Added unit-tests to validate new methods

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

@@ -541,7 +541,7 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
             return false;
         }
 
-        private HtmlSymbol AcceptAllButLastDoubleHypens()
+        protected HtmlSymbol AcceptAllButLastDoubleHypens()
         {
             var lastDoubleHyphen = CurrentSymbol;
             AcceptWhile(s =>
@@ -552,11 +552,12 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
                     return true;
                 }
 
-                NextToken();
-                EnsureCurrent();
                 return false;
             });
 
+            NextToken();
+            EnsureCurrent();
+
             if (At(HtmlSymbolType.Text) && IsDashSymbol(CurrentSymbol))
             {
                 // Doing this here to maintain the order of symbols
@@ -572,12 +573,12 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
             return lastDoubleHyphen;
         }
 
-        private static bool IsDashSymbol(HtmlSymbol symbol)
+        internal static bool IsDashSymbol(HtmlSymbol symbol)
         {
             return string.Equals(symbol.Content, "-", StringComparison.Ordinal);
         }
 
-        private bool IsHtmlCommentAhead()
+        protected bool IsHtmlCommentAhead()
         {
             /*
              * From HTML5 Specification, available at http://www.w3.org/TR/html52/syntax.html#comments

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

@@ -300,12 +300,16 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
         protected internal bool NextIs(Func<TSymbol, bool> condition)
         {
             var cur = CurrentSymbol;
-            NextToken();
-            var result = condition(CurrentSymbol);
-            PutCurrentBack();
-            PutBack(cur);
-            EnsureCurrent();
-            return result;
+            if (NextToken())
+            {
+                var result = condition(CurrentSymbol);
+                PutCurrentBack();
+                PutBack(cur);
+                EnsureCurrent();
+                return result;
+            }
+
+            return false;
         }
 
         protected internal bool Was(TSymbolType type)

