Warn for empty outer tags when context switching from C# to markup

src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/legacyTest/TestFiles/ParserTests/HtmlTagsTest/EmptyTag.diag.txt

@@ -0,0 +1 @@
+(1,3): Warning RZ1022: Outer tag is missing a name. The first character of a markup block must be an HTML tag with a valid name.

src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/DefaultRazorProjectEngineIntegrationTest.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Immutable;
+using Roslyn.Test.Utilities;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Razor.Language;
@@ -91,6 +92,45 @@ public void Process_GeneratesCodeDocumentWithValidCSharpDocument()
         Assert.Empty(csharpDocument.Diagnostics);
     }
 
+    [Fact, WorkItem("https://github.com/dotnet/razor/issues/12810")]
+    public void Process_EmptyOuterTag_DoesNotReportWarningAtRazorWarningLevel10()
+    {
+        // Arrange
+        var projectItem = new TestRazorProjectItem("Index.cshtml")
+        {
+            Content = "@{<>foo</>}"
+        };
+        var configuration = RazorConfiguration.Default with { RazorWarningLevel = 10 };
+        var projectEngine = RazorProjectEngine.Create(configuration, TestRazorProjectFileSystem.Empty);
+
+        // Act
+        var codeDocument = projectEngine.Process(projectItem);
+
+        // Assert
+        Assert.Empty(codeDocument.GetRequiredCSharpDocument().Diagnostics);
+    }
+
+    [Fact, WorkItem("https://github.com/dotnet/razor/issues/12810")]
+    public void Process_EmptyOuterTag_ReportsWarningAtRazorWarningLevel11()
+    {
+        // Arrange
+        var projectItem = new TestRazorProjectItem("Index.cshtml")
+        {
+            Content = "@{<>foo</>}"
+        };
+        var configuration = RazorConfiguration.Default with { RazorWarningLevel = 11 };
+        var projectEngine = RazorProjectEngine.Create(configuration, TestRazorProjectFileSystem.Empty);
+
+        // Act
+        var codeDocument = projectEngine.Process(projectItem);
+
+        // Assert
+        var diagnostic = Assert.Single(codeDocument.GetRequiredCSharpDocument().Diagnostics);
+        Assert.Equal("RZ1022", diagnostic.Id);
+        Assert.Equal(RazorDiagnosticSeverity.Warning, diagnostic.Severity);
+        Assert.Equal(11, diagnostic.WarningLevel);
+    }
+
     [Fact]
     public void Process_WithImportsAndTagHelpers_SetsOnCodeDocument()
     {

src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/Legacy/HtmlErrorTest.cs

@@ -3,6 +3,7 @@
 
 #nullable disable
 
+using Roslyn.Test.Utilities;
 using Xunit;
 
 namespace Microsoft.AspNetCore.Razor.Language.Legacy;
@@ -50,4 +51,16 @@ public void WithUnfinishedTagAtEOFErrorsWithIncompleteTag()
     {
         ParseDocumentTest("@{<foo bar=baz");
     }
+
+    [Fact, WorkItem("https://github.com/dotnet/razor/issues/12810")]
+    public void EmptyOuterTagProducesWarningInMarkupBlock()
+    {
+        ParseDocumentTest("@{<>foo</>}");
+    }
+
+    [Fact, WorkItem("https://github.com/dotnet/razor/issues/12810")]
+    public void EmptyOuterTagProducesWarningInTemplateExpression()
+    {
+        ParseDocumentTest("@Html.Repeat(10, @<>Foo #@item</>)");
+    }
 }

src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/TestFiles/ParserTests/HtmlErrorTest/EmptyOuterTagProducesWarningInMarkupBlock.cspans.txt

@@ -0,0 +1,9 @@
+Markup span at (0:0,0 [0] ) - Parent: Markup block at (0:0,0 [11] )
+Transition span at (0:0,0 [1] ) - Parent: Statement block at (0:0,0 [11] )
+MetaCode span at (1:0,1 [1] ) - Parent: Statement block at (0:0,0 [11] )
+Markup span at (2:0,2 [2] ) - Parent: Tag block at (2:0,2 [2] )
+Markup span at (4:0,4 [3] ) - Parent: Markup block at (2:0,2 [8] )
+Markup span at (7:0,7 [3] ) - Parent: Tag block at (7:0,7 [3] )
+Code span at (10:0,10 [0] ) - Parent: Statement block at (0:0,0 [11] )
+MetaCode span at (10:0,10 [1] ) - Parent: Statement block at (0:0,0 [11] )
+Markup span at (11:0,11 [0] ) - Parent: Markup block at (0:0,0 [11] )

src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/TestFiles/ParserTests/HtmlErrorTest/EmptyOuterTagProducesWarningInMarkupBlock.diag.txt

