b) != expectedBody {
 				t.Errorf("Expected part body to be:\n%v\nbut got:\n%v", expectedBody, string(b))

textproto/bench_test.go

@@ -228,7 +228,7 @@ func BenchmarkTextprotoReadHeader(b *testing.B) {
 			r := bufio.NewReader(strings.NewReader(blob))
 
 			var err error
-			for i := 0; i < b.N; i++ {
+			for b.Loop() {
 				r.Reset(strings.NewReader(blob))
 				hdr, err = ReadHeader(r)
 				if err != nil {
@@ -254,7 +254,7 @@ func BenchmarkStdlibReadHeader(b *testing.B) {
 			tr := textproto.NewReader(r)
 
 			var err error
-			for i := 0; i < b.N; i++ {
+			for b.Loop() {
 				r.Reset(strings.NewReader(blob))
 				mimeHdr, err = tr.ReadMIMEHeader()
 				if err != nil {