Add IsExternalInit for relevant TFM's

src/ReactiveUI.SourceGenerators/Core/Helpers/AttributeDefinitions.cs

@@ -259,5 +259,32 @@ namespace ReactiveUI.SourceGenerators.WinForms;
 internal sealed class RoutedControlHostAttribute(string? baseType) : Attribute;
 #nullable restore
 #pragma warning restore
+""";
+
+    public const string IsExternalInitType = "System.Runtime.CompilerServices.IsExternalInit";
+    public const string IsExternalInit = """
+// Copyright (c) 2024 .NET Foundation and Contributors. All rights reserved.
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for full license information.
+
+// <auto-generated />
+#pragma warning disable
+
+#if !NET5_0_OR_GREATER
+
+namespace System.Runtime.CompilerServices;
+
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+
+/// <summary>
+/// Reserved to be used by the compiler for tracking metadata. This class should not be used by developers in source code.
+/// </summary>
+[ExcludeFromCodeCoverage]
+[DebuggerNonUserCode]
+static class IsExternalInit;
+
+#endif
 """;
 }

src/ReactiveUI.SourceGenerators/Reactive/ReactiveGenerator.cs

@@ -33,6 +33,9 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
 
             // Add the ReactiveAttribute to the compilation
             ctx.AddSource($"{AttributeDefinitions.ReactiveAttributeType}.g.cs", SourceText.From(AttributeDefinitions.ReactiveAttribute, Encoding.UTF8));
+
+            // Add the IsExternalInit type to the compilation
+            ctx.AddSource($"{AttributeDefinitions.IsExternalInitType}.g.cs", SourceText.From(AttributeDefinitions.IsExternalInit, Encoding.UTF8));
         });
 
         // Gather info for all annotated command methods (starting from method declarations with at least one attribute)