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, 5 insertions, 9 deletions

diff --git a/src/html/template/attr.go b/src/html/template/attr.go
index 22922e6038..6c52211fed 100644
--- a/src/html/template/attr.go
+++ b/src/html/template/attr.go
@@ -143,12 +143,12 @@ func attrType(name string) contentType {
 		// widely applied.
 		// Treat data-action as URL below.
 		name = name[5:]
-	} else if colon := strings.IndexRune(name, ':'); colon != -1 {
-		if name[:colon] == "xmlns" {
+	} else if prefix, short, ok := strings.Cut(name, ":"); ok {
+		if prefix == "xmlns" {
 			return contentTypeURL
 		}
 		// Treat svg:href and xlink:href as href below.
-		name = name[colon+1:]
+		name = short
 	}
 	if t, ok := attrTypeMap[name]; ok {
 		return t
diff --git a/src/html/template/js.go b/src/html/template/js.go
index ea9c18346b..32a4fbd30a 100644
--- a/src/html/template/js.go
+++ b/src/html/template/js.go
@@ -398,9 +398,7 @@ func isJSType(mimeType string) bool {
 	//   https://tools.ietf.org/html/rfc4329#section-3
 	//   https://www.ietf.org/rfc/rfc4627.txt
 	// discard parameters
-	if i := strings.Index(mimeType, ";"); i >= 0 {
-		mimeType = mimeType[:i]
-	}
+	mimeType, _, _ = strings.Cut(mimeType, ";")
 	mimeType = strings.ToLower(mimeType)
 	mimeType = strings.TrimSpace(mimeType)
 	switch mimeType {
diff --git a/src/html/template/url.go b/src/html/template/url.go
index 6f8185a4e9..4b39fddf07 100644
--- a/src/html/template/url.go
+++ b/src/html/template/url.go
@@ -46,9 +46,7 @@ func urlFilter(args ...interface{}) string {
 // isSafeURL is true if s is a relative URL or if URL has a protocol in
 // (http, https, mailto).
 func isSafeURL(s string) bool {
-	if i := strings.IndexRune(s, ':'); i >= 0 && !strings.ContainsRune(s[:i], '/') {
-
-		protocol := s[:i]
+	if protocol, _, ok := strings.Cut(s, ":"); ok && !strings.Contains(protocol, "/") {
 		if !strings.EqualFold(protocol, "http") && !strings.EqualFold(protocol, "https") && !strings.EqualFold(protocol, "mailto") {
 			return false
 		}