feat: warn about invalid templ comment syntax, add docs

docs/docs/03-syntax-and-usage/11-comments.md

@@ -0,0 +1,40 @@
+# Comments
+
+# HTML comments
+
+Inside templ statements, use HTML comments.
+
+```templ title="template.templ"
+templ template() {
+	<!-- Single line -->
+	<!--
+		Single or multiline.
+	-->
+}
+```
+
+Comments are rendered to the template output.
+
+```html title="Output"
+<!-- Single line -->
+<!--
+	Single or multiline.
+-->
+```
+
+As per HTML, nested comments are not supported.
+
+# Go comments
+
+Outside of templ statements, use Go comments.
+
+```templ
+package main
+
+// Use standard Go comments outside templ statements.
+var greeting = "Hello!"
+
+templ hello(name string) {
+	<p>{greeting} { name }</p>
+}
+```

parser/v2/commentparser.go

@@ -1,13 +1,11 @@
 package parser
 
 import (
-	"fmt"
-
 	"github.com/a-h/parse"
 )
 
 var htmlCommentStart = parse.String("<!--")
-var htmlCommentEnd = parse.String("-->")
+var htmlCommentEnd = parse.String("--")
 
 type commentParser struct {
 }
@@ -22,11 +20,16 @@ func (p commentParser) Parse(pi *parse.Input) (c Comment, ok bool, err error) {
 
 	// Once we've got the comment start sequence, parse anything until the end
 	// sequence as the comment contents.
-	if c.Contents, ok, err = Must(parse.StringUntil(htmlCommentEnd), fmt.Sprintf("expected end comment sequence not present")).Parse(pi); err != nil || !ok {
+	if c.Contents, ok, err = Must(parse.StringUntil(htmlCommentEnd), "expected end comment literal '-->' not found").Parse(pi); err != nil || !ok {
 		return
 	}
 	// Cut the end element.
 	_, _, _ = htmlCommentEnd.Parse(pi)
 
+	// Cut the gt.
+	if _, ok, err = Must(gt, "comment contains invalid sequence '--'").Parse(pi); err != nil || !ok {
+		return
+	}
+
 	return c, true, nil
 }

parser/v2/commentparser_test.go

@@ -73,13 +73,22 @@ func TestCommentParserErrors(t *testing.T) {
 		{
 			name:  "unclosed HTML comment",
 			input: `<!-- unclosed HTML comment`,
-			expected: parse.Error("expected end comment sequence not present",
+			expected: parse.Error("expected end comment literal '-->' not found",
 				parse.Position{
 					Index: 26,
 					Line:  0,
 					Col:   26,
 				}),
 		},
+		{
+			name:  "comment in comment",
+			input: `<!-- <-- other --> -->`,
+			expected: parse.Error("comment contains invalid sequence '--'", parse.Position{
+				Index: 8,
+				Line:  0,
+				Col:   8,
+			}),
+		},
 	}
 	for _, tt := range tests {
 		tt := tt