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, 11 insertions, 19 deletions

diff --git a/src/os/exec/exec.go b/src/os/exec/exec.go
index 0c49575511..9551c22d6e 100644
--- a/src/os/exec/exec.go
+++ b/src/os/exec/exec.go
@@ -748,12 +748,11 @@ func dedupEnvCase(caseInsensitive bool, env []string) []string {
 	out := make([]string, 0, len(env))
 	saw := make(map[string]int, len(env)) // key => index into out
 	for _, kv := range env {
-		eq := strings.Index(kv, "=")
-		if eq < 0 {
+		k, _, ok := strings.Cut(kv, "=")
+		if !ok {
 			out = append(out, kv)
 			continue
 		}
-		k := kv[:eq]
 		if caseInsensitive {
 			k = strings.ToLower(k)
 		}
@@ -775,11 +774,10 @@ func addCriticalEnv(env []string) []string {
 		return env
 	}
 	for _, kv := range env {
-		eq := strings.Index(kv, "=")
-		if eq < 0 {
+		k, _, ok := strings.Cut(kv, "=")
+		if !ok {
 			continue
 		}
-		k := kv[:eq]
 		if strings.EqualFold(k, "SYSTEMROOT") {
 			// We already have it.
 			return env
diff --git a/src/os/exec/exec_test.go b/src/os/exec/exec_test.go
index d854e0de84..459ba39dff 100644
--- a/src/os/exec/exec_test.go
+++ b/src/os/exec/exec_test.go
@@ -166,12 +166,10 @@ func TestCatGoodAndBadFile(t *testing.T) {
 	if _, ok := err.(*exec.ExitError); !ok {
 		t.Errorf("expected *exec.ExitError from cat combined; got %T: %v", err, err)
 	}
-	s := string(bs)
-	sp := strings.SplitN(s, "\n", 2)
-	if len(sp) != 2 {
-		t.Fatalf("expected two lines from cat; got %q", s)
+	errLine, body, ok := strings.Cut(string(bs), "\n")
+	if !ok {
+		t.Fatalf("expected two lines from cat; got %q", bs)
 	}
-	errLine, body := sp[0], sp[1]
 	if !strings.HasPrefix(errLine, "Error: open /bogus/file.foo") {
 		t.Errorf("expected stderr to complain about file; got %q", errLine)
 	}
diff --git a/src/os/os_test.go b/src/os/os_test.go
index 506f1fb0ee..62173d9bf4 100644
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -1761,8 +1761,8 @@ func TestHostname(t *testing.T) {
 	// and the /bin/hostname only returns the first component
 	want := runBinHostname(t)
 	if hostname != want {
-		i := strings.Index(hostname, ".")
-		if i < 0 || hostname[0:i] != want {
+		host, _, ok := strings.Cut(hostname, ".")
+		if !ok || host != want {
 			t.Errorf("Hostname() = %q, want %q", hostname, want)
 		}
 	}
diff --git a/src/os/user/cgo_lookup_unix.go b/src/os/user/cgo_lookup_unix.go
index abc9e9ce6d..5c972e77cf 100644
--- a/src/os/user/cgo_lookup_unix.go
+++ b/src/os/user/cgo_lookup_unix.go
@@ -125,9 +125,7 @@ func buildUser(pwd *C.struct_passwd) *User {
 	// say: "It is expected to be a comma separated list of
 	// personal data where the first item is the full name of the
 	// user."
-	if i := strings.Index(u.Name, ","); i >= 0 {
-		u.Name = u.Name[:i]
-	}
+	u.Name, _, _ = strings.Cut(u.Name, ",")
 	return u
 }
 
diff --git a/src/os/user/lookup_unix.go b/src/os/user/lookup_unix.go
index ac4f1502af..1cabdb5b71 100644
--- a/src/os/user/lookup_unix.go
+++ b/src/os/user/lookup_unix.go
@@ -174,9 +174,7 @@ func matchUserIndexValue(value string, idx int) lineFunc {
 		// say: "It is expected to be a comma separated list of
 		// personal data where the first item is the full name of the
 		// user."
-		if i := strings.Index(u.Name, ","); i >= 0 {
-			u.Name = u.Name[:i]
-		}
+		u.Name, _, _ = strings.Cut(u.Name, ",")
 		return u, nil
 	}
 }