Add utility for verifying razor diagnostics

Author: jjonescz

src/Compiler/test/Microsoft.NET.Sdk.Razor.SourceGenerators.Tests/RazorSourceGeneratorTestsBase.cs

@@ -545,4 +545,18 @@ public static void AssertPair(this RazorEventListener.RazorEvent e, string expec
         Assert.Equal(payload1, e.Payload[0]);
         Assert.Equal(payload2, e.Payload[1]);
     }
+
+    public static void VerifyRazor(
+        this IEnumerable<Diagnostic> diagnostics,
+        Project project,
+        params DiagnosticDescription[] expected)
+    {
+        var mappedDiagnostics = diagnostics.PretendTheseAreCSharpDiagnostics(path =>
+        {
+            var document = project.AdditionalDocuments.Single(d => d.Name == path);
+            Assert.True(document.TryGetText(out var text));
+            return text;
+        });
+        mappedDiagnostics.Verify(expected);
+    }
 }

src/Shared/Microsoft.AspNetCore.Razor.Test.Common/DiagnosticExtensions.cs

@@ -0,0 +1,38 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.Text;
+
+namespace Microsoft.CodeAnalysis;
+
+public static class RazorDiagnosticExtensions
+{
+    /// <summary>
+    /// Provides better experience when verifying diagnostics in tests - includes squiggled text and code snippet.
+    /// </summary>
+    public static IEnumerable<Diagnostic> PretendTheseAreCSharpDiagnostics(
+        this IEnumerable<Diagnostic> diagnostics,
+        Func<string, SourceText> filePathToContent)
+    {
+        var texts = new Dictionary<string, SourceText>();
+        return diagnostics.Select(d =>
+        {
+            if (d.Location is { Kind: LocationKind.ExternalFile } originalLocation)
+            {
+                var mappedSpan = originalLocation.GetMappedLineSpan();
+                var path = mappedSpan.Path;
+                var text = texts.GetOrAdd(path, filePathToContent);
+                var syntaxTree = CSharpSyntaxTree.ParseText(text, path: path);
+                var span = text.Lines.GetTextSpan(mappedSpan.Span);
+                var location = Location.Create(syntaxTree, span);
+                return Diagnostic.Create(d.Descriptor, location);
+            }
+
+            return d;
+        });
+    }
+}