Fix partial methods support with Roslyn 4.0

Also ensure a consistent order for forwarded attributes

Author: Sergio0694

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

@@ -1016,18 +1016,20 @@ static void GatherForwardedAttributes(
                 }
             }
 
-            // Gather attributes from the method declaration
-            GatherForwardedAttributes(methodSymbol, semanticModel, token, in diagnostics, in fieldAttributesInfo, in propertyAttributesInfo);
-
             // If the method is a partial definition, also gather attributes from the implementation part
-            if (methodSymbol is { IsPartialDefinition: true, PartialImplementationPart: { } partialImplementation })
+            if (methodSymbol is { IsPartialDefinition: true } or { PartialDefinitionPart: not null })
             {
+                IMethodSymbol partialDefinition = methodSymbol.PartialDefinitionPart ?? methodSymbol;
+                IMethodSymbol partialImplementation = methodSymbol.PartialImplementationPart ?? methodSymbol;
+
+                // We always give priority to the partial definition, to ensure a predictable and testable ordering
+                GatherForwardedAttributes(partialDefinition, semanticModel, token, in diagnostics, in fieldAttributesInfo, in propertyAttributesInfo);
                 GatherForwardedAttributes(partialImplementation, semanticModel, token, in diagnostics, in fieldAttributesInfo, in propertyAttributesInfo);
             }
-            else if (methodSymbol is { IsPartialDefinition: false, PartialDefinitionPart: { } partialDefinition })
+            else
             {
-                // If the method is a partial implementation, also gather attributes from the definition part
-                GatherForwardedAttributes(partialDefinition, semanticModel, token, in diagnostics, in fieldAttributesInfo, in propertyAttributesInfo);
+                // If the method is not a partial definition/implementation, just gather attributes from the method with no modifications
+                GatherForwardedAttributes(methodSymbol, semanticModel, token, in diagnostics, in fieldAttributesInfo, in propertyAttributesInfo);
             }
 
             fieldAttributes = fieldAttributesInfo.ToImmutable();

src/CommunityToolkit.Mvvm.SourceGenerators/Polyfills/SyntaxValueProviderExtensions.cs

@@ -59,6 +59,15 @@ public static IncrementalValuesProvider<T> ForAttributeWithMetadataName<T>(
                         return null;
                     }
 
+                    // Edge case: if the symbol is a partial method, skip the implementation part and only process the partial method
+                    // definition. This is needed because attributes will be reported as available on both the definition and the
+                    // implementation part. To avoid generating duplicate files, we only give priority to the definition part.
+                    // On Roslyn 4.3+, ForAttributeWithMetadataName will already only return the symbol the attribute was located on.
+                    if (symbol is IMethodSymbol { IsPartialDefinition: false, PartialDefinitionPart: not null })
+                    {
+                        return null;
+                    }
+
                     // Create the GeneratorAttributeSyntaxContext value to pass to the input transform. The attributes array
                     // will only ever have a single value, but that's fine with the attributes the various generators look for.
                     GeneratorAttributeSyntaxContext syntaxContext = new(

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

@@ -322,14 +322,14 @@ 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")]
-                    [global::MyApp.ValueAttribute(0)]
                     [global::MyApp.ValueAttribute(2)]
+                    [global::MyApp.ValueAttribute(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]
-                    [global::MyApp.ValueAttribute(1)]
                     [global::MyApp.ValueAttribute(3)]
+                    [global::MyApp.ValueAttribute(1)]
                     public global::CommunityToolkit.Mvvm.Input.IRelayCommand Test1Command => test1Command ??= new global::CommunityToolkit.Mvvm.Input.RelayCommand(new global::System.Action(Test1));
                 }
             }