all: replace HasPrefix and slicing with CutPrefix

src/archive/zip/reader.go

@@ -796,8 +796,8 @@ func toValidName(name string) string {
 
 	p = strings.TrimPrefix(p, "/")
 
-	for strings.HasPrefix(p, "../") {
-		p = p[len("../"):]
+	for cut := true; cut; {
+		p, cut = strings.CutPrefix(p, "../")
 	}
 
 	return p

src/cmd/cgo/ast.go

@@ -288,11 +288,11 @@ func (f *File) saveExport(x interface{}, context astContext) {
 		return
 	}
 	for _, c := range n.Doc.List {
-		if !strings.HasPrefix(c.Text, "//export ") {
+		name, cut := strings.CutPrefix(c.Text, "//export ")
+		if !cut {
 			continue
 		}
 
-		name := strings.TrimSpace(c.Text[9:])
 		if name == "" {
 			error_(c.Pos(), "export missing name")
 		}

src/cmd/dist/buildtool.go

@@ -255,10 +255,10 @@ func bootstrapBuildTools() {
 
 	// Copy binaries into tool binary directory.
 	for _, name := range bootstrapDirs {
-		if !strings.HasPrefix(name, "cmd/") {
+		name, cut := strings.CutPrefix(name, "cmd/")
+		if !cut {
 			continue
 		}
-		name = name[len("cmd/"):]
 		if !strings.Contains(name, "/") {
 			copyfile(pathf("%s/%s%s", tooldir, name, exe), pathf("%s/bin/%s%s", workspace, name, exe), writeExec)
 		}

src/cmd/go/internal/gover/gomod.go

@@ -31,10 +31,10 @@ func GoModLookup(gomod []byte, key string) string {
 }
 
 func parseKey(line []byte, key string) (string, bool) {
-	if !strings.HasPrefix(string(line), key) {
+	s, cut := strings.CutPrefix(string(line), key)
+	if !cut {
 		return "", false
 	}
-	s := strings.TrimPrefix(string(line), key)
 	if len(s) == 0 || (s[0] != ' ' && s[0] != '\t') {
 		return "", false
 	}

src/cmd/go/internal/load/godebug.go

@@ -132,10 +132,11 @@ func godebugForGoVersion(v string) map[string]string {
 		v = v[:j]
 	}
 
-	if !strings.HasPrefix(v, "1.") {
+	nv, cut := strings.CutPrefix(v, "1.")
+	if !cut {
 		return nil
 	}
-	n, err := strconv.Atoi(v[len("1."):])
+	n, err := strconv.Atoi(nv)
 	if err != nil {
 		return nil
 	}

src/cmd/go/internal/modfetch/codehost/git.go

@@ -290,10 +290,10 @@ func (r *gitRepo) Tags(ctx context.Context, prefix string) (*Tags, error) {
 		List: []Tag{},
 	}
 	for ref, hash := range refs {
-		if !strings.HasPrefix(ref, "refs/tags/") {
+		tag, cut := strings.CutPrefix(ref, "refs/tags/")
+		if !cut {
 			continue
 		}
-		tag := ref[len("refs/tags/"):]
 		if !strings.HasPrefix(tag, prefix) {
 			continue
 		}
@@ -721,19 +721,19 @@ func (r *gitRepo) RecentTag(ctx context.Context, rev, prefix string, allowed fun
 			line = strings.TrimSpace(line)
 			// git do support lstrip in for-each-ref format, but it was added in v2.13.0. Stripping here
 			// instead gives support for git v2.7.0.
-			if !strings.HasPrefix(line, "refs/tags/") {
+			line, cut := strings.CutPrefix(line, "refs/tags/")
+			if !cut {
 				continue
 			}
-			line = line[len("refs/tags/"):]
 
-			if !strings.HasPrefix(line, prefix) {
+			semtag, cut := strings.CutPrefix(line, prefix)
+			if !cut {
 				continue
 			}
 			if !allowed(line) {
 				continue
 			}
 
