exp/template/html: define isComment helper

Non semantics-changing refactoring in preparation for comment elision.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5071043

src/pkg/exp/template/html/context.go

@@ -89,8 +89,8 @@ const (
 	// stateBeforeValue occurs after the equals sign but before the value.
 	// It occurs between the ^'s in ` name =^ ^value`.
 	stateBeforeValue
-	// stateComment occurs inside an <!-- HTML comment -->.
-	stateComment
+	// stateHTMLCmt occurs inside an <!-- HTML comment -->.
+	stateHTMLCmt
 	// stateRCDATA occurs inside an RCDATA element (<textarea> or <title>)
 	// as described at http://dev.w3.org/html5/spec/syntax.html#elements-0
 	stateRCDATA
@@ -137,7 +137,7 @@ var stateNames = [...]string{
 	stateAttrName:    "stateAttrName",
 	stateAfterName:   "stateAfterName",
 	stateBeforeValue: "stateBeforeValue",
-	stateComment:     "stateComment",
+	stateHTMLCmt:     "stateHTMLCmt",
 	stateRCDATA:      "stateRCDATA",
 	stateAttr:        "stateAttr",
 	stateURL:         "stateURL",
@@ -165,6 +165,16 @@ func (s state) String() string {
 	return fmt.Sprintf("illegal state %d", int(s))
 }
 
+// isComment is true for any state that contains content meant for template
+// authors & maintainers, not for end-users or machines.
+func isComment(s state) bool {
+	switch s {
+	case stateHTMLCmt, stateJSBlockCmt, stateJSLineCmt, stateCSSBlockCmt, stateCSSLineCmt:
+		return true
+	}
+	return false
+}
+
 // delim is the delimiter that will end the current HTML attribute.
 type delim uint8
 

src/pkg/exp/template/html/escape.go

@@ -187,11 +187,6 @@ func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
 		s = append(s, "exp_template_html_jsstrescaper")
 	case stateJSRegexp:
 		s = append(s, "exp_template_html_jsregexpescaper")
-	case stateComment, stateJSBlockCmt, stateJSLineCmt, stateCSSBlockCmt, stateCSSLineCmt:
-		return context{
-			state: stateError,
-			err:   errorf(ErrInsideComment, n.Line, "%s appears inside a comment", n),
-		}
 	case stateCSS:
 		s = append(s, "exp_template_html_cssvaluefilter")
 	case stateText:
@@ -204,6 +199,12 @@ func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
 		c.state = stateAttrName
 		s = append(s, "exp_template_html_htmlnamefilter")
 	default:
+		if isComment(c.state) {
+			return context{
+				state: stateError,
+				err:   errorf(ErrInsideComment, n.Line, "%s appears inside a comment", n),
+			}
+		}
 		panic("unexpected state " + c.state.String())
 	}
 	switch c.delim {

src/pkg/exp/template/html/escape_test.go

@@ -1123,15 +1123,15 @@ func TestEscapeText(t *testing.T) {
 		},
 		{
 			`<!-- foo`,
-			context{state: stateComment},
+			context{state: stateHTMLCmt},
 		},
 		{
 			`<!-->`,
-			context{state: stateComment},
+			context{state: stateHTMLCmt},
 		},
 		{
 			`<!--->`,
-			context{state: stateComment},
+			context{state: stateHTMLCmt},
 		},
 		{
 			`<!-- foo -->`,
@@ -1167,7 +1167,7 @@ func TestEscapeText(t *testing.T) {
 		},
 		{
 			`<script>foo</script><!--`,
-			context{state: stateComment},
+			context{state: stateHTMLCmt},
 		},
 		{
 			`<script>document.write("<p>foo</p>");`,

src/pkg/exp/template/html/transition.go

@@ -21,7 +21,7 @@ var transitionFunc = [...]func(context, []byte) (context, []byte){
 	stateAttrName:    tAttrName,
 	stateAfterName:   tAfterName,
 	stateBeforeValue: tBeforeValue,
-	stateComment:     tComment,
+	stateHTMLCmt:     tHTMLCmt,
 	stateRCDATA:      tSpecialTagEnd,
 	stateAttr:        tAttr,
 	stateURL:         tURL,
@@ -52,7 +52,7 @@ func tText(c context, s []byte) (context, []byte) {
 		if i == -1 || i+1 == len(s) {
 			return c, nil
 		} else if i+4 <= len(s) && bytes.Equal(commentStart, s[i:i+4]) {
-			return context{state: stateComment}, s[i+4:]
+			return context{state: stateHTMLCmt}, s[i+4:]
 		}
 		i++
 		if s[i] == '/' {
@@ -168,8 +168,8 @@ func tBeforeValue(c context, s []byte) (context, []byte) {
 	return c, s[i:]
 }
 
-// tComment is the context transition function for stateComment.
-func tComment(c context, s []byte) (context, []byte) {
+// tHTMLCmt is the context transition function for stateHTMLCmt.
+func tHTMLCmt(c context, s []byte) (context, []byte) {
 	i := bytes.Index(s, commentEnd)
 	if i != -1 {
 		return context{}, s[i+3:]