Implement AssertEx.EqualOrDiff

Based on work by @AArnott in dotnet/roslyn-sdk#240

eng/Versions.props

@@ -30,6 +30,7 @@
     <BasicUndoVersion>0.9.3</BasicUndoVersion>
     <BenchmarkDotNetVersion>0.11.1</BenchmarkDotNetVersion>
     <CommandLineParserVersion>2.0.273-beta</CommandLineParserVersion>
+    <DiffPlexVersion>1.4.4</DiffPlexVersion>
     <DropAppVersion>17.144.28413-buildid7983345</DropAppVersion>
     <EnvDTEVersion>8.0.2</EnvDTEVersion>
     <EnvDTE80Version>8.0.0</EnvDTE80Version>

src/Test/Utilities/Portable/Assert/AssertEx.cs

@@ -9,6 +9,9 @@
 using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Text;
+using DiffPlex;
+using DiffPlex.DiffBuilder;
+using DiffPlex.DiffBuilder.Model;
 using Microsoft.CodeAnalysis.Test.Utilities;
 using Xunit;
 
@@ -210,6 +213,48 @@ public static void Equal<T>(ImmutableArray<T> expected, ImmutableArray<T> actual
             Assert.True(false, assertMessage);
         }
 
+        /// <summary>
+        /// Asserts that two strings are equal, and prints a diff between the two if they are not.
+        /// </summary>
+        /// <param name="expected">The expected string. This is presented as the "baseline/before" side in the diff.</param>
+        /// <param name="actual">The actual string. This is presented as the changed or "after" side in the diff.</param>
+        /// <param name="message">The message to precede the diff, if the values are not equal.</param>
+        public static void EqualOrDiff(string expected, string actual, string message = null)
+        {
+            if (expected == actual)
+            {
+                return;
+            }
+
+            var diffBuilder = new InlineDiffBuilder(new Differ());
+            var diff = diffBuilder.BuildDiffModel(expected, actual, ignoreWhitespace: false);
+            var messageBuilder = new StringBuilder();
+            messageBuilder.AppendLine(
+                string.IsNullOrEmpty(message)
+                    ? "Actual and expected values differ. Expected shown in baseline of diff:"
+                    : message);
+
+            foreach (var line in diff.Lines)
+            {
+                switch (line.Type)
+                {
+                    case ChangeType.Inserted:
+                        messageBuilder.Append("+");
+                        break;
+                    case ChangeType.Deleted:
+                        messageBuilder.Append("-");
+                        break;
+                    default:
+                        messageBuilder.Append(" ");
+                        break;
+                }
+
+                messageBuilder.AppendLine(line.Text);
+            }
+
+            Assert.True(false, messageBuilder.ToString());
+        }
+
         public static void NotEqual<T>(IEnumerable<T> expected, IEnumerable<T> actual, IEqualityComparer<T> comparer = null, string message = null,
             string itemSeparator = null, Func<T, string> itemInspector = null)
         {

src/Test/Utilities/Portable/Roslyn.Test.Utilities.csproj

@@ -92,6 +92,7 @@
     </EmbeddedResource>
   </ItemGroup>
   <ItemGroup>
+    <PackageReference Include="DiffPlex" Version="$(DiffPlexVersion)" />
     <PackageReference Include="Microsoft.CodeAnalysis.Test.Resources.Proprietary" Version="$(MicrosoftCodeAnalysisTestResourcesProprietaryVersion)" />
     <PackageReference Include="Microsoft.CSharp" Version="$(MicrosoftCSharpVersion)" />
     <PackageReference Include="Microsoft.DiaSymReader.Converter.Xml" Version="$(MicrosoftDiaSymReaderConverterXmlVersion)" />