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

@@ -0,0 +1,171 @@
+using System.Collections.Generic;
+using Microsoft.AspNetCore.Razor.Language.Legacy;
+using Xunit;
+
+namespace Microsoft.AspNetCore.Razor.Language.Test.Legacy
+{
+    public class HtmlMarkupParserTests
+    {
+        private static readonly HtmlSymbol doubleHyphenSymbol = new HtmlSymbol("--", HtmlSymbolType.DoubleHyphen);
+
+        public static IEnumerable<object[]> NonDashSymbols
+        {
+            get
+            {
+                yield return new[] { new HtmlSymbol("--", HtmlSymbolType.DoubleHyphen) };
+                yield return new[] { new HtmlSymbol("asdf", HtmlSymbolType.Text) };
+                yield return new[] { new HtmlSymbol(">", HtmlSymbolType.CloseAngle) };
+                yield return new[] { new HtmlSymbol("<", HtmlSymbolType.OpenAngle) };
+                yield return new[] { new HtmlSymbol("!", HtmlSymbolType.Bang) };
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(NonDashSymbols))]
+        public void IsDashSymbol_ReturnsFalseForNonDashSymbol(object symbol)
+        {
+            // Arrange
+            HtmlSymbol convertedSymbol = (HtmlSymbol)symbol;
+
+            // Act & Assert
+            Assert.False(HtmlMarkupParser.IsDashSymbol(convertedSymbol));
+        }
+
+        [Fact]
+        public void IsDashSymbol_ReturnsTrueForADashSymbol()
+        {
+            // Arrange
+            HtmlSymbol dashSymbol = new HtmlSymbol("-", HtmlSymbolType.Text);
+
+            // Act & Assert
+            Assert.True(HtmlMarkupParser.IsDashSymbol(dashSymbol));
+        }
+
+        [Fact]
+        public void AcceptAllButLastDoubleHypens_ReturnsTheOnlyDoubleHyphenSymbol()
+        {
+            // Arrange
+            TestHtmlMarkupParser sut = CreateTestParserForContent("-->");
+
+            // Act
+            HtmlSymbol symbol = sut.AcceptAllButLastDoubleHypens();
+
+            // Assert
+            Assert.Equal(doubleHyphenSymbol, symbol);
+            Assert.True(sut.At(HtmlSymbolType.CloseAngle));
+            Assert.Equal(doubleHyphenSymbol, sut.PreviousSymbol);
+        }
+
+        [Fact]
+        public void AcceptAllButLastDoubleHypens_ReturnsTheDoubleHyphenSymbolAfterAcceptingTheDash()
+        {
+            // Arrange
+            TestHtmlMarkupParser sut = CreateTestParserForContent("--->");
+
+            // Act
+            HtmlSymbol symbol = sut.AcceptAllButLastDoubleHypens();
+
+            // Assert
+            Assert.Equal(doubleHyphenSymbol, symbol);
+            Assert.True(sut.At(HtmlSymbolType.CloseAngle));
+            Assert.True(HtmlMarkupParser.IsDashSymbol(sut.PreviousSymbol));
+        }
+
+        [Fact]
+        public void IsHtmlCommentAhead_ReturnsTrueForEmptyCommentTag()
+        {
+            // Arrange
+            TestHtmlMarkupParser sut = CreateTestParserForContent("---->");
+
+            // Act & Assert
+            Assert.True(sut.IsHtmlCommentAhead());
+        }
+
+        [Fact]
+        public void IsHtmlCommentAhead_ReturnsTrueForValidCommentTag()
+        {
+            // Arrange
+            TestHtmlMarkupParser sut = CreateTestParserForContent("-- Some comment content in here -->");
+
+            // Act & Assert
+            Assert.True(sut.IsHtmlCommentAhead());
+        }
+
+        [Fact]
+        public void IsHtmlCommentAhead_ReturnsTrueForValidCommentTagWithExtraDashesAtClosingTag()
+        {
+            // Arrange
+            TestHtmlMarkupParser sut = CreateTestParserForContent("-- Some comment content in here ----->");
+
+            // Act & Assert
+            Assert.True(sut.IsHtmlCommentAhead());
+        }
+
+        [Fact]
+        public void IsHtmlCommentAhead_ReturnsTrueForValidCommentTagWithExtraInfoAfter()
+        {
+            // Arrange
+            TestHtmlMarkupParser sut = CreateTestParserForContent("-- comment --> the first part is a valid comment without the Open angle and bang symbols");
+
+            // Act & Assert
+            Assert.True(sut.IsHtmlCommentAhead());
+        }
+
+        [Fact]
+        public void IsHtmlCommentAhead_ReturnsFalseForNotClosedComment()
+        {
+            // Arrange
+            TestHtmlMarkupParser sut = CreateTestParserForContent("-- not closed comment");
+
+            // Act & Assert
+            Assert.False(sut.IsHtmlCommentAhead());
+        }
+
+        [Fact]
+        public void IsHtmlCommentAhead_ReturnsFalseForCommentWithoutLastClosingAngle()
+        {
+            // Arrange
+            TestHtmlMarkupParser sut = CreateTestParserForContent("-- not closed comment--");
+
+            // Act & Assert
+            Assert.False(sut.IsHtmlCommentAhead());
+        }
+
+        private class TestHtmlMarkupParser : HtmlMarkupParser
+        {
+            public new HtmlSymbol PreviousSymbol
+            {
+                get => base.PreviousSymbol;
+            }
+
+            public new bool IsHtmlCommentAhead()
+            {
+                return base.IsHtmlCommentAhead();
+            }
+
+            public TestHtmlMarkupParser(ParserContext context) : base(context)
+            {
+                this.EnsureCurrent();
+            }
+
+            public new HtmlSymbol AcceptAllButLastDoubleHypens()
+            {
+                return base.AcceptAllButLastDoubleHypens();
+            }
+
+            public override void BuildSpan(SpanBuilder span, SourceLocation start, string content)
+            {
+                base.BuildSpan(span, start, content);
+            }
+        }
+
+        private static TestHtmlMarkupParser CreateTestParserForContent(string content)
+        {
+            var source = TestRazorSourceDocument.Create(content);
+            var options = RazorParserOptions.CreateDefault();
+            var context = new ParserContext(source, options);
+
+            return new TestHtmlMarkupParser(context);
+        }
+    }
+}