Fix #1399 - crash on start-end syntax for void element

We weren't correctly recovering when a void element is written as a
start-end pair. This change cleans up some of the plumbing around
end-tag handling and adds recognition for this case.

Added a new bespoke diagnostic for the void element case.

src/Microsoft.AspNetCore.Blazor.Razor.Extensions/BlazorDiagnosticFactory.cs

@@ -61,6 +61,16 @@ namespace Microsoft.AspNetCore.Blazor.Razor
             return RazorDiagnostic.Create(MismatchedClosingTag, span ?? SourceSpan.Undefined, expectedTagName, tagName);
         }
 
+        public static readonly RazorDiagnosticDescriptor UnexpectedClosingTagForVoidElement = new RazorDiagnosticDescriptor(
+            "BL9983",
+            () => "Unexpected closing tag '{0}'. The element '{0}' is a void element, and should be used without a closing tag.",
+            RazorDiagnosticSeverity.Error);
+
+        public static RazorDiagnostic Create_UnexpectedClosingTagForVoidElement(SourceSpan? span, string tagName)
+        {
+            return RazorDiagnostic.Create(UnexpectedClosingTagForVoidElement, span ?? SourceSpan.Undefined, tagName);
+        }
+
         public static readonly RazorDiagnosticDescriptor InvalidHtmlContent = new RazorDiagnosticDescriptor(
             "BL9984",
             () => "Found invalid HTML content. Text '{0}'",

src/Microsoft.AspNetCore.Blazor.Razor.Extensions/ComponentDocumentRewritePass.cs

@@ -23,7 +23,7 @@ namespace Microsoft.AspNetCore.Blazor.Razor
     {
         // Per the HTML spec, the following elements are inherently self-closing
         // For example, <img> is the same as <img /> (and therefore it cannot contain descendants)
-        public readonly static HashSet<string> VoidElements = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
+        public static readonly HashSet<string> VoidElements = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
         {
             "area", "base", "br", "col", "embed", "hr", "img", "input", "link", "meta", "param", "source", "track", "wbr",
         };
@@ -182,7 +182,6 @@ namespace Microsoft.AspNetCore.Blazor.Razor
                                     }
 
                                 case HtmlTokenType.StartTag:
-                                case HtmlTokenType.EndTag:
                                     {
                                         var tag = token.AsTag();
 
@@ -218,8 +217,13 @@ namespace Microsoft.AspNetCore.Blazor.Razor
                                             stack.Pop();
                                         }
 
-                                        if (token.Type == HtmlTokenType.EndTag)
+                                        break;
+                                    }
+
+                                case HtmlTokenType.EndTag:
                                     {
+                                        var tag = token.AsTag();
+
                                         var popped = stack.Pop();
                                         if (stack.Count == 0)
                                         {
@@ -227,9 +231,12 @@ namespace Microsoft.AspNetCore.Blazor.Razor
                                             // Put back the current node so we don't crash.
                                             stack.Push(popped);
 
-                                                var tagName = parser.GetTagNameOriginalCasing(token.AsTag());
+                                            var tagName = parser.GetTagNameOriginalCasing(tag);
                                             var span = new SourceSpan(start, end.AbsoluteIndex - start.AbsoluteIndex);
-                                                var diagnostic = BlazorDiagnosticFactory.Create_UnexpectedClosingTag(span, tagName);
+
+                                            var diagnostic = VoidElements.Contains(tagName)
+                                                ? BlazorDiagnosticFactory.Create_UnexpectedClosingTagForVoidElement(span, tagName)
+                                                : BlazorDiagnosticFactory.Create_UnexpectedClosingTag(span, tagName);
                                             popped.Children.Add(new HtmlElementIntermediateNode()
                                             {
                                                 Diagnostics =
@@ -260,7 +267,6 @@ namespace Microsoft.AspNetCore.Blazor.Razor
                                                 popped.Source.Value.CharacterIndex,
                                                 length);
                                         }
-                                        }
 
                                         break;
                                     }

test/Microsoft.AspNetCore.Blazor.Build.Test/DiagnosticRazorIntegrationTest.cs

@@ -1,4 +1,4 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using Xunit;

test/Microsoft.AspNetCore.Blazor.Razor.Extensions.Test/ComponentDocumentRewritePassTest.cs

@@ -255,6 +255,36 @@ namespace Microsoft.AspNetCore.Blazor.Razor
                 c => NodeAssert.Content(c, "Hello, World!"));
         }
 
+        [Fact]
+        public void Execute_RewritesHtml_UnbalancedClosing_MisuseOfVoidElement()
+        {
+            // Arrange
+            var document = CreateDocument(@"<input></input>");
+
+            var documentNode = Lower(document);
+
+            // Act
+            Pass.Execute(document, documentNode);
+
+            // Assert
+            var method = documentNode.FindPrimaryMethod();
+            Assert.Collection(
+                method.Children,
+                c => Assert.IsType<CSharpCodeIntermediateNode>(c),
+                c => NodeAssert.Element(c, "input"),
+                c => NodeAssert.Element(c, "input"));
+
+            var input2 = NodeAssert.Element(method.Children[2], "input");
+            Assert.Equal(7, input2.Source.Value.AbsoluteIndex);
+            Assert.Equal(0, input2.Source.Value.LineIndex);
+            Assert.Equal(7, input2.Source.Value.CharacterIndex);
+            Assert.Equal(8, input2.Source.Value.Length);
+
+            var diagnostic = Assert.Single(input2.Diagnostics);
+            Assert.Same(BlazorDiagnosticFactory.UnexpectedClosingTagForVoidElement.Id, diagnostic.Id);
+            Assert.Equal(input2.Source, diagnostic.Span);
+        }
+
         [Fact]
         public void Execute_RewritesHtml_UnbalancedClosingTagAtTopLevel()
         {