all: use bytes.Cut, strings.Cut

Many uses of Index/IndexByte/IndexRune/Split/SplitN
can be written more clearly using the new Cut functions.
Do that. Also rewrite to other functions if that's clearer.

For #46336.

Change-Id: I68d024716ace41a57a8bf74455c62279bde0f448
Reviewed-on: https://go-review.googlesource.com/c/go/+/351711
Trust: Russ Cox <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

misc/cgo/errors/errors_test.go

@@ -36,14 +36,13 @@ func check(t *testing.T, file string) {
 				continue
 			}
 
-			frags := bytes.SplitAfterN(line, []byte("ERROR HERE: "), 2)
-			if len(frags) == 1 {
+			_, frag, ok := bytes.Cut(line, []byte("ERROR HERE: "))
+			if !ok {
 				continue
 			}
-			frag := fmt.Sprintf(":%d:.*%s", i+1, frags[1])
-			re, err := regexp.Compile(frag)
+			re, err := regexp.Compile(fmt.Sprintf(":%d:.*%s", i+1, frag))
 			if err != nil {
-				t.Errorf("Invalid regexp after `ERROR HERE: `: %#q", frags[1])
+				t.Errorf("Invalid regexp after `ERROR HERE: `: %#q", frag)
 				continue
 			}
 			errors = append(errors, re)

misc/cgo/testcshared/cshared_test.go

@@ -200,7 +200,7 @@ func adbRun(t *testing.T, env []string, adbargs ...string) string {
 	args := append(adbCmd(), "exec-out")
 	// Propagate LD_LIBRARY_PATH to the adb shell invocation.
 	for _, e := range env {
-		if strings.Index(e, "LD_LIBRARY_PATH=") != -1 {
+		if strings.Contains(e, "LD_LIBRARY_PATH=") {
 			adbargs = append([]string{e}, adbargs...)
 			break
 		}
@@ -326,7 +326,7 @@ func createHeaders() error {
 			base, name := filepath.Split(args[0])
 			args[0] = filepath.Join(base, "llvm-dlltool")
 			var machine string
-			switch strings.SplitN(name, "-", 2)[0] {
+			switch prefix, _, _ := strings.Cut(name, "-"); prefix {
 			case "i686":
 				machine = "i386"
 			case "x86_64":

misc/cgo/testsanitizers/cc_test.go

@@ -344,7 +344,7 @@ func (c *config) checkCSanitizer() (skip bool, err error) {
 		if os.IsNotExist(err) {
 			return true, fmt.Errorf("%#q failed to produce executable: %v", strings.Join(cmd.Args, " "), err)
 		}
-		snippet := bytes.SplitN(out, []byte{'\n'}, 2)[0]
+		snippet, _, _ := bytes.Cut(out, []byte("\n"))
 		return true, fmt.Errorf("%#q generated broken executable: %v\n%s", strings.Join(cmd.Args, " "), err, snippet)
 	}
 

misc/ios/go_ios_exec.go

@@ -148,9 +148,8 @@ func runOnDevice(appdir string) error {
 	// Device IDs as listed with ios-deploy -c.
 	deviceID = os.Getenv("GOIOS_DEVICE_ID")
 
-	parts := strings.SplitN(appID, ".", 2)
-	if len(parts) == 2 {
-		bundleID = parts[1]
+	if _, id, ok := strings.Cut(appID, "."); ok {
+		bundleID = id
 	}
 
 	if err := signApp(appdir); err != nil {
@@ -291,11 +290,10 @@ func findDevImage() (string, error) {
 	var iosVer, buildVer string
 	lines := bytes.Split(out, []byte("\n"))
 	for _, line := range lines {
-		spl := bytes.SplitN(line, []byte(": "), 2)
-		if len(spl) != 2 {
+		key, val, ok := strings.Cut(string(line), ": ")
+		if !ok {
 			continue
 		}
-		key, val := string(spl[0]), string(spl[1])
 		switch key {
 		case "ProductVersion":
 			iosVer = val

misc/linkcheck/linkcheck.go

@@ -81,10 +81,8 @@ func crawl(url string, sourceURL string) {
 	}
 	mu.Lock()
 	defer mu.Unlock()
-	var frag string
-	if i := strings.Index(url, "#"); i >= 0 {
-		frag = url[i+1:]
-		url = url[:i]
+	if u, frag, ok := strings.Cut(url, "#"); ok {
+		url = u
 		if frag != "" {
 			uf := urlFrag{url, frag}
 			neededFrags[uf] = append(neededFrags[uf], sourceURL)

src/archive/tar/strconv.go

@@ -14,7 +14,7 @@ import (
 
 // hasNUL reports whether the NUL character exists within s.
 func hasNUL(s string) bool {
-	return strings.IndexByte(s, 0) >= 0
+	return strings.Contains(s, "\x00")
 }
 
