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 <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

4 files changed, 16 insertions, 18 deletions

diff --git a/src/crypto/ecdsa/ecdsa_test.go b/src/crypto/ecdsa/ecdsa_test.go
index 556818acf4..c8390b2cc9 100644
--- a/src/crypto/ecdsa/ecdsa_test.go
+++ b/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":
diff --git a/src/crypto/tls/handshake_client_test.go b/src/crypto/tls/handshake_client_test.go
index b6eb488a4d..2158f3247b 100644
--- a/src/crypto/tls/handshake_client_test.go
+++ b/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
diff --git a/src/crypto/tls/handshake_test.go b/src/crypto/tls/handshake_test.go
index 9bfb1177f2..90ac9bd11e 100644
--- a/src/crypto/tls/handshake_test.go
+++ b/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 {
diff --git a/src/crypto/x509/pem_decrypt.go b/src/crypto/x509/pem_decrypt.go
index 781cb3de83..682923ac53 100644
--- a/src/crypto/x509/pem_decrypt.go
+++ b/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")