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>

7 files changed, 47 insertions, 64 deletions

diff --git a/src/runtime/debug/mod.go b/src/runtime/debug/mod.go
index 0381bdcc53..05cad61155 100644
--- a/src/runtime/debug/mod.go
+++ b/src/runtime/debug/mod.go
@@ -70,11 +70,10 @@ func readBuildInfo(data string) (*BuildInfo, bool) {
 	)
 	// Reverse of cmd/go/internal/modload.PackageBuildInfo
 	for len(data) > 0 {
-		i := strings.IndexByte(data, '\n')
-		if i < 0 {
+		line, data, ok = strings.Cut(data, "\n")
+		if !ok {
 			break
 		}
-		line, data = data[:i], data[i+1:]
 		switch {
 		case strings.HasPrefix(line, pathLine):
 			elem := line[len(pathLine):]
diff --git a/src/runtime/pprof/pprof_test.go b/src/runtime/pprof/pprof_test.go
index f39d65c0b5..03ff55b541 100644
--- a/src/runtime/pprof/pprof_test.go
+++ b/src/runtime/pprof/pprof_test.go
@@ -1234,11 +1234,10 @@ func TestGoroutineCounts(t *testing.T) {
 
 func containsInOrder(s string, all ...string) bool {
 	for _, t := range all {
-		i := strings.Index(s, t)
-		if i < 0 {
+		var ok bool
+		if _, s, ok = strings.Cut(s, t); !ok {
 			return false
 		}
-		s = s[i+len(t):]
 	}
 	return true
 }
@@ -1318,18 +1317,18 @@ func TestEmptyCallStack(t *testing.T) {
 // stackContainsLabeled takes a spec like funcname;key=value and matches if the stack has that key
 // and value and has funcname somewhere in the stack.
 func stackContainsLabeled(spec string, count uintptr, stk []*profile.Location, labels map[string][]string) bool {
-	semi := strings.Index(spec, ";")
-	if semi == -1 {
+	base, kv, ok := strings.Cut(spec, ";")
+	if !ok {
 		panic("no semicolon in key/value spec")
 	}
-	kv := strings.SplitN(spec[semi+1:], "=", 2)
-	if len(kv) != 2 {
+	k, v, ok := strings.Cut(kv, "=")
+	if !ok {
 		panic("missing = in key/value spec")
 	}
-	if !contains(labels[kv[0]], kv[1]) {
+	if !contains(labels[k], v) {
 		return false
 	}
-	return stackContains(spec[:semi], count, stk, labels)
+	return stackContains(base, count, stk, labels)
 }
 
 func TestCPUProfileLabel(t *testing.T) {
diff --git a/src/runtime/pprof/proto.go b/src/runtime/pprof/proto.go
index 6862513956..54e7a80183 100644
--- a/src/runtime/pprof/proto.go
+++ b/src/runtime/pprof/proto.go
@@ -13,6 +13,7 @@ import (
 	"os"
 	"runtime"
 	"strconv"
+	"strings"
 	"time"
 	"unsafe"
 )
@@ -581,6 +582,9 @@ func (b *profileBuilder) readMapping() {
 	}
 }
 
+var space = []byte(" ")
+var newline = []byte("\n")
+
 func parseProcSelfMaps(data []byte, addMapping func(lo, hi, offset uint64, file, buildID string)) {
 	// $ cat /proc/self/maps
 	// 00400000-0040b000 r-xp 00000000 fc:01 787766                             /bin/cat
@@ -607,37 +611,24 @@ func parseProcSelfMaps(data []byte, addMapping func(lo, hi, offset uint64, file,
 	// next removes and returns the next field in the line.
 	// It also removes from line any spaces following the field.
 	next := func() []byte {
-		j := bytes.IndexByte(line, ' ')
-		if j < 0 {
-			f := line
-			line = nil
-			return f
-		}
-		f := line[:j]
-		line = line[j+1:]
-		for len(line) > 0 && line[0] == ' ' {
-			line = line[1:]
-		}
+		var f []byte
+		f, line, _ = bytes.Cut(line, space)
+		line = bytes.TrimLeft(line, " ")
 		return f
 	}
 
 	for len(data) > 0 {
-		i := bytes.IndexByte(data, '\n')
-		if i < 0 {
-			line, data = data, nil
-		} else {
-			line, data = data[:i], data[i+1:]
-		}
+		line, data, _ = bytes.Cut(data, newline)
 		addr := next()
-		i = bytes.IndexByte(addr, '-')
-		if i < 0 {
+		loStr, hiStr, ok := strings.Cut(string(addr), "-")
+		if !ok {
 			continue
 		}
-		lo, err := strconv.ParseUint(string(addr[:i]), 16, 64)
+		lo, err := strconv.ParseUint(loStr, 16, 64)
 		if err != nil {
 			continue
 		}
-		hi, err := strconv.ParseUint(string(addr[i+1:]), 16, 64)
+		hi, err := strconv.ParseUint(hiStr, 16, 64)
 		if err != nil {
 			continue
 		}
diff --git a/src/runtime/pprof/proto_test.go b/src/runtime/pprof/proto_test.go
index d052b9fa42..4a9749a83f 100644
--- a/src/runtime/pprof/proto_test.go
+++ b/src/runtime/pprof/proto_test.go
@@ -274,11 +274,10 @@ func TestProcSelfMaps(t *testing.T) {
 
 	f := func(t *testing.T, input string) {
 		for tx, tt := range strings.Split(input, "\n\n") {
-			i := strings.Index(tt, "->\n")
-			if i < 0 {
+			in, out, ok := strings.Cut(tt, "->\n")
+			if !ok {
 				t.Fatal("malformed test case")
 			}
-			in, out := tt[:i], tt[i+len("->\n"):]
 			if len(out) > 0 && out[len(out)-1] != '\n' {
 				out += "\n"
 			}
diff --git a/src/runtime/runtime-gdb_test.go b/src/runtime/runtime-gdb_test.go
index 8c76a9123c..4a0f489c2f 100644
--- a/src/runtime/runtime-gdb_test.go
+++ b/src/runtime/runtime-gdb_test.go
@@ -267,7 +267,7 @@ func testGdbPython(t *testing.T, cgo bool) {
 		t.Fatalf("gdb exited with error: %v", err)
 	}
 
-	firstLine := bytes.SplitN(got, []byte("\n"), 2)[0]
+	firstLine, _, _ := bytes.Cut(got, []byte("\n"))
 	if string(firstLine) != "Loading Go Runtime support." {
 		// This can happen when using all.bash with
 		// GOROOT_FINAL set, because the tests are run before
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)
 }