 // isASCII reports whether the input is an ASCII C-style string.
@@ -201,10 +201,7 @@ func parsePAXTime(s string) (time.Time, error) {
 	const maxNanoSecondDigits = 9
 
 	// Split string into seconds and sub-seconds parts.
-	ss, sn := s, ""
-	if pos := strings.IndexByte(s, '.'); pos >= 0 {
-		ss, sn = s[:pos], s[pos+1:]
-	}
+	ss, sn, _ := strings.Cut(s, ".")
 
 	// Parse the seconds.
 	secs, err := strconv.ParseInt(ss, 10, 64)
@@ -254,48 +251,32 @@ func formatPAXTime(ts time.Time) (s string) {
 // return the remainder as r.
 func parsePAXRecord(s string) (k, v, r string, err error) {
 	// The size field ends at the first space.
-	sp := strings.IndexByte(s, ' ')
-	if sp == -1 {
+	nStr, rest, ok := strings.Cut(s, " ")
+	if !ok {
 		return "", "", s, ErrHeader
 	}
 
 	// Parse the first token as a decimal integer.
-	n, perr := strconv.ParseInt(s[:sp], 10, 0) // Intentionally parse as native int
-	if perr != nil || n < 5 || int64(len(s)) < n {
+	n, perr := strconv.ParseInt(nStr, 10, 0) // Intentionally parse as native int
+	if perr != nil || n < 5 || n > int64(len(s)) {
 		return "", "", s, ErrHeader
 	}
-
-	afterSpace := int64(sp + 1)
-	beforeLastNewLine := n - 1
-	// In some cases, "length" was perhaps padded/malformed, and
-	// trying to index past where the space supposedly is goes past
-	// the end of the actual record.
-	// For example:
-	//    "0000000000000000000000000000000030 mtime=1432668921.098285006\n30 ctime=2147483649.15163319"
-	//                                  ^     ^
-	//                                  |     |
-	//                                  |  afterSpace=35
-	//                                  |
-	//                          beforeLastNewLine=29
-	// yet indexOf(firstSpace) MUST BE before endOfRecord.
-	//
-	// See https://golang.org/issues/40196.
-	if afterSpace >= beforeLastNewLine {
+	n -= int64(len(nStr) + 1) // convert from index in s to index in rest
+	if n <= 0 {
 		return "", "", s, ErrHeader
 	}
 
 	// Extract everything between the space and the final newline.
-	rec, nl, rem := s[afterSpace:beforeLastNewLine], s[beforeLastNewLine:n], s[n:]
+	rec, nl, rem := rest[:n-1], rest[n-1:n], rest[n:]
 	if nl != "\n" {
 		return "", "", s, ErrHeader
 	}
 
 	// The first equals separates the key from the value.
-	eq := strings.IndexByte(rec, '=')
-	if eq == -1 {
+	k, v, ok = strings.Cut(rec, "=")
+	if !ok {
 		return "", "", s, ErrHeader
 	}
-	k, v = rec[:eq], rec[eq+1:]
 
 	if !validPAXRecord(k, v) {
 		return "", "", s, ErrHeader
@@ -333,7 +314,7 @@ func formatPAXRecord(k, v string) (string, error) {
 // for the PAX version of the USTAR string fields.
 // The key must not contain an '=' character.
 func validPAXRecord(k, v string) bool {
-	if k == "" || strings.IndexByte(k, '=') >= 0 {
+	if k == "" || strings.Contains(k, "=") {
 		return false
 	}
 	switch k {

src/archive/tar/writer_test.go

@@ -988,9 +988,7 @@ func TestIssue12594(t *testing.T) {
 		var blk block
 		copy(blk[:], b.Bytes())
 		prefix := string(blk.toUSTAR().prefix())
-		if i := strings.IndexByte(prefix, 0); i >= 0 {
-			prefix = prefix[:i] // Truncate at the NUL terminator
-		}
+		prefix, _, _ = strings.Cut(prefix, "\x00") // Truncate at the NUL terminator
 		if blk.getFormat() == FormatGNU && len(prefix) > 0 && strings.HasPrefix(name, prefix) {
 			t.Errorf("test %d, found prefix in GNU format: %s", i, prefix)
 		}

