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>

3 files changed, 9 insertions, 10 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 {