@@ -0,0 +1 @@
+(1,3): Warning RZ1022: Outer tag is missing a name. The first character of a markup block must be an HTML tag with a valid name.

src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/TestFiles/ParserTests/HtmlErrorTest/EmptyOuterTagProducesWarningInMarkupBlock.stree.txt

@@ -0,0 +1,32 @@
+RazorDocument - [0..11)::11 - [@{<>foo</>}]
+    MarkupBlock - [0..11)::11
+        MarkupTextLiteral - [0..0)::0 - [] - Gen<Markup>
+            Marker;[];
+        CSharpCodeBlock - [0..11)::11
+            CSharpStatement - [0..11)::11
+                CSharpTransition - [0..1)::1 - Gen<None>
+                    Transition;[@];
+                CSharpStatementBody - [1..11)::10
+                    RazorMetaCode - [1..2)::1 - Gen<None>
+                        LeftBrace;[{];
+                    CSharpCodeBlock - [2..10)::8
+                        MarkupBlock - [2..10)::8
+                            MarkupElement - [2..10)::8
+                                MarkupStartTag - [2..4)::2 - [<>] - Gen<Markup>
+                                    OpenAngle;[<];
+                                    Text;[<Missing>];
+                                    CloseAngle;[>];
+                                MarkupTextLiteral - [4..7)::3 - [foo] - Gen<Markup>
+                                    Text;[foo];
+                                MarkupEndTag - [7..10)::3 - [</>] - Gen<Markup>
+                                    OpenAngle;[<];
+                                    ForwardSlash;[/];
+                                    Text;[<Missing>];
+                                    CloseAngle;[>];
+                        CSharpStatementLiteral - [10..10)::0 - [] - Gen<Stmt>
+                            Marker;[];
+                    RazorMetaCode - [10..11)::1 - Gen<None>
+                        RightBrace;[}];
+        MarkupTextLiteral - [11..11)::0 - [] - Gen<Markup>
+            Marker;[];
+    EndOfFile;[];

src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/TestFiles/ParserTests/HtmlErrorTest/EmptyOuterTagProducesWarningInTemplateExpression.cspans.txt

@@ -0,0 +1,11 @@
+Markup span at (0:0,0 [0] ) - Parent: Markup block at (0:0,0 [34] )
+Transition span at (0:0,0 [1] ) - Parent: Expression block at (0:0,0 [34] )
+Code span at (1:0,1 [16] ) - Parent: Expression block at (0:0,0 [34] )
+Transition span at (17:0,17 [1] ) - Parent: Markup block at (17:0,17 [16] )
+Markup span at (18:0,18 [2] ) - Parent: Tag block at (18:0,18 [2] )
+Markup span at (20:0,20 [5] ) - Parent: Markup block at (17:0,17 [16] )
+Transition span at (25:0,25 [1] ) - Parent: Expression block at (25:0,25 [5] )
+Code span at (26:0,26 [4] ) - Parent: Expression block at (25:0,25 [5] )
+Markup span at (30:0,30 [3] ) - Parent: Tag block at (30:0,30 [3] )
+Code span at (33:0,33 [1] ) - Parent: Expression block at (0:0,0 [34] )
+Markup span at (34:0,34 [0] ) - Parent: Markup block at (0:0,0 [34] )

src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/TestFiles/ParserTests/HtmlErrorTest/EmptyOuterTagProducesWarningInTemplateExpression.diag.txt

@@ -0,0 +1 @@
+(1,19): Warning RZ1022: Outer tag is missing a name. The first character of a markup block must be an HTML tag with a valid name.

src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/TestFiles/ParserTests/HtmlErrorTest/EmptyOuterTagProducesWarningInTemplateExpression.stree.txt

