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>

2 files changed, 22 insertions, 27 deletions

diff --git a/src/runtime/testdata/testprog/numcpu_freebsd.go b/src/runtime/testdata/testprog/numcpu_freebsd.go
index aff36ec702..7209f67959 100644
--- a/src/runtime/testdata/testprog/numcpu_freebsd.go
+++ b/src/runtime/testdata/testprog/numcpu_freebsd.go
@@ -85,19 +85,18 @@ func getList() ([]string, error) {
 	if err != nil {
 		return nil, fmt.Errorf("fail to execute '%s': %s", cmdline, err)
 	}
-	pos := bytes.IndexRune(output, '\n')
-	if pos == -1 {
+	output, _, ok := bytes.Cut(output, []byte("\n"))
+	if !ok {
 		return nil, fmt.Errorf("invalid output from '%s', '\\n' not found: %s", cmdline, output)
 	}
-	output = output[0:pos]
 
-	pos = bytes.IndexRune(output, ':')
-	if pos == -1 {
+	_, cpus, ok := bytes.Cut(output, []byte(":"))
+	if !ok {
 		return nil, fmt.Errorf("invalid output from '%s', ':' not found: %s", cmdline, output)
 	}
 
 	var list []string
-	for _, val := range bytes.Split(output[pos+1:], []byte(",")) {
+	for _, val := range bytes.Split(cpus, []byte(",")) {
 		index := string(bytes.TrimSpace(val))
 		if len(index) == 0 {
 			continue
diff --git a/src/runtime/testdata/testprog/traceback_ancestors.go b/src/runtime/testdata/testprog/traceback_ancestors.go
index 0ee402c4bd..1d0d00bab7 100644
--- a/src/runtime/testdata/testprog/traceback_ancestors.go
+++ b/src/runtime/testdata/testprog/traceback_ancestors.go
@@ -33,30 +33,27 @@ func printStack() {
 	for {
 		n := runtime.Stack(buf, true)
 		if n < len(buf) {
-			tb := string(buf[:n])
+			all := string(buf[:n])
+			var saved string
 
 			// Delete any ignored goroutines, if present.
-			pos := 0
-			for pos < len(tb) {
-				next := pos + strings.Index(tb[pos:], "\n\n")
-				if next < pos {
-					next = len(tb)
-				} else {
-					next += len("\n\n")
-				}
+			for all != "" {
+				var g string
+				g, all, _ = strings.Cut(all, "\n\n")
 
-				if strings.HasPrefix(tb[pos:], "goroutine ") {
-					id := tb[pos+len("goroutine "):]
-					id = id[:strings.IndexByte(id, ' ')]
+				if strings.HasPrefix(g, "goroutine ") {
+					id, _, _ := strings.Cut(strings.TrimPrefix(g, "goroutine "), " ")
 					if ignoreGoroutines[id] {
-						tb = tb[:pos] + tb[next:]
-						next = pos
+						continue
 					}
 				}
-				pos = next
+				if saved != "" {
+					saved += "\n\n"
+				}
+				saved += g
 			}
 
-			fmt.Print(tb)
+			fmt.Print(saved)
 			return
 		}
 		buf = make([]byte, 2*len(buf))
@@ -89,11 +86,10 @@ func recurseThenCallGo(w chan struct{}, frames int, goroutines int, main bool) {
 func goroutineID() string {
 	buf := make([]byte, 128)
 	runtime.Stack(buf, false)
-	const prefix = "goroutine "
-	if !bytes.HasPrefix(buf, []byte(prefix)) {
+	prefix := []byte("goroutine ")
+	if !bytes.HasPrefix(buf, prefix) {
 		panic(fmt.Sprintf("expected %q at beginning of traceback:\n%s", prefix, buf))
 	}
-	buf = buf[len(prefix):]
-	n := bytes.IndexByte(buf, ' ')
-	return string(buf[:n])
+	id, _, _ := bytes.Cut(bytes.TrimPrefix(buf, prefix), []byte(" "))
+	return string(id)
 }