Improve syntactical filtering for messaging/validator generators

CommunityToolkit.Mvvm.SourceGenerators/ComponentModel/ObservableValidatorValidateAllPropertiesGenerator.cs

@@ -25,7 +25,7 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
         IncrementalValuesProvider<ValidationInfo> validationInfo =
             context.SyntaxProvider
             .CreateSyntaxProvider(
-                static (node, _) => node is ClassDeclarationSyntax,
+                static (node, _) => node is ClassDeclarationSyntax classDeclaration && classDeclaration.HasOrPotentiallyHasBaseTypes(),
                 static (context, token) =>
                 {
                     if (!context.SemanticModel.Compilation.HasLanguageVersionAtLeastEqualTo(LanguageVersion.CSharp8))

CommunityToolkit.Mvvm.SourceGenerators/Extensions/TypeDeclarationSyntaxExtensions.cs

@@ -0,0 +1,41 @@
+// 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 more information.
+
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+
+namespace CommunityToolkit.Mvvm.SourceGenerators.Extensions;
+
+/// <summary>
+/// Extension methods for the <see cref="SyntaxNode"/> type.
+/// </summary>
+internal static class TypeDeclarationSyntaxExtensions
+{
+    /// <summary>
+    /// Checks whether a given <see cref="TypeDeclarationSyntax"/> has or could possibly have any base types, using only syntax.
+    /// </summary>
+    /// <param name="typeDeclaration">The input <see cref="TypeDeclarationSyntax"/> instance to check.</param>
+    /// <returns>Whether <paramref name="typeDeclaration"/> has or could possibly have any base types.</returns>
+    public static bool HasOrPotentiallyHasBaseTypes(this TypeDeclarationSyntax typeDeclaration)
+    {
+        // If the base types list is not empty, the type can definitely has implemented interfaces
+        if (typeDeclaration.BaseList is { Types.Count: > 0 })
+        {
+            return true;
+        }
+
+        // If the base types list is empty, check if the type is partial. If it is, it means
+        // that there could be another partial declaration with a non-empty base types list.
+        foreach (SyntaxToken modifier in typeDeclaration.Modifiers)
+        {
+            if (modifier.IsKind(SyntaxKind.PartialKeyword))
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+}

CommunityToolkit.Mvvm.SourceGenerators/Messaging/IMessengerRegisterAllGenerator.cs

@@ -26,7 +26,7 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
         IncrementalValuesProvider<RecipientInfo> recipientInfo =
             context.SyntaxProvider
             .CreateSyntaxProvider(
-                static (node, _) => node is ClassDeclarationSyntax,
+                static (node, _) => node is ClassDeclarationSyntax classDeclaration && classDeclaration.HasOrPotentiallyHasBaseTypes(),
                 static (context, token) =>
                 {
                     if (!context.SemanticModel.Compilation.HasLanguageVersionAtLeastEqualTo(LanguageVersion.CSharp8))