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>

12 files changed, 51 insertions, 72 deletions

diff --git a/src/net/http/cgi/child.go b/src/net/http/cgi/child.go
index 0114da377b..bdb35a64e5 100644
--- a/src/net/http/cgi/child.go
+++ b/src/net/http/cgi/child.go
@@ -39,8 +39,8 @@ func Request() (*http.Request, error) {
 func envMap(env []string) map[string]string {
 	m := make(map[string]string)
 	for _, kv := range env {
-		if idx := strings.Index(kv, "="); idx != -1 {
-			m[kv[:idx]] = kv[idx+1:]
+		if k, v, ok := strings.Cut(kv, "="); ok {
+			m[k] = v
 		}
 	}
 	return m
diff --git a/src/net/http/cgi/host.go b/src/net/http/cgi/host.go
index eff67caf4e..e7124a2ab0 100644
--- a/src/net/http/cgi/host.go
+++ b/src/net/http/cgi/host.go
@@ -273,12 +273,11 @@ func (h *Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 			break
 		}
 		headerLines++
-		parts := strings.SplitN(string(line), ":", 2)
-		if len(parts) < 2 {
+		header, val, ok := strings.Cut(string(line), ":")
+		if !ok {
 			h.printf("cgi: bogus header line: %s", string(line))
 			continue
 		}
-		header, val := parts[0], parts[1]
 		if !httpguts.ValidHeaderFieldName(header) {
 			h.printf("cgi: invalid header name: %q", header)
 			continue
diff --git a/src/net/http/cgi/host_test.go b/src/net/http/cgi/host_test.go
index fb869a6728..f8abc88c89 100644
--- a/src/net/http/cgi/host_test.go
+++ b/src/net/http/cgi/host_test.go
@@ -62,12 +62,12 @@ readlines:
 		}
 		linesRead++
 		trimmedLine := strings.TrimRight(line, "\r\n")
-		split := strings.SplitN(trimmedLine, "=", 2)
-		if len(split) != 2 {
-			t.Fatalf("Unexpected %d parts from invalid line number %v: %q; existing map=%v",
-				len(split), linesRead, line, m)
+		k, v, ok := strings.Cut(trimmedLine, "=")
+		if !ok {
+			t.Fatalf("Unexpected response from invalid line number %v: %q; existing map=%v",
+				linesRead, line, m)
 		}
-		m[split[0]] = split[1]
+		m[k] = v
 	}
 
 	for key, expected := range expectedMap {
diff --git a/src/net/http/client_test.go b/src/net/http/client_test.go
index 05ed2268b5..e741c3746f 100644
--- a/src/net/http/client_test.go
+++ b/src/net/http/client_test.go
@@ -431,11 +431,10 @@ func testRedirectsByMethod(t *testing.T, method string, table []redirectTest, wa
 		if v := urlQuery.Get("code"); v != "" {
 			location := ts.URL
 			if final := urlQuery.Get("next"); final != "" {
-				splits := strings.Split(final, ",")
-				first, rest := splits[0], splits[1:]
+				first, rest, _ := strings.Cut(final, ",")
 				location = fmt.Sprintf("%s?code=%s", location, first)
-				if len(rest) > 0 {
-					location = fmt.Sprintf("%s&next=%s", location, strings.Join(rest, ","))
+				if rest != "" {
+					location = fmt.Sprintf("%s&next=%s", location, rest)
 				}
 			}
 			code, _ := strconv.Atoi(v)
diff --git a/src/net/http/cookie.go b/src/net/http/cookie.go
index ca2c1c2506..02b40315de 100644
--- a/src/net/http/cookie.go
+++ b/src/net/http/cookie.go
@@ -67,15 +67,14 @@ func readSetCookies(h Header) []*Cookie {
 			continue
 		}
 		parts[0] = textproto.TrimString(parts[0])
-		j := strings.Index(parts[0], "=")
-		if j < 0 {
+		name, value, ok := strings.Cut(parts[0], "=")
+		if !ok {
 			continue
 		}
-		name, value := parts[0][:j], parts[0][j+1:]
 		if !isCookieNameValid(name) {
 			continue
 		}
-		value, ok := parseCookieValue(value, true)
+		value, ok = parseCookieValue(value, true)
 		if !ok {
 			continue
 		}
@@ -90,10 +89,7 @@ func readSetCookies(h Header) []*Cookie {
 				continue
 			}
 