@@ -0,0 +1,49 @@
+RazorDocument - [0..34)::34 - [@Html.Repeat(10, @<>Foo #@item</>)]
+    MarkupBlock - [0..34)::34
+        MarkupTextLiteral - [0..0)::0 - [] - Gen<Markup>
+            Marker;[];
+        CSharpCodeBlock - [0..34)::34
+            CSharpImplicitExpression - [0..34)::34
+                CSharpTransition - [0..1)::1 - Gen<None>
+                    Transition;[@];
+                CSharpImplicitExpressionBody - [1..34)::33
+                    CSharpCodeBlock - [1..34)::33
+                        CSharpExpressionLiteral - [1..17)::16 - [Html.Repeat(10, ] - Gen<Expr>
+                            Identifier;[Html];
+                            Dot;[.];
+                            Identifier;[Repeat];
+                            LeftParenthesis;[(];
+                            NumericLiteral;[10];
+                            Comma;[,];
+                            Whitespace;[ ];
+                        CSharpTemplateBlock - [17..33)::16
+                            MarkupBlock - [17..33)::16
+                                MarkupTransition - [17..18)::1 - Gen<None>
+                                    Transition;[@];
+                                MarkupElement - [18..33)::15
+                                    MarkupStartTag - [18..20)::2 - [<>] - Gen<Markup>
+                                        OpenAngle;[<];
+                                        Text;[<Missing>];
+                                        CloseAngle;[>];
+                                    MarkupTextLiteral - [20..25)::5 - [Foo #] - Gen<Markup>
+                                        Text;[Foo];
+                                        Whitespace;[ ];
+                                        Text;[#];
+                                    CSharpCodeBlock - [25..30)::5
+                                        CSharpImplicitExpression - [25..30)::5
+                                            CSharpTransition - [25..26)::1 - Gen<None>
+                                                Transition;[@];
+                                            CSharpImplicitExpressionBody - [26..30)::4
+                                                CSharpCodeBlock - [26..30)::4
+                                                    CSharpExpressionLiteral - [26..30)::4 - [item] - Gen<Expr>
+                                                        Identifier;[item];
+                                    MarkupEndTag - [30..33)::3 - [</>] - Gen<Markup>
+                                        OpenAngle;[<];
+                                        ForwardSlash;[/];
+                                        Text;[<Missing>];
+                                        CloseAngle;[>];
+                        CSharpExpressionLiteral - [33..34)::1 - [)] - Gen<Expr>
+                            RightParenthesis;[)];
+        MarkupTextLiteral - [34..34)::0 - [] - Gen<Markup>
+            Marker;[];
+    EndOfFile;[];

src/Razor/src/Compiler/Microsoft.AspNetCore.Razor.Language/test/TestFiles/ParserTests/HtmlTagsTest/EmptyTag.diag.txt

@@ -0,0 +1 @@
+(1,3): Warning RZ1022: Outer tag is missing a name. The first character of a markup block must be an HTML tag with a valid name.

src/Razor/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/Legacy/HtmlMarkupParser.cs

@@ -658,6 +658,17 @@ private MarkupStartTagSyntax ParseStartTag(
             return ParseStartTextTag(openAngleToken, out tagMode, out isWellFormed);
         }
 
+        if (mode == ParseMode.MarkupInCodeBlock &&
+            _tagTracker.Count == 0 &&
+            tagName.Length == 0 &&
+            At(SyntaxKind.CloseAngle))
+        {
+            // The outer tag of a markup block is missing a name.
+            Context.ErrorSink.OnError(
+                RazorDiagnosticFactory.CreateParsing_OuterTagMissingName(
+                    new SourceSpan(tagStartLocation, contentLength: 1)));
+        }
+
         var tagNameToken = At(SyntaxKind.Text) ? EatCurrentToken() : SyntaxFactory.MissingToken(SyntaxKind.Text);
 
         var attributes = EmptySyntaxList;

src/Razor/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/RazorDiagnosticFactory.cs

@@ -207,7 +207,8 @@ public static RazorDiagnostic CreateParsing_MarkupBlockMustStartWithTag(SourceSp
     internal static readonly RazorDiagnosticDescriptor Parsing_OuterTagMissingName =
         new($"{DiagnosticPrefix}1022",
             Resources.ParseError_OuterTagMissingName,
-            RazorDiagnosticSeverity.Error);
+            RazorDiagnosticSeverity.Warning,
+            warningLevel: 11);
 
     public static RazorDiagnostic CreateParsing_OuterTagMissingName(SourceSpan location)
         => RazorDiagnostic.Create(Parsing_OuterTagMissingName, location);