src/archive/zip/writer_test.go

@@ -362,7 +362,7 @@ func TestWriterDirAttributes(t *testing.T) {
 	}
 
 	binary.LittleEndian.PutUint32(sig[:], uint32(dataDescriptorSignature))
-	if bytes.Index(b, sig[:]) != -1 {
+	if bytes.Contains(b, sig[:]) {
 		t.Error("there should be no data descriptor")
 	}
 }

src/cmd/doc/dirs.go

@@ -221,11 +221,7 @@ func findCodeRoots() []Dir {
 	cmd.Stderr = os.Stderr
 	out, _ := cmd.Output()
 	for _, line := range strings.Split(string(out), "\n") {
-		i := strings.Index(line, "\t")
-		if i < 0 {
-			continue
-		}
-		path, dir := line[:i], line[i+1:]
+		path, dir, _ := strings.Cut(line, "\t")
 		if dir != "" {
 			list = append(list, Dir{importPath: path, dir: dir, inModule: true})
 		}

src/cmd/doc/pkg.go

@@ -315,9 +315,7 @@ func (pkg *Package) oneLineNodeDepth(node ast.Node, depth int) string {
 			recv = "(" + recv + ") "
 		}
 		fnc := pkg.oneLineNodeDepth(n.Type, depth)
-		if strings.Index(fnc, "func") == 0 {
-			fnc = fnc[4:]
-		}
+		fnc = strings.TrimPrefix(fnc, "func")
 		return fmt.Sprintf("func %s%s%s", recv, name, fnc)
 
 	case *ast.TypeSpec:

src/cmd/fix/typecheck.go

@@ -544,8 +544,8 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string, a
 			if strings.HasPrefix(t, "[") || strings.HasPrefix(t, "map[") {
 				// Lazy: assume there are no nested [] in the array
 				// length or map key type.
-				if i := strings.Index(t, "]"); i >= 0 {
-					typeof[n] = t[i+1:]
+				if _, elem, ok := strings.Cut(t, "]"); ok {
+					typeof[n] = elem
 				}
 			}
 
@@ -575,8 +575,7 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string, a
 			t := expand(typeof[n])
 			if strings.HasPrefix(t, "[") { // array or slice
 				// Lazy: assume there are no nested [] in the array length.
-				if i := strings.Index(t, "]"); i >= 0 {
-					et := t[i+1:]
+				if _, et, ok := strings.Cut(t, "]"); ok {
 					for _, e := range n.Elts {
 						if kv, ok := e.(*ast.KeyValueExpr); ok {
 							e = kv.Value
@@ -589,8 +588,7 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string, a
 			}
 			if strings.HasPrefix(t, "map[") { // map
 				// Lazy: assume there are no nested [] in the map key type.
-				if i := strings.Index(t, "]"); i >= 0 {
-					kt, vt := t[4:i], t[i+1:]
+				if kt, vt, ok := strings.Cut(t[len("map["):], "]"); ok {
 					for _, e := range n.Elts {
 						if kv, ok := e.(*ast.KeyValueExpr); ok {
 							if typeof[kv.Key] == "" {
@@ -629,12 +627,10 @@ func typecheck1(cfg *TypeConfig, f interface{}, typeof map[interface{}]string, a
 				key, value = "int", "rune"
 			} else if strings.HasPrefix(t, "[") {
 				key = "int"
-				if i := strings.Index(t, "]"); i >= 0 {
-					value = t[i+1:]
-				}
+				_, value, _ = strings.Cut(t, "]")
 			} else if strings.HasPrefix(t, "map[") {
-				if i := strings.Index(t, "]"); i >= 0 {
-					key, value = t[4:i], t[i+1:]
+				if k, v, ok := strings.Cut(t[len("map["):], "]"); ok {
+					key, value = k, v
 				}
 			}
 			changed := false

