Fix [RelayCommand] attribute over methods with no attributes

src/CommunityToolkit.Mvvm.SourceGenerators/CommunityToolkit.Mvvm.SourceGenerators.projitems

@@ -55,6 +55,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Extensions\INamedTypeSymbolExtensions.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Extensions\IncrementalGeneratorInitializationContextExtensions.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Extensions\IncrementalValuesProviderExtensions.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Extensions\MethodDeclarationSyntaxExtensions.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Extensions\SymbolInfoExtensions.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Extensions\ISymbolExtensions.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Extensions\SourceProductionContextExtensions.cs" />

src/CommunityToolkit.Mvvm.SourceGenerators/Extensions/MethodDeclarationSyntaxExtensions.cs

@@ -0,0 +1,33 @@
+// 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="MethodDeclarationSyntax"/> type.
+/// </summary>
+internal static class MethodDeclarationSyntaxExtensions
+{
+    /// <summary>
+    /// Checks whether a given <see cref="MethodDeclarationSyntax"/> has or could potentially have any attribute lists.
+    /// </summary>
+    /// <param name="methodDeclaration">The input <see cref="MethodDeclarationSyntax"/> to check.</param>
+    /// <returns>Whether <paramref name="methodDeclaration"/> has or potentially has any attribute lists.</returns>
+    public static bool HasOrPotentiallyHasAttributeLists(this MethodDeclarationSyntax methodDeclaration)
+    {
+        // If the declaration has any attribute lists, there's nothing left to do
+        if (methodDeclaration.AttributeLists.Count > 0)
+        {
+            return true;
+        }
+
+        // If there are no attributes, check whether the method declaration has the partial keyword. If it
+        // does, there could potentially be attribute lists on the other partial definition/implementation.
+        return methodDeclaration.Modifiers.Any(SyntaxKind.PartialKeyword);
+    }
+}

src/CommunityToolkit.Mvvm.SourceGenerators/Extensions/SyntaxNodeExtensions.cs

@@ -26,8 +26,7 @@ internal static class SyntaxNodeExtensions
     public static bool IsFirstSyntaxDeclarationForSymbol(this SyntaxNode syntaxNode, ISymbol symbol)
     {
         return
-            symbol.DeclaringSyntaxReferences.Length > 0 &&
-            symbol.DeclaringSyntaxReferences[0] is SyntaxReference syntaxReference &&
+            symbol.DeclaringSyntaxReferences is [SyntaxReference syntaxReference, ..] &&
             syntaxReference.SyntaxTree == syntaxNode.SyntaxTree &&
             syntaxReference.Span == syntaxNode.Span;
     }

src/CommunityToolkit.Mvvm.SourceGenerators/Input/RelayCommandGenerator.cs

@@ -27,7 +27,7 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
             context.SyntaxProvider
             .ForAttributeWithMetadataName(
                 "CommunityToolkit.Mvvm.Input.RelayCommandAttribute",
-                static (node, _) => node is MethodDeclarationSyntax { Parent: ClassDeclarationSyntax, AttributeLists.Count: > 0 },
+                static (node, _) => node is MethodDeclarationSyntax { Parent: ClassDeclarationSyntax } methodDeclaration && methodDeclaration.HasOrPotentiallyHasAttributeLists(),
                 static (context, token) =>
                 {
                     if (!context.SemanticModel.Compilation.HasLanguageVersionAtLeastEqualTo(LanguageVersion.CSharp8))

tests/CommunityToolkit.Mvvm.SourceGenerators.UnitTests/Test_SourceGeneratorsCodegen.cs

@@ -268,6 +268,76 @@ partial class MyViewModel
         VerifyGenerateSources(source, new[] { new RelayCommandGenerator() }, ("MyApp.MyViewModel.Test.g.cs", result));
     }
 
+    // See https://github.com/CommunityToolkit/dotnet/issues/632
+    [TestMethod]
+    public void RelayCommandMethodWithPartialDeclarations_TriggersCorrectly()
+    {
+        string source = """
+            using CommunityToolkit.Mvvm.Input;
+
+            #nullable enable
+
+            namespace MyApp;
+
+            partial class MyViewModel
+            {
+                [RelayCommand]
+                private partial void Test1()
+                {
+                }
+
+                private partial void Test1();
+
+                private partial void Test2()
+                {
+                }
+
+                [RelayCommand]
+                private partial void Test2();
+            }
+            """;
+
+        string result1 = """
+            // <auto-generated/>
+            #pragma warning disable
+            #nullable enable
+            namespace MyApp
+            {
+                partial class MyViewModel
+                {
+                    /// <summary>The backing field for <see cref="Test1Command"/>.</summary>
+                    [global::System.CodeDom.Compiler.GeneratedCode("CommunityToolkit.Mvvm.SourceGenerators.RelayCommandGenerator", "8.1.0.0")]
+                    private global::CommunityToolkit.Mvvm.Input.RelayCommand? test1Command;
+                    /// <summary>Gets an <see cref="global::CommunityToolkit.Mvvm.Input.IRelayCommand"/> instance wrapping <see cref="Test1"/>.</summary>
+                    [global::System.CodeDom.Compiler.GeneratedCode("CommunityToolkit.Mvvm.SourceGenerators.RelayCommandGenerator", "8.1.0.0")]
+                    [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+                    public global::CommunityToolkit.Mvvm.Input.IRelayCommand Test1Command => test1Command ??= new global::CommunityToolkit.Mvvm.Input.RelayCommand(new global::System.Action(Test1));
+                }
+            }
+            """;
+
+        string result2 = """
+            // <auto-generated/>
+            #pragma warning disable
+            #nullable enable
+            namespace MyApp
+            {
+                partial class MyViewModel
+                {
+                    /// <summary>The backing field for <see cref="Test2Command"/>.</summary>
+                    [global::System.CodeDom.Compiler.GeneratedCode("CommunityToolkit.Mvvm.SourceGenerators.RelayCommandGenerator", "8.1.0.0")]
+                    private global::CommunityToolkit.Mvvm.Input.RelayCommand? test2Command;
+                    /// <summary>Gets an <see cref="global::CommunityToolkit.Mvvm.Input.IRelayCommand"/> instance wrapping <see cref="Test2"/>.</summary>
+                    [global::System.CodeDom.Compiler.GeneratedCode("CommunityToolkit.Mvvm.SourceGenerators.RelayCommandGenerator", "8.1.0.0")]
+                    [global::System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
+                    public global::CommunityToolkit.Mvvm.Input.IRelayCommand Test2Command => test2Command ??= new global::CommunityToolkit.Mvvm.Input.RelayCommand(new global::System.Action(Test2));
+                }
+            }
+            """;
+
+        VerifyGenerateSources(source, new[] { new RelayCommandGenerator() }, ("MyApp.MyViewModel.Test1.g.cs", result1), ("MyApp.MyViewModel.Test2.g.cs", result2));
+    }
+
     // See https://github.com/CommunityToolkit/dotnet/issues/632
     [TestMethod]
     public void RelayCommandMethodWithForwardedAttributesOverPartialDeclarations_MergesAttributes()