WIP

TestsHelper.SourceGenerator.Tests/DictionaryHelper.cs

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+using System.Collections.Generic;
+
+namespace TestsHelper.SourceGenerator.Tests;
+
+public static class DictionaryHelper
+{
+    public static Dictionary<TKey, TValue> MergeRight<TKey, TValue>(params Dictionary<TKey, TValue>[] dictionaries) 
+        where TKey : notnull
+    {
+        Dictionary<TKey, TValue> dictionary = new Dictionary<TKey, TValue>();
+
+        foreach (Dictionary<TKey,TValue> d in dictionaries)
+        {
+            foreach ((TKey key, TValue value) in d)
+            {
+                dictionary[key] = value;
+            }
+        }
+        return dictionary;
+    }
+}

TestsHelper.SourceGenerator.Tests/Diff.cs

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+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+using System.Text;
+using DiffPlex;
+using DiffPlex.Chunkers;
+using DiffPlex.DiffBuilder;
+using DiffPlex.DiffBuilder.Model;
+
+namespace TestsHelper.SourceGenerator.Tests;
+
+public static class Diff
+{
+    private static readonly IChunker _lineChunker = new LineChunker();
+    private static readonly IChunker _lineEndingsPreservingChunker = new LineEndingsPreservingChunker();
+    private static readonly InlineDiffBuilder _diffBuilder = new InlineDiffBuilder(new Differ());
+
+    public static bool IsDifferent(string expected, string actual, [NotNullWhen(true)] out string? displayMessage)
+    {
+        displayMessage = default;
+        if (expected == actual) return false;
+
+        DiffPaneModel? diff = _diffBuilder.BuildDiffModel(expected, actual, ignoreWhitespace: false, ignoreCase: false, _lineChunker);
+        var messageBuilder = new StringBuilder();
+        messageBuilder.AppendLine("Actual and expected values differ. Expected shown in baseline of diff:");
+
+        if (!diff.Lines.Any(line => line.Type is ChangeType.Inserted or ChangeType.Deleted))
+        {
+            // We have a failure only caused by line ending differences; recalculate with line endings visible
+            diff = _diffBuilder.BuildDiffModel(expected, actual, ignoreWhitespace: false, ignoreCase: false,
+                _lineEndingsPreservingChunker);
+        }
+
+        foreach (DiffPiece? 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.Replace("\r", "<CR>").Replace("\n", "<LF>"));
+        }
+
+        displayMessage = messageBuilder.ToString();
+        return true;
+    }
+}

TestsHelper.SourceGenerator.Tests/DiffConstraint.cs

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+using NUnit.Framework.Constraints;
+
+namespace TestsHelper.SourceGenerator.Tests;
+
+public class DiffConstraint : Constraint
+{
+    private readonly string _expected;
+
+    public DiffConstraint(string expected)
+    {
+        _expected = expected;
+    }
+
+    public override ConstraintResult ApplyTo<TActual>(TActual actual)
+    {
+        if (actual is string actualValue)
+        {
+            if (Diff.IsDifferent(_expected, actualValue, out var message))
+            {
+                return new Result(this, actual, ConstraintStatus.Failure, message);
+            }
+
+            return new Result(this, actual, ConstraintStatus.Success, string.Empty);
+        }
+
+        return new Result(this, actual, ConstraintStatus.Failure, "Expected is not string");
+    }
+
+    private sealed class Result : ConstraintResult
+    {
+        private readonly string _diffDisplay;
+
+        public Result(IConstraint constraint, object actualValue, ConstraintStatus status, string diffDisplay)
+            : base(constraint, actualValue, status)
+        {
+            _diffDisplay = diffDisplay;
+        }
+
+        public override void WriteMessageTo(MessageWriter writer)
+        {
+            writer.WriteMessageLine(_diffDisplay);
+        }
+    }
+}