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>

5 files changed, 13 insertions, 18 deletions

diff --git a/misc/cgo/errors/errors_test.go b/misc/cgo/errors/errors_test.go
index 68a30a44fe..e90ed1e058 100644
--- a/misc/cgo/errors/errors_test.go
+++ b/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)
diff --git a/misc/cgo/testcshared/cshared_test.go b/misc/cgo/testcshared/cshared_test.go
index 19ad8c76a8..84b92d502f 100644
--- a/misc/cgo/testcshared/cshared_test.go
+++ b/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":
diff --git a/misc/cgo/testsanitizers/cc_test.go b/misc/cgo/testsanitizers/cc_test.go
index 384b6250e1..7af30ab557 100644
--- a/misc/cgo/testsanitizers/cc_test.go
+++ b/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)
 	}
 
diff --git a/misc/ios/go_ios_exec.go b/misc/ios/go_ios_exec.go
index 9e63717d92..34a734cda7 100644
--- a/misc/ios/go_ios_exec.go
+++ b/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
diff --git a/misc/linkcheck/linkcheck.go b/misc/linkcheck/linkcheck.go
index 570b430da4..efe400965b 100644
--- a/misc/linkcheck/linkcheck.go
+++ b/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)