Add 'RequiresCSharpLanguageVersionPreviewAnalyzer'

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

@@ -75,3 +75,11 @@ MVVMTK0039 | CommunityToolkit.Mvvm.SourceGenerators.RelayCommandGenerator | Warn
 Rule ID | Category | Severity | Notes
 --------|----------|----------|-------
 MVVMTK0040 | CommunityToolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator | Error | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0040
+
+## Release 8.4.0
+
+### New Rules
+
+Rule ID | Category | Severity | Notes
+--------|----------|----------|-------
+MVVMTK0041 | CommunityToolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator | Error | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0041

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

@@ -46,6 +46,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\ClassUsingAttributeInsteadOfInheritanceAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\FieldWithOrphanedDependentObservablePropertyAttributesAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\FieldReferenceForObservablePropertyFieldAnalyzer.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\RequiresCSharpLanguageVersionPreviewAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\UnsupportedCSharpLanguageVersionAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Suppressors\RelayCommandAttributeWithFieldOrPropertyTargetDiagnosticSuppressor.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Suppressors\ObservablePropertyAttributeWithSupportedTargetDiagnosticSuppressor.cs" />

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

@@ -0,0 +1,65 @@
+// 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.
+
+#if ROSLYN_4_11_0_OR_GREATER
+
+using System.Collections.Immutable;
+using CommunityToolkit.Mvvm.SourceGenerators.Extensions;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+using static CommunityToolkit.Mvvm.SourceGenerators.Diagnostics.DiagnosticDescriptors;
+
+namespace CommunityToolkit.Mvvm.SourceGenerators;
+
+/// <summary>
+/// A diagnostic analyzer that generates errors when a property using [ObservableProperty] on a partial property is in a project with the C# language version not set to preview.
+/// </summary>
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public sealed class RequiresCSharpLanguageVersionPreviewAnalyzer : DiagnosticAnalyzer
+{
+    /// <inheritdoc/>
+    public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } = [CSharpLanguageVersionIsNotPreviewForObservableProperty];
+
+    /// <inheritdoc/>
+    public override void Initialize(AnalysisContext context)
+    {
+        context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.Analyze | GeneratedCodeAnalysisFlags.ReportDiagnostics);
+        context.EnableConcurrentExecution();
+
+        context.RegisterCompilationStartAction(static context =>
+        {
+            // If the language version is set to preview, we'll never emit diagnostics
+            if (context.Compilation.IsLanguageVersionPreview())
+            {
+                return;
+            }
+
+            // Get the symbol for [ObservableProperty]
+            if (context.Compilation.GetTypeByMetadataName("CommunityToolkit.Mvvm.ComponentModel.ObservablePropertyAttribute") is not INamedTypeSymbol observablePropertySymbol)
+            {
+                return;
+            }
+
+            context.RegisterSymbolAction(context =>
+            {
+                // We only want to target partial property definitions (also include non-partial ones for diagnostics)
+                if (context.Symbol is not IPropertySymbol { PartialDefinitionPart: null })
+                {
+                    return;
+                }
+
+                // If the property is using [ObservableProperty], emit the diagnostic
+                if (context.Symbol.TryGetAttributeWithType(observablePropertySymbol, out AttributeData? observablePropertyAttribute))
+                {
+                    context.ReportDiagnostic(Diagnostic.Create(
+                        CSharpLanguageVersionIsNotPreviewForObservableProperty,
+                        observablePropertyAttribute.GetLocation(),
+                        context.Symbol));
+                }
+            }, SymbolKind.Property);
+        });
+    }
+}
+
+#endif

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

@@ -674,4 +674,20 @@ internal static class DiagnosticDescriptors
         isEnabledByDefault: true,
         description: "The backing fields of auto-properties cannot be annotated with [ObservableProperty] (the attribute can only be used directly on fields, and the generator will then handle generating the corresponding property).",
         helpLinkUri: "https://aka.ms/mvvmtoolkit/errors/mvvmtk0040");
+
+    /// <summary>
+    /// Gets a <see cref="DiagnosticDescriptor"/> for a CanvasEffect property with invalid accessors.
+    /// <para>
+    /// Format: <c>"Using [ObservableProperty] on partial properties requires the C# language version to be set to 'preview', as support for the 'field' keyword is needed by the source generators to emit valid code (add <LangVersion>preview</LangVersion> to your .csproj/.props file)"</c>.
+    /// </para>
+    /// </summary>
+    public static readonly DiagnosticDescriptor CSharpLanguageVersionIsNotPreviewForObservableProperty = new(
+        id: "MVVMTK0041",
+        title: "C# language version is not 'preview'",
+        messageFormat: """Using [ObservableProperty] on partial properties requires the C# language version to be set to 'preview', as support for the 'field' keyword is needed by the source generators to emit valid code (add <LangVersion>preview</LangVersion> to your .csproj/.props file)""",
+        category: typeof(ObservablePropertyGenerator).FullName,
+        defaultSeverity: DiagnosticSeverity.Error,
+        isEnabledByDefault: true,
+        description: "The C# language version must be set to 'preview' when using [ObservableProperty] on partial properties for the source generators to emit valid code (the <LangVersion>preview</LangVersion> option must be set in the .csproj/.props file).",
+        helpLinkUri: "https://aka.ms/mvvmtoolkit/errors/mvvmtk0041");
 }