Add a way for tooling to determine if a span corresponds to a TagHelper.

- This enables the editor to ask us if an HTML parent happens to be a TagHelper.

#1778

src/Microsoft.VisualStudio.Editor.Razor/RazorSyntaxFactsServiceExtensions.cs

@@ -0,0 +1,37 @@
+// 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 System;
+using Microsoft.AspNetCore.Razor.Language;
+
+namespace Microsoft.VisualStudio.Editor.Razor
+{
+    public static class RazorSyntaxFactsServiceExtensions
+    {
+        public static bool IsTagHelperSpan(this RazorSyntaxFactsService syntaxFactsService, RazorSyntaxTree syntaxTree, SourceSpan span)
+        {
+            if (syntaxFactsService == null)
+            {
+                throw new ArgumentNullException(nameof(syntaxFactsService));
+            }
+
+            if (syntaxTree == null)
+            {
+                // Extra hardening for the case that tooling hasn't retrieved a SyntaxTree yet.
+                return false;
+            }
+
+            var tagHelperSpans = syntaxFactsService.GetTagHelperSpans(syntaxTree);
+            for (var i = 0; i < tagHelperSpans.Count; i++)
+            {
+                var tagHelperSpan = tagHelperSpans[i].Span;
+                if (tagHelperSpan.AbsoluteIndex == span.AbsoluteIndex &&  tagHelperSpan.Length == span.Length)
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+    }
+}

test/Microsoft.VisualStudio.Editor.Razor.Test/RazorSyntaxFactsServiceExtensionsTest.cs

@@ -0,0 +1,82 @@
+// 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 System;
+using Microsoft.AspNetCore.Razor.Language;
+using Xunit;
+
+namespace Microsoft.VisualStudio.Editor.Razor
+{
+    public class RazorSyntaxFactsServiceExtensionsTest
+    {
+        [Fact]
+        public void IsTagHelperSpan_ReturnsTrue()
+        {
+            // Arrange
+            var syntaxTree = GetSyntaxTree(
+@"<div>
+    <taghelper />
+</div>");
+            var location = new SourceSpan(9 + Environment.NewLine.Length, 13);
+            var service = new DefaultRazorSyntaxFactsService();
+
+            // Act
+            var result = service.IsTagHelperSpan(syntaxTree, location);
+
+            // Assert
+            Assert.True(result);
+        }
+
+        [Fact]
+        public void IsTagHelperSpan_ReturnsFalse()
+        {
+            // Arrange
+            var syntaxTree = GetSyntaxTree(
+@"<div>
+    <taghelper></taghelper>
+</div>");
+            var location = new SourceSpan(0, 4);
+            var service = new DefaultRazorSyntaxFactsService();
+
+            // Act
+            var result = service.IsTagHelperSpan(syntaxTree, location);
+
+            // Assert
+            Assert.False(result);
+        }
+
+        [Fact]
+        public void IsTagHelperSpan_NullSyntaxTree_ReturnsFalse()
+        {
+            // Arrange
+            var location = new SourceSpan(0, 4);
+            var service = new DefaultRazorSyntaxFactsService();
+
+            // Act
+            var result = service.IsTagHelperSpan(null, location);
+
+            // Assert
+            Assert.False(result);
+        }
+
+        private static RazorSyntaxTree GetSyntaxTree(string source)
+        {
+            var taghelper = TagHelperDescriptorBuilder.Create("TestTagHelper", "TestAssembly")
+                .TagMatchingRuleDescriptor(rule => rule.RequireTagName("taghelper"))
+                .TypeName("TestTagHelper")
+                .Build();
+            var engine = RazorEngine.CreateDesignTime(builder =>
+            {
+                builder.AddTagHelpers(taghelper);
+            });
+
+            var sourceDocument = RazorSourceDocument.Create(source, "test.cshtml");
+            var addTagHelperImport = RazorSourceDocument.Create("@addTagHelper *, TestAssembly", "import.cshtml");
+            var codeDocument = RazorCodeDocument.Create(sourceDocument, new[] { addTagHelperImport });
+
+            engine.Process(codeDocument);
+
+            return codeDocument.GetSyntaxTree();
+        }
+    }
+}