Add JsonExtensionData support

Author: xoofx

src/Tomlyn.Tests/NewApiExtensionDataTests.cs

@@ -0,0 +1,71 @@
+using System.Collections.Generic;
+using System.Text.Json.Serialization;
+using NUnit.Framework;
+
+namespace Tomlyn.Tests;
+
+public sealed class NewApiExtensionDataTests
+{
+    private sealed class ExtensionDataModel
+    {
+        public int Known { get; set; }
+
+        [JsonExtensionData]
+        public Dictionary<string, object?> Extra { get; set; } = new();
+    }
+
+    private sealed class NullableExtensionDataModel
+    {
+        public int Known { get; set; }
+
+        [JsonExtensionData]
+        public Dictionary<string, object?>? Extra { get; set; }
+    }
+
+    [Test]
+    public void Deserialize_StoresUnknownKeys_IntoExtensionData()
+    {
+        var model = TomlSerializer.Deserialize<ExtensionDataModel>("Known = 1\nUnknown = \"x\"\n");
+        Assert.That(model, Is.Not.Null);
+        Assert.That(model!.Known, Is.EqualTo(1));
+        Assert.That(model.Extra.ContainsKey("Unknown"), Is.True);
+        Assert.That(model.Extra["Unknown"], Is.EqualTo("x"));
+    }
+
+    [Test]
+    public void Serialize_MergesExtensionData_IntoSurroundingTable()
+    {
+        var model = new ExtensionDataModel
+        {
+            Known = 1,
+            Extra = new Dictionary<string, object?> { ["Unknown"] = "x" },
+        };
+
+        var toml = TomlSerializer.Serialize(model);
+        Assert.That(toml, Does.Contain("Known"));
+        Assert.That(toml, Does.Contain("Unknown"));
+    }
+
+    [Test]
+    public void Serialize_ThrowsOnExtensionDataKeyConflict()
+    {
+        var model = new ExtensionDataModel
+        {
+            Known = 1,
+            Extra = new Dictionary<string, object?> { ["Known"] = 2 },
+        };
+
+        Assert.Throws<TomlException>(() => TomlSerializer.Serialize(model));
+    }
+
+    [Test]
+    public void Deserialize_InitializesNullExtensionDataDictionary_WhenSettable()
+    {
+        var model = TomlSerializer.Deserialize<NullableExtensionDataModel>("Known = 1\nUnknown = 2\n");
+        Assert.That(model, Is.Not.Null);
+        Assert.That(model!.Extra, Is.Not.Null);
+        Assert.That(model.Extra!.ContainsKey("Unknown"), Is.True);
+        Assert.That(model.Extra["Unknown"], Is.EqualTo(2L));
+    }
+}
+