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, 36 insertions, 52 deletions

diff --git a/src/encoding/asn1/common.go b/src/encoding/asn1/common.go
index 1c712e1eff..40115df8b4 100644
--- a/src/encoding/asn1/common.go
+++ b/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
diff --git a/src/encoding/json/tags.go b/src/encoding/json/tags.go
index c38fd5102f..b490328f4c 100644
--- a/src/encoding/json/tags.go
+++ b/src/encoding/json/tags.go
@@ -15,10 +15,8 @@ type tagOptions string
 // parseTag splits a struct field's json tag into its name and
 // comma-separated options.
 func parseTag(tag string) (string, tagOptions) {
-	if idx := strings.Index(tag, ","); idx != -1 {
-		return tag[:idx], tagOptions(tag[idx+1:])
-	}
-	return tag, tagOptions("")
+	tag, opt, _ := strings.Cut(tag, ",")
+	return tag, tagOptions(opt)
 }
 
 // Contains reports whether a comma-separated list of options
@@ -30,15 +28,11 @@ func (o tagOptions) Contains(optionName string) bool {
 	}
 	s := string(o)
 	for s != "" {
-		var next string
-		i := strings.Index(s, ",")
-		if i >= 0 {
-			s, next = s[:i], s[i+1:]
-		}
-		if s == optionName {
+		var name string
+		name, s, _ = strings.Cut(s, ",")
+		if name == optionName {
 			return true
 		}
-		s = next
 	}
 	return false
 }
diff --git a/src/encoding/pem/pem.go b/src/encoding/pem/pem.go
index a7272da5ad..e7adf88382 100644
--- a/src/encoding/pem/pem.go
+++ b/src/encoding/pem/pem.go
@@ -78,6 +78,7 @@ func removeSpacesAndTabs(data []byte) []byte {
 var pemStart = []byte("\n-----BEGIN ")
 var pemEnd = []byte("\n-----END ")
 var pemEndOfLine = []byte("-----")
+var colon = []byte(":")
 
 // Decode will find the next PEM formatted block (certificate, private key
 // etc) in the input. It returns that block and the remainder of the input. If
@@ -89,8 +90,8 @@ func Decode(data []byte) (p *Block, rest []byte) {
 	rest = data
 	if bytes.HasPrefix(data, pemStart[1:]) {
 		rest = rest[len(pemStart)-1 : len(data)]
-	} else if i := bytes.Index(data, pemStart); i >= 0 {
-		rest = rest[i+len(pemStart) : len(data)]
+	} else if _, after, ok := bytes.Cut(data, pemStart); ok {
+		rest = after
 	} else {
 		return nil, data
 	}
@@ -114,13 +115,12 @@ func Decode(data []byte) (p *Block, rest []byte) {
 		}
 		line, next := getLine(rest)
 
-		i := bytes.IndexByte(line, ':')
-		if i == -1 {
+		key, val, ok := bytes.Cut(line, colon)
+		if !ok {
 			break
 		}
 
 		// TODO(agl): need to cope with values that spread across lines.
-		key, val := line[:i], line[i+1:]
 		key = bytes.TrimSpace(key)
 		val = bytes.TrimSpace(val)
 		p.Headers[string(key)] = string(val)
diff --git a/src/encoding/xml/typeinfo.go b/src/encoding/xml/typeinfo.go
index 162724ef1a..51e976cf01 100644
--- a/src/encoding/xml/typeinfo.go
+++ b/src/encoding/xml/typeinfo.go
@@ -115,8 +115,8 @@ func structFieldInfo(typ reflect.Type, f *reflect.StructField) (*fieldInfo, erro
 
 	// Split the tag from the xml namespace if necessary.
 	tag := f.Tag.Get("xml")
-	if i := strings.Index(tag, " "); i >= 0 {
-		finfo.xmlns, tag = tag[:i], tag[i+1:]
+	if ns, t, ok := strings.Cut(tag, " "); ok {
+		finfo.xmlns, tag = ns, t
 	}
 
 	// Parse flags.
diff --git a/src/encoding/xml/xml.go b/src/encoding/xml/xml.go
index c14954df15..33d0b417b9 100644
--- a/src/encoding/xml/xml.go
+++ b/src/encoding/xml/xml.go
@@ -1164,11 +1164,11 @@ func (d *Decoder) nsname() (name Name, ok bool) {
 	}
 	if strings.Count(s, ":") > 1 {
 		name.Local = s
-	} else if i := strings.Index(s, ":"); i < 1 || i > len(s)-2 {
+	} else if space, local, ok := strings.Cut(s, ":"); !ok || space == "" || local == "" {
 		name.Local = s
 	} else {
-		name.Space = s[0:i]
-		name.Local = s[i+1:]
+		name.Space = space
+		name.Local = local
 	}
 	return name, true
 }
@@ -2012,25 +2012,26 @@ func emitCDATA(w io.Writer, s []byte) error {
 	if _, err := w.Write(cdataStart); err != nil {
 		return err
 	}
+
 	for {
-		i := bytes.Index(s, cdataEnd)
-		if i >= 0 && i+len(cdataEnd) <= len(s) {
-			// Found a nested CDATA directive end.
-			if _, err := w.Write(s[:i]); err != nil {
-				return err
-			}
-			if _, err := w.Write(cdataEscape); err != nil {
-				return err
-			}
-			i += len(cdataEnd)
-		} else {
-			if _, err := w.Write(s); err != nil {
-				return err
-			}
+		before, after, ok := bytes.Cut(s, cdataEnd)
+		if !ok {
 			break
 		}
-		s = s[i:]
+		// Found a nested CDATA directive end.
+		if _, err := w.Write(before); err != nil {
+			return err
+		}
+		if _, err := w.Write(cdataEscape); err != nil {
+			return err
+		}
+		s = after
+	}
+
+	if _, err := w.Write(s); err != nil {
+		return err
 	}
+
 	_, err := w.Write(cdataEnd)
 	return err
 }
@@ -2041,20 +2042,16 @@ func procInst(param, s string) string {
 	// TODO: this parsing is somewhat lame and not exact.
 	// It works for all actual cases, though.
 	param = param + "="
-	idx := strings.Index(s, param)
-	if idx == -1 {
-		return ""
-	}
-	v := s[idx+len(param):]
+	_, v, _ := strings.Cut(s, param)
 	if v == "" {
 		return ""
 	}
 	if v[0] != '\'' && v[0] != '"' {
 		return ""
 	}
-	idx = strings.IndexRune(v[1:], rune(v[0]))
-	if idx == -1 {
+	unquote, _, ok := strings.Cut(v[1:], v[:1])
+	if !ok {
 		return ""
 	}
-	return v[1 : idx+1]
+	return unquote
 }