-			semtag := line[len(prefix):]
 			if semver.Compare(semtag, highest) > 0 {
 				highest = semtag
 			}

src/cmd/go/internal/modfetch/coderepo.go

@@ -565,10 +565,10 @@ func (r *codeRepo) convert(ctx context.Context, info *codehost.RevInfo, statVers
 	// If the tag is invalid, retracted, or a pseudo-version, tagToVersion returns
 	// an empty version.
 	tagToVersion := func(tag string) (v string, tagIsCanonical bool) {
-		if !strings.HasPrefix(tag, tagPrefix) {
+		trimmed, cut := strings.CutPrefix(tag, tagPrefix)
+		if !cut {
 			return "", false
 		}
-		trimmed := tag[len(tagPrefix):]
 		// Tags that look like pseudo-versions would be confusing. Ignore them.
 		if module.IsPseudoVersion(tag) {
 			return "", false

src/cmd/go/internal/modindex/build.go

@@ -171,10 +171,11 @@ func (ctxt *Context) hasSubdir(root, dir string) (rel string, ok bool) {
 func hasSubdir(root, dir string) (rel string, ok bool) {
 	root = str.WithFilePathSeparator(filepath.Clean(root))
 	dir = filepath.Clean(dir)
-	if !strings.HasPrefix(dir, root) {
+	path, cut := strings.CutPrefix(dir, root)
+	if !cut {
 		return "", false
 	}
-	return filepath.ToSlash(dir[len(root):]), true
+	return filepath.ToSlash(path), true
 }
 
 // gopath returns the list of Go path directories.

src/cmd/go/internal/modindex/scan.go

@@ -65,10 +65,10 @@ func indexModule(modroot string) ([]byte, error) {
 		if !d.IsDir() {
 			return nil
 		}
-		if !strings.HasPrefix(path, root) {
+		rel, cut := strings.CutPrefix(path, root)
+		if !cut {
 			panic(fmt.Errorf("path %v in walk doesn't have modroot %v as prefix", path, modroot))
 		}
-		rel := path[len(root):]
 		packages = append(packages, importRaw(modroot, rel))
 		return nil
 	})

src/cmd/internal/script/scripttest/run.go

@@ -75,10 +75,10 @@ func RunToolScriptTest(t *testing.T, repls []ToolReplacement, scriptsdir string,
 		found := false
 		for k := range env {
 			ev := env[k]
-			if !strings.HasPrefix(ev, "PATH=") {
+			oldpath, cut := strings.CutPrefix(ev, "PATH=")
+			if !cut {
 				continue
 			}
-			oldpath := ev[5:]
 			env[k] = "PATH=" + dir + string(filepath.ListSeparator) + oldpath
 			found = true
 			break

src/cmd/link/internal/loadpe/ldpe.go

@@ -807,10 +807,10 @@ func (state *peLoaderState) preprocessSymbols() error {
 // "__imp_XYZ" but no XYZ can be found.
 func LookupBaseFromImport(s loader.Sym, ldr *loader.Loader, arch *sys.Arch) (loader.Sym, error) {
 	sname := ldr.SymName(s)
-	if !strings.HasPrefix(sname, "__imp_") {
+	basename, cut := strings.CutPrefix(sname, "__imp_")
+	if !cut {
 		return 0, nil
 	}
-	basename := sname[len("__imp_"):]
 	if arch.Family == sys.I386 && basename[0] == '_' {
 		basename = basename[1:] // _Name => Name
 	}

