New info diagnostic for WinRT AOT compatibility when C# is not 'preview'

src/CommunityToolkit.Mvvm.SourceGenerators/AnalyzerReleases.Shipped.md

@@ -92,3 +92,4 @@ MVVMTK0047 | CommunityToolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator
 MVVMTK0048 | CommunityToolkit.Mvvm.SourceGenerators.RelayCommandGenerator | Warning | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0048
 MVVMTK0049 | CommunityToolkit.Mvvm.SourceGenerators.INotifyPropertyChangedGenerator | Warning | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0049
 MVVMTK0050 | CommunityToolkit.Mvvm.SourceGenerators.ObservableObjectGenerator | Warning | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0050
+MVVMTK0051 | CommunityToolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator | Info | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0050

src/CommunityToolkit.Mvvm.SourceGenerators/Diagnostics/Analyzers/WinRTObservablePropertyOnFieldsIsNotAotCompatibleAnalyzer.cs

@@ -6,6 +6,7 @@
 
 using System.Collections.Immutable;
 using System.Linq;
+using System.Threading;
 using CommunityToolkit.Mvvm.SourceGenerators.Extensions;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.Diagnostics;
@@ -20,7 +21,9 @@ namespace CommunityToolkit.Mvvm.SourceGenerators;
 public sealed class WinRTObservablePropertyOnFieldsIsNotAotCompatibleAnalyzer : DiagnosticAnalyzer
 {
     /// <inheritdoc/>
-    public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } = ImmutableArray.Create(WinRTObservablePropertyOnFieldsIsNotAotCompatible);
+    public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } = ImmutableArray.Create(
+        WinRTObservablePropertyOnFieldsIsNotAotCompatible,
+        WinRTObservablePropertyOnFieldsIsNotAotCompatibleCompilationEndInfo);
 
     /// <inheritdoc/>
     public override void Initialize(AnalysisContext context)
@@ -42,6 +45,9 @@ public override void Initialize(AnalysisContext context)
                 return;
             }
 
+            // Track whether we produced any diagnostics, for the compilation end scenario
+            AttributeData? firstObservablePropertyAttribute = null;
+
             context.RegisterSymbolAction(context =>
             {
                 // Ensure we do have a valid field
@@ -51,7 +57,7 @@ public override void Initialize(AnalysisContext context)
                 }
 
                 // Emit a diagnostic if the field is using the [ObservableProperty] attribute
-                if (fieldSymbol.HasAttributeWithType(observablePropertySymbol))
+                if (fieldSymbol.TryGetAttributeWithType(observablePropertySymbol, out AttributeData? observablePropertyAttribute))
                 {
                     context.ReportDiagnostic(Diagnostic.Create(
                         WinRTObservablePropertyOnFieldsIsNotAotCompatible,
@@ -61,8 +67,32 @@ public override void Initialize(AnalysisContext context)
                             .Add(FieldReferenceForObservablePropertyFieldAnalyzer.PropertyNameKey, ObservablePropertyGenerator.Execute.GetGeneratedPropertyName(fieldSymbol)),
                         fieldSymbol.ContainingType,
                         fieldSymbol.Name));
+
+                    // Notify that we did produce at least one diagnostic. Note: callbacks can run in parallel, so the order
+                    // is not guaranteed. As such, there's no point in using an interlocked compare exchange operation here,
+                    // since we couldn't rely on the value being written actually being the "first" occurrence anyway.
+                    // So we can just do a normal volatile read for better performance.
+                    Volatile.Write(ref firstObservablePropertyAttribute, observablePropertyAttribute);
                 }
             }, SymbolKind.Field);
+
+            // If C# preview is already in use, we can stop here. The last diagnostic is only needed when partial properties
+            // cannot be used, to inform developers that they'll need to bump the language version to enable the code fixer.
+            if (context.Compilation.IsLanguageVersionPreview())
+            {
+                return;
+            }
+
+            context.RegisterCompilationEndAction(context =>
+            {
+                // If we have produced at least one diagnostic, also emit the info message
+                if (Volatile.Read(ref firstObservablePropertyAttribute) is { } observablePropertyAttribute)
+                {
+                    context.ReportDiagnostic(Diagnostic.Create(
+                        WinRTObservablePropertyOnFieldsIsNotAotCompatibleCompilationEndInfo,
+                        observablePropertyAttribute.GetLocation()));
+                }
+            });
         });
     }
 }

