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>

1 files changed, 5 insertions, 11 deletions

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
 }