src/encoding/json/v2/fields.go

@@ -493,11 +493,11 @@ func parseFieldOptions(sf reflect.StructField) (out fieldOptions, ignored bool,
 		}
 		switch opt {
 		case "case":
-			if !strings.HasPrefix(tag, ":") {
+			tag, cut := strings.CutPrefix(tag, ":")
+			if !cut {
 				err = cmp.Or(err, fmt.Errorf("Go struct field %s is missing value for `case` tag option; specify `case:ignore` or `case:strict` instead", sf.Name))
 				break
 			}
-			tag = tag[len(":"):]
 			opt, n, err2 := consumeTagOption(tag)
 			if err2 != nil {
 				err = cmp.Or(err, fmt.Errorf("Go struct field %s has malformed value for `case` tag option: %v", sf.Name, err2))
@@ -527,11 +527,11 @@ func parseFieldOptions(sf reflect.StructField) (out fieldOptions, ignored bool,
 		case "string":
 			out.string = true
 		case "format":
-			if !strings.HasPrefix(tag, ":") {
+			tag, cut := strings.CutPrefix(tag, ":")
+			if !cut {
 				err = cmp.Or(err, fmt.Errorf("Go struct field %s is missing value for `format` tag option", sf.Name))
 				break
 			}
-			tag = tag[len(":"):]
 			opt, n, err2 := consumeTagOption(tag)
 			if err2 != nil {
 				err = cmp.Or(err, fmt.Errorf("Go struct field %s has malformed value for `format` tag option: %v", sf.Name, err2))

src/go/build/constraint/expr.go

@@ -178,12 +178,12 @@ func splitGoBuild(line string) (expr string, ok bool) {
 		return "", false
 	}
 
-	if !strings.HasPrefix(line, "//go:build") {
+	line, cut := strings.CutPrefix(line, "//go:build")
+	if !cut {
 		return "", false
 	}
 
 	line = strings.TrimSpace(line)
-	line = line[len("//go:build"):]
 
 	// If strings.TrimSpace finds more to trim after removing the //go:build prefix,
 	// it means that the prefix was followed by a space, making this a //go:build line
@@ -373,17 +373,17 @@ func splitPlusBuild(line string) (expr string, ok bool) {
 		return "", false
 	}
 
-	if !strings.HasPrefix(line, "//") {
+	line, cut := strings.CutPrefix(line, "//")
+	if !cut {
 		return "", false
 	}
-	line = line[len("//"):]
 	// Note the space is optional; "//+build" is recognized too.
 	line = strings.TrimSpace(line)
 
-	if !strings.HasPrefix(line, "+build") {
+	line, cut = strings.CutPrefix(line, "+build")
+	if !cut {
 		return "", false
 	}
-	line = line[len("+build"):]
 
 	// If strings.TrimSpace finds more to trim after removing the +build prefix,
 	// it means that the prefix was followed by a space, making this a +build line

src/internal/buildcfg/cfg.go

@@ -87,18 +87,27 @@ func gofips140() string {
 // isFIPSVersion reports whether v is a valid FIPS version,
 // of the form vX.Y.Z.
 func isFIPSVersion(v string) bool {
-	if !strings.HasPrefix(v, "v") {
+	v, cut := strings.CutPrefix(v, "v")
+	if !cut {
 		return false
 	}
-	v, ok := skipNum(v[len("v"):])
-	if !ok || !strings.HasPrefix(v, ".") {
+	v, ok := skipNum(v)
+	if !ok {
 		return false
 	}
-	v, ok = skipNum(v[len("."):])
-	if !ok || !strings.HasPrefix(v, ".") {
+	v, cut = strings.CutPrefix(v, ".")
+	if !cut {
 		return false
 	}
-	v, ok = skipNum(v[len("."):])
+	v, ok = skipNum(v)
+	if !ok {
+		return false
+	}
+	v, cut = strings.CutPrefix(v, ".")
+	if !cut {
+		return false
+	}
+	v, ok = skipNum(v)
 	return ok && v == ""
 }
 

src/internal/trace/raw/textreader.go

@@ -37,10 +37,11 @@ func NewTextReader(r io.Reader) (*TextReader, error) {
 		return nil, fmt.Errorf("failed to parse header")
 	}
 	gover, line := readToken(line)
-	if !strings.HasPrefix(gover, "Go1.") {
+	ver, cut := strings.CutPrefix(gover, "Go1.")
+	if !cut {
 		return nil, fmt.Errorf("failed to parse header Go version")
 	}
-	rawv, err := strconv.ParseUint(gover[len("Go1."):], 10, 64)
+	rawv, err := strconv.ParseUint(ver, 10, 64)
 	if err != nil {
 		return nil, fmt.Errorf("failed to parse header Go version: %v", err)
 	}