src/CommunityToolkit.Mvvm.SourceGenerators/Diagnostics/DiagnosticDescriptors.cs

@@ -844,4 +844,21 @@ internal static class DiagnosticDescriptors
         isEnabledByDefault: true,
         description: "Using the [ObservableObject] attribute on types is not AOT compatible in WinRT scenarios (such as UWP XAML and WinUI 3 apps), and they should derive from ObservableObject instead (as it allows the CsWinRT generators to correctly produce the necessary WinRT marshalling code).",
         helpLinkUri: "https://aka.ms/mvvmtoolkit/errors/mvvmtk0050");
+
+    /// <summary>
+    /// Gets a <see cref="DiagnosticDescriptor"/> for when <c>[ObservableProperty]</c> is used on a field in WinRT scenarios.
+    /// <para>
+    /// Format: <c>"This project produced one or more 'MVVMTK0045' warnings due to [ObservableProperty] being used on fields, which is not AOT compatible in WinRT scenarios, but it can't enable partial properties and the associated code fixer because 'LangVersion' is not set to 'preview' (setting 'LangVersion=preview' is required to use [ObservableProperty] on partial properties and address these warnings)"</c>.
+    /// </para>
+    /// </summary>
+    public static readonly DiagnosticDescriptor WinRTObservablePropertyOnFieldsIsNotAotCompatibleCompilationEndInfo = new(
+        id: "MVVMTK0051",
+        title: "Using [ObservableProperty] with WinRT and AOT requires 'LangVersion=preview'",
+        messageFormat: """This project produced one or more 'MVVMTK0045' warnings due to [ObservableProperty] being used on fields, which is not AOT compatible in WinRT scenarios, but it can't enable partial properties and the associated code fixer because 'LangVersion' is not set to 'preview' (setting 'LangVersion=preview' is required to use [ObservableProperty] on partial properties and address these warnings)""",
+        category: typeof(ObservablePropertyGenerator).FullName,
+        defaultSeverity: DiagnosticSeverity.Info,
+        isEnabledByDefault: true,
+        description: "Project producing one or more 'MVVMTK0045' warnings due to [ObservableProperty] being used on fields, which is not AOT compatible in WinRT scenarios, should set 'LangVersion' to 'preview' to enable partial properties and the associated code fixer because (setting 'LangVersion=preview' is required to use [ObservableProperty] on partial properties and address these warnings).",
+        helpLinkUri: "https://aka.ms/mvvmtoolkit/errors/mvvmtk0051",
+        customTags: WellKnownDiagnosticTags.CompilationEnd);
 }

tests/CommunityToolkit.Mvvm.SourceGenerators.Roslyn4110.UnitTests/Test_SourceGeneratorsDiagnostics.cs

@@ -434,6 +434,28 @@ await CSharpAnalyzerWithLanguageVersionTest<WinRTObservablePropertyOnFieldsIsNot
             editorconfig: [("_MvvmToolkitIsUsingWindowsRuntimePack", true), ("CsWinRTAotOptimizerEnabled", "auto")]);
     }
 
+    [TestMethod]
+    public async Task WinRTObservablePropertyOnFieldsIsNotAotCompatibleAnalyzer_TargetingWindows_CsWinRTAotOptimizerEnabled_Auto_NotCSharpPreview_Warns_WithCompilationWarning()
+    {
+        const string source = """
+            using CommunityToolkit.Mvvm.ComponentModel;
+            
+            namespace MyApp
+            {
+                public partial class SampleViewModel : ObservableObject
+                {            
+                    [{|MVVMTK0051:ObservableProperty|}]            
+                    private string {|MVVMTK0045:name|};
+                }
+            }
+            """;
+
+        await CSharpAnalyzerWithLanguageVersionTest<WinRTObservablePropertyOnFieldsIsNotAotCompatibleAnalyzer>.VerifyAnalyzerAsync(
+            source,
+            LanguageVersion.CSharp12,
+            editorconfig: [("_MvvmToolkitIsUsingWindowsRuntimePack", true), ("CsWinRTAotOptimizerEnabled", "auto")]);
+    }
+
     [TestMethod]
     public async Task WinRTObservablePropertyOnFieldsIsNotAotCompatibleAnalyzer_TargetingWindows_CsWinRTAotOptimizerEnabled_True_NoXaml_Level1_DoesNotWarn()
     {