-			attr, val := parts[i], ""
-			if j := strings.Index(attr, "="); j >= 0 {
-				attr, val = attr[:j], attr[j+1:]
-			}
+			attr, val, _ := strings.Cut(parts[i], "=")
 			lowerAttr, isASCII := ascii.ToLower(attr)
 			if !isASCII {
 				continue
@@ -256,19 +252,12 @@ func readCookies(h Header, filter string) []*Cookie {
 
 		var part string
 		for len(line) > 0 { // continue since we have rest
-			if splitIndex := strings.Index(line, ";"); splitIndex > 0 {
-				part, line = line[:splitIndex], line[splitIndex+1:]
-			} else {
-				part, line = line, ""
-			}
+			part, line, _ = strings.Cut(line, ";")
 			part = textproto.TrimString(part)
-			if len(part) == 0 {
+			if part == "" {
 				continue
 			}
-			name, val := part, ""
-			if j := strings.Index(part, "="); j >= 0 {
-				name, val = name[:j], name[j+1:]
-			}
+			name, val, _ := strings.Cut(part, "=")
 			if !isCookieNameValid(name) {
 				continue
 			}
@@ -379,7 +368,7 @@ func sanitizeCookieValue(v string) string {
 	if len(v) == 0 {
 		return v
 	}
-	if strings.IndexByte(v, ' ') >= 0 || strings.IndexByte(v, ',') >= 0 {
+	if strings.ContainsAny(v, " ,") {
 		return `"` + v + `"`
 	}
 	return v
diff --git a/src/net/http/fs.go b/src/net/http/fs.go
index 57e731e481..19b2894bf2 100644
--- a/src/net/http/fs.go
+++ b/src/net/http/fs.go
@@ -881,11 +881,11 @@ func parseRange(s string, size int64) ([]httpRange, error) {
 		if ra == "" {
 			continue
 		}
-		i := strings.Index(ra, "-")
-		if i < 0 {
+		start, end, ok := strings.Cut(ra, "-")
+		if !ok {
 			return nil, errors.New("invalid range")
 		}
-		start, end := textproto.TrimString(ra[:i]), textproto.TrimString(ra[i+1:])
+		start, end = textproto.TrimString(start), textproto.TrimString(end)
 		var r httpRange
 		if start == "" {
 			// If no start is specified, end specifies the
diff --git a/src/net/http/internal/chunked.go b/src/net/http/internal/chunked.go
index f06e5725f3..923e6a6d2c 100644
--- a/src/net/http/internal/chunked.go
+++ b/src/net/http/internal/chunked.go
@@ -152,6 +152,8 @@ func isASCIISpace(b byte) bool {
 	return b == ' ' || b == '\t' || b == '\n' || b == '\r'
 }
 
+var semi = []byte(";")
+
 // removeChunkExtension removes any chunk-extension from p.
 // For example,
 //     "0" => "0"
@@ -159,14 +161,11 @@ func isASCIISpace(b byte) bool {
 //     "0;token=val" => "0"
 //     `0;token="quoted string"` => "0"
 func removeChunkExtension(p []byte) ([]byte, error) {
-	semi := bytes.IndexByte(p, ';')
-	if semi == -1 {
-		return p, nil
-	}
+	p, _, _ = bytes.Cut(p, semi)
 	// TODO: care about exact syntax of chunk extensions? We're
 	// ignoring and stripping them anyway. For now just never
 	// return an error.
-	return p[:semi], nil
+	return p, nil
 }
 
 // NewChunkedWriter returns a new chunkedWriter that translates writes into HTTP
diff --git a/src/net/http/main_test.go b/src/net/http/main_test.go
index 6564627998..632a308a5c 100644
--- a/src/net/http/main_test.go
+++ b/src/net/http/main_test.go
@@ -31,11 +31,8 @@ func interestingGoroutines() (gs []string) {
 	buf := make([]byte, 2<<20)
 	buf = buf[:runtime.Stack(buf, true)]
 	for _, g := range strings.Split(string(buf), "\n\n") {
-		sl := strings.SplitN(g, "\n", 2)
-		if len(sl) != 2 {
-			continue
-		}
-		stack := strings.TrimSpace(sl[1])
+		_, stack, _ := strings.Cut(g, "\n")
+		stack = strings.TrimSpace(stack)
 		if stack == "" ||
 			strings.Contains(stack, "testing.(*M).before.func1") ||
 			strings.Contains(stack, "os/signal.signal_recv") ||
diff --git a/src/net/http/request.go b/src/net/http/request.go
index 0eb7042d7b..76c2317d28 100644
--- a/src/net/http/request.go
+++ b/src/net/http/request.go
@@ -940,7 +940,7 @@ func NewRequestWithContext(ctx context.Context, method, url string, body io.Read
 func (r *Request) BasicAuth() (username, password string, ok bool) {
 	auth := r.Header.Get("Authorization")
 	if auth == "" {
-		return
+		return "", "", false
 	}
 	return parseBasicAuth(auth)
 }
@@ -951,18 +951,18 @@ func parseBasicAuth(auth string) (username, password string, ok bool) {
 	const prefix = "Basic "
 	// Case insensitive prefix match. See Issue 22736.
 	if len(auth) < len(prefix) || !ascii.EqualFold(auth[:len(prefix)], prefix) {
-		return
+		return "", "", false
 	}
 	c, err := base64.StdEncoding.DecodeString(auth[len(prefix):])
 	if err != nil {
-		return
+		return "", "", false
 	}
 	cs := string(c)
-	s := strings.IndexByte(cs, ':')
-	if s < 0 {
-		return
+	username, password, ok = strings.Cut(cs, ":")
+	if !ok {
+		return "", "", false
 	}
-	return cs[:s], cs[s+1:], true
+	return username, password, true
 }
 