src/mime/mediatype.go

@@ -103,10 +103,11 @@ func checkMediaTypeDisposition(s string) error {
 	if rest == "" {
 		return nil
 	}
-	if !strings.HasPrefix(rest, "/") {
+	rest, cut := strings.CutPrefix(rest, "/")
+	if !cut {
 		return errors.New("mime: expected slash after first token")
 	}
-	subtype, rest := consumeToken(rest[1:])
+	subtype, rest := consumeToken(rest)
 	if subtype == "" {
 		return errors.New("mime: expected token after slash")
 	}
@@ -309,11 +310,11 @@ func consumeValue(v string) (value, rest string) {
 
 func consumeMediaParam(v string) (param, value, rest string) {
 	rest = strings.TrimLeftFunc(v, unicode.IsSpace)
-	if !strings.HasPrefix(rest, ";") {
+	rest, cut := strings.CutPrefix(rest, ";")
+	if !cut {
 		return "", "", v
 	}
 
-	rest = rest[1:] // consume semicolon
 	rest = strings.TrimLeftFunc(rest, unicode.IsSpace)
 	param, rest = consumeToken(rest)
 	param = strings.ToLower(param)
@@ -322,10 +323,10 @@ func consumeMediaParam(v string) (param, value, rest string) {
 	}
 
 	rest = strings.TrimLeftFunc(rest, unicode.IsSpace)
-	if !strings.HasPrefix(rest, "=") {
+	rest, cut = strings.CutPrefix(rest, "=")
+	if !cut {
 		return "", "", v
 	}
-	rest = rest[1:] // consume equals sign
 	rest = strings.TrimLeftFunc(rest, unicode.IsSpace)
 	value, rest2 := consumeValue(rest)
 	if value == "" && rest2 == rest {

src/net/http/fs.go

@@ -1017,12 +1017,13 @@ func parseRange(s string, size int64) ([]httpRange, error) {
 		return nil, nil // header not present
 	}
 	const b = "bytes="
-	if !strings.HasPrefix(s, b) {
+	s, cut := strings.CutPrefix(s, b)
+	if !cut {
 		return nil, errors.New("invalid range")
 	}
 	var ranges []httpRange
 	noOverlap := false
-	for ra := range strings.SplitSeq(s[len(b):], ",") {
+	for ra := range strings.SplitSeq(s, ",") {
 		ra = textproto.TrimString(ra)
 		if ra == "" {
 			continue

src/net/http/h2_bundle.go

@@ -6760,10 +6760,10 @@ const http2TrailerPrefix = "Trailer:"
 // fields to be trailers.
 func (rws *http2responseWriterState) promoteUndeclaredTrailers() {
 	for k, vv := range rws.handlerHeader {
-		if !strings.HasPrefix(k, http2TrailerPrefix) {
+		trailerKey, cut := strings.CutPrefix(k, http2TrailerPrefix)
+		if !cut {
 			continue
 		}
-		trailerKey := strings.TrimPrefix(k, http2TrailerPrefix)
 		rws.declareTrailer(trailerKey)
 		rws.handlerHeader[CanonicalHeaderKey(trailerKey)] = vv
 	}

src/net/http/httptest/recorder.go

@@ -224,14 +224,15 @@ func (rw *ResponseRecorder) Result() *http.Response {
 		}
 	}
 	for k, vv := range rw.HeaderMap {
-		if !strings.HasPrefix(k, http.TrailerPrefix) {
+		k, cut := strings.CutPrefix(k, http.TrailerPrefix)
+		if !cut {
 			continue
 		}
 		if res.Trailer == nil {
 			res.Trailer = make(http.Header)
 		}
 		for _, v := range vv {
-			res.Trailer.Add(strings.TrimPrefix(k, http.TrailerPrefix), v)
+			res.Trailer.Add(k, v)
 		}
 	}
 	return res