src/cmd/vet/vet_test.go

@@ -241,8 +241,8 @@ func errorCheck(outStr string, wantAuto bool, fullshort ...string) (err error) {
 			// Assume errmsg says "file:line: foo".
 			// Cut leading "file:line: " to avoid accidental matching of file name instead of message.
 			text := errmsg
-			if i := strings.Index(text, " "); i >= 0 {
-				text = text[i+1:]
+			if _, suffix, ok := strings.Cut(text, " "); ok {
+				text = suffix
 			}
 			if we.re.MatchString(text) {
 				matched = true

src/crypto/ecdsa/ecdsa_test.go

@@ -219,9 +219,9 @@ func TestVectors(t *testing.T) {
 
 		if line[0] == '[' {
 			line = line[1 : len(line)-1]
-			parts := strings.SplitN(line, ",", 2)
+			curve, hash, _ := strings.Cut(line, ",")
 
-			switch parts[0] {
+			switch curve {
 			case "P-224":
 				pub.Curve = elliptic.P224()
 			case "P-256":
@@ -234,7 +234,7 @@ func TestVectors(t *testing.T) {
 				pub.Curve = nil
 			}
 
-			switch parts[1] {
+			switch hash {
 			case "SHA-1":
 				h = sha1.New()
 			case "SHA-224":

src/crypto/tls/handshake_client_test.go

@@ -97,18 +97,18 @@ func (o *opensslOutputSink) Write(data []byte) (n int, err error) {
 	o.all = append(o.all, data...)
 
 	for {
-		i := bytes.IndexByte(o.line, '\n')
-		if i < 0 {
+		line, next, ok := bytes.Cut(o.line, []byte("\n"))
+		if !ok {
 			break
 		}
 
-		if bytes.Equal([]byte(opensslEndOfHandshake), o.line[:i]) {
+		if bytes.Equal([]byte(opensslEndOfHandshake), line) {
 			o.handshakeComplete <- struct{}{}
 		}
-		if bytes.Equal([]byte(opensslReadKeyUpdate), o.line[:i]) {
+		if bytes.Equal([]byte(opensslReadKeyUpdate), line) {
 			o.readKeyUpdate <- struct{}{}
 		}
-		o.line = o.line[i+1:]
+		o.line = next
 	}
 
 	return len(data), nil

src/crypto/tls/handshake_test.go

@@ -191,18 +191,17 @@ func parseTestData(r io.Reader) (flows [][]byte, err error) {
 		// Otherwise the line is a line of hex dump that looks like:
 		// 00000170  fc f5 06 bf (...)  |.....X{&?......!|
 		// (Some bytes have been omitted from the middle section.)
-
-		if i := strings.IndexByte(line, ' '); i >= 0 {
-			line = line[i:]
-		} else {
+		_, after, ok := strings.Cut(line, " ")
+		if !ok {
 			return nil, errors.New("invalid test data")
 		}
+		line = after
 
-		if i := strings.IndexByte(line, '|'); i >= 0 {
-			line = line[:i]
-		} else {
+		before, _, ok := strings.Cut(line, "|")
+		if !ok {
 			return nil, errors.New("invalid test data")
 		}
+		line = before
 
 		hexBytes := strings.Fields(line)
 		for _, hexByte := range hexBytes {

src/crypto/x509/pem_decrypt.go

@@ -127,12 +127,11 @@ func DecryptPEMBlock(b *pem.Block, password []byte) ([]byte, error) {
 		return nil, errors.New("x509: no DEK-Info header in block")
 	}
 
-	idx := strings.Index(dek, ",")
-	if idx == -1 {
+	mode, hexIV, ok := strings.Cut(dek, ",")
+	if !ok {
 		return nil, errors.New("x509: malformed DEK-Info header")
 	}
 
-	mode, hexIV := dek[:idx], dek[idx+1:]
 	ciph := cipherByName(mode)
 	if ciph == nil {
 		return nil, errors.New("x509: unknown encryption mode")

src/encoding/asn1/common.go

@@ -94,14 +94,7 @@ type fieldParameters struct {
 func parseFieldParameters(str string) (ret fieldParameters) {
 	var part string
 	for len(str) > 0 {
-		// This loop uses IndexByte and explicit slicing
-		// instead of strings.Split(str, ",") to reduce allocations.
-		i := strings.IndexByte(str, ',')
-		if i < 0 {
-			part, str = str, ""
-		} else {
-			part, str = str[:i], str[i+1:]
-		}
+		part, str, _ = strings.Cut(str, ",")
 		switch {
 		case part == "optional":
 			ret.optional = true