Added extra tests for non-covered scenarios:

- For older version of Razor the HTML comments will be complained about by TahHelperRewriter
- RazorParserFeatureFlags tests now ensure that AllowHtmlCommentsInTagHelpers is true in 2.1 version  and false in older versions
- Added extra test for IsHtmlCommentAhead to make sure Razor code transition is allowed in comment tag
- Moved the unallowed html comment ending to a static array.

src/Microsoft.AspNetCore.Razor.Language/Legacy/HtmlMarkupParser.cs

@@ -12,6 +12,8 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
     {
         private const string ScriptTagName = "script";
 
+        private static readonly HtmlSymbol[] nonAllowedHtmlCommentEnding = new[] { new HtmlSymbol("-", HtmlSymbolType.Text), new HtmlSymbol("!", HtmlSymbolType.Bang), new HtmlSymbol("<", HtmlSymbolType.OpenAngle) };
+
         private static readonly char[] ValidAfterTypeAttributeNameCharacters = { ' ', '\t', '\r', '\n', '\f', '=' };
         private SourceLocation _lastTagStart = SourceLocation.Zero;
         private HtmlSymbol _bufferedOpenAngle;
@@ -649,16 +651,15 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
         internal static bool IsCommentContentDisallowed(IEnumerable<HtmlSymbol> sequence)
         {
             var reversedSequence = sequence.Reverse();
-            var disallowEnding = new[] { new HtmlSymbol("-", HtmlSymbolType.Text), new HtmlSymbol("!", HtmlSymbolType.Bang), new HtmlSymbol("<", HtmlSymbolType.OpenAngle) };
             var index = 0;
             foreach (var item in reversedSequence)
             {
-                if (!item.Equals(disallowEnding[index++]))
+                if (!item.Equals(nonAllowedHtmlCommentEnding[index++]))
                 {
                     return false;
                 }
 
-                if (index == disallowEnding.Length)
+                if (index == nonAllowedHtmlCommentEnding.Length)
                 {
                     return true;
                 }

test/Microsoft.AspNetCore.Razor.Language.Test/Legacy/HtmlMarkupParserTests.cs

@@ -133,6 +133,16 @@ namespace Microsoft.AspNetCore.Razor.Language.Test.Legacy
             Assert.False(sut.IsHtmlCommentAhead());
         }
 
+        [Fact]
+        public void IsHtmlCommentAhead_ReturnsTrueForCommentWithCodeInside()
+        {
+            // Arrange
+            var sut = CreateTestParserForContent("-- not closed @DateTime.Now comment-->");
+
+            // Act & Assert
+            Assert.True(sut.IsHtmlCommentAhead());
+        }
+
         [Fact]
         public void IsCommentContentDisallowed_ReturnsFalseForAllowedContent()
         {

test/Microsoft.AspNetCore.Razor.Language.Test/Legacy/TagHelperParseTreeRewriterTest.cs

@@ -1111,7 +1111,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
         [Fact]
         public void Rewrite_AllowsSimpleHtmlCommentsAsChildren()
         {
-            // Arrangestring documentContent,
+            // Arrange
             IEnumerable<string> allowedChildren = new List<string> { "b" };
             string literal = "asdf";
             string commentOutput = "Hello World";
@@ -1148,10 +1148,61 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
                 Array.Empty<RazorDiagnostic>());
         }
 
+        [Fact]
+        public void Rewrite_DoesntAllowSimpleHtmlCommentsAsChildrenWhenFeatureFlagIsOff()
+        {
+            // Arrange
+            Func<string, string, string, int, int, RazorDiagnostic> nestedTagError =
+                    (childName, parentName, allowed, location, length) =>
+                    RazorDiagnosticFactory.CreateTagHelper_InvalidNestedTag(
+                        new SourceSpan(absoluteIndex: location, lineIndex: 0, characterIndex: location, length: length), childName, parentName, allowed);
+            Func<string, string, int, int, RazorDiagnostic> nestedContentError =
+                (parentName, allowed, location, length) =>
+                RazorDiagnosticFactory.CreateTagHelper_CannotHaveNonTagContent(
+                    new SourceSpan(absoluteIndex: location, lineIndex: 0, characterIndex: location, length: length), parentName, allowed);
+
+            IEnumerable<string> allowedChildren = new List<string> { "b" };
+            string comment1 = "Hello";
+            string expectedOutput = $"<p><!--{comment1}--></p>";
+
+            var pTagHelperBuilder = TagHelperDescriptorBuilder
+                .Create("PTagHelper", "SomeAssembly")
+                .TagMatchingRuleDescriptor(rule => rule.RequireTagName("p"));
+            foreach (var childTag in allowedChildren)
+            {
+                pTagHelperBuilder.AllowChildTag(childTag);
+            }
+
+            var descriptors = new TagHelperDescriptor[]
+            {
+                pTagHelperBuilder.Build()
+            };
+
+            var factory = new SpanFactory();
+            var blockFactory = new BlockFactory(factory);
+
+            var expectedMarkup = new MarkupBlock(
+                new MarkupTagHelperBlock("p",
+                    blockFactory.HtmlCommentBlock(comment1)));
+
+            // Act & Assert
+            EvaluateData(
+                descriptors,
+                expectedOutput,
+                expectedMarkup,
+                new[]
+                {
+                    nestedContentError("p", "b", 3, 4),
+                    nestedContentError("p", "b", 7, 5),
+                    nestedContentError("p", "b", 12, 3),
+                },
+                featureFlags: RazorParserFeatureFlags.Create(RazorLanguageVersion.Version_2_0));
+        }
+
         [Fact]
         public void Rewrite_FailsForContentWithCommentsAsChildren()
         {
-            // Arrangestring documentContent,
+            // Arrange
             Func<string, string, string, int, int, RazorDiagnostic> nestedTagError =
                     (childName, parentName, allowed, location, length) =>
                     RazorDiagnosticFactory.CreateTagHelper_InvalidNestedTag(
@@ -1203,7 +1254,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
         [Fact]
         public void Rewrite_AllowsRazorCommentsAsChildren()
         {
-            // Arrangestring documentContent,
+            // Arrange
             IEnumerable<string> allowedChildren = new List<string> { "b" };
             string literal = "asdf";
             string commentOutput = $"@*{literal}*@";
@@ -1248,7 +1299,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
         [Fact]
         public void Rewrite_AllowsRazorMarkupInHtmlComment()
         {
-            // Arrangestring documentContent,
+            // Arrange
             IEnumerable<string> allowedChildren = new List<string> { "b" };
             string literal = "asdf";
             string part1 = "Hello ";

test/Microsoft.AspNetCore.Razor.Language.Test/RazorParserFeatureFlagsTest.cs

@@ -16,6 +16,7 @@ namespace Microsoft.AspNetCore.Razor.Language
 
             // Assert
             Assert.True(context.AllowMinimizedBooleanTagHelperAttributes);
+            Assert.True(context.AllowHtmlCommentsInTagHelpers);
         }
 
         [Fact]
@@ -26,6 +27,7 @@ namespace Microsoft.AspNetCore.Razor.Language
 
             // Assert
             Assert.False(context.AllowMinimizedBooleanTagHelperAttributes);
+            Assert.False(context.AllowHtmlCommentsInTagHelpers);
         }
     }
 }