 // SetBasicAuth sets the request's Authorization header to use HTTP
@@ -980,13 +980,12 @@ func (r *Request) SetBasicAuth(username, password string) {
 
 // parseRequestLine parses "GET /foo HTTP/1.1" into its three parts.
 func parseRequestLine(line string) (method, requestURI, proto string, ok bool) {
-	s1 := strings.Index(line, " ")
-	s2 := strings.Index(line[s1+1:], " ")
-	if s1 < 0 || s2 < 0 {
-		return
+	method, rest, ok1 := strings.Cut(line, " ")
+	requestURI, proto, ok2 := strings.Cut(rest, " ")
+	if !ok1 || !ok2 {
+		return "", "", "", false
 	}
-	s2 += s1 + 1
-	return line[:s1], line[s1+1 : s2], line[s2+1:], true
+	return method, requestURI, proto, true
 }
 
 var textprotoReaderPool sync.Pool
diff --git a/src/net/http/response.go b/src/net/http/response.go
index b8985da3c8..297394eabe 100644
--- a/src/net/http/response.go
+++ b/src/net/http/response.go
@@ -165,16 +165,14 @@ func ReadResponse(r *bufio.Reader, req *Request) (*Response, error) {
 		}
 		return nil, err
 	}
-	if i := strings.IndexByte(line, ' '); i == -1 {
+	proto, status, ok := strings.Cut(line, " ")
+	if !ok {
 		return nil, badStringError("malformed HTTP response", line)
-	} else {
-		resp.Proto = line[:i]
-		resp.Status = strings.TrimLeft(line[i+1:], " ")
-	}
-	statusCode := resp.Status
-	if i := strings.IndexByte(resp.Status, ' '); i != -1 {
-		statusCode = resp.Status[:i]
 	}
+	resp.Proto = proto
+	resp.Status = strings.TrimLeft(status, " ")
+
+	statusCode, _, _ := strings.Cut(resp.Status, " ")
 	if len(statusCode) != 3 {
 		return nil, badStringError("malformed HTTP status code", statusCode)
 	}
@@ -182,7 +180,6 @@ func ReadResponse(r *bufio.Reader, req *Request) (*Response, error) {
 	if err != nil || resp.StatusCode < 0 {
 		return nil, badStringError("malformed HTTP status code", statusCode)
 	}
-	var ok bool
 	if resp.ProtoMajor, resp.ProtoMinor, ok = ParseHTTPVersion(resp.Proto); !ok {
 		return nil, badStringError("malformed HTTP version", resp.Proto)
 	}
diff --git a/src/net/http/server.go b/src/net/http/server.go
index 4d0ce5619f..55fd4ae22f 100644
--- a/src/net/http/server.go
+++ b/src/net/http/server.go
@@ -2282,7 +2282,7 @@ func cleanPath(p string) string {
 // stripHostPort returns h without any trailing ":<port>".
 func stripHostPort(h string) string {
 	// If no port on host, return unchanged
-	if strings.IndexByte(h, ':') == -1 {
+	if !strings.Contains(h, ":") {
 		return h
 	}
 	host, _, err := net.SplitHostPort(h)
diff --git a/src/net/http/transport.go b/src/net/http/transport.go
index 309194e8e5..17f0047b59 100644
--- a/src/net/http/transport.go
+++ b/src/net/http/transport.go
@@ -1715,12 +1715,12 @@ func (t *Transport) dialConn(ctx context.Context, cm connectMethod) (pconn *pers
 			return nil, err
 		}
 		if resp.StatusCode != 200 {
-			f := strings.SplitN(resp.Status, " ", 2)
+			_, text, ok := strings.Cut(resp.Status, " ")
 			conn.Close()
-			if len(f) < 2 {
+			if !ok {
 				return nil, errors.New("unknown status code")
 			}
-			return nil, errors.New(f[1])
+			return nil, errors.New(text)
 		}
 	}