Add analyzer for [field: ObservableProperty] uses from auto-properties

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

@@ -67,3 +67,11 @@ Rule ID | Category | Severity | Notes
 MVVMTK0037 | CommunityToolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator | Error | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0037
 MVVMTK0038 | CommunityToolkit.Mvvm.SourceGenerators.RelayCommandGenerator | Error | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0038
 MVVMTK0039 | CommunityToolkit.Mvvm.SourceGenerators.RelayCommandGenerator | Warning | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0039
+
+## Release 8.2.2
+
+### New Rules
+
+Rule ID | Category | Severity | Notes
+--------|----------|----------|-------
+MVVMTK0040 | CommunityToolkit.Mvvm.SourceGenerators.ObservablePropertyGenerator | Error | See https://aka.ms/mvvmtoolkit/errors/mvvmtk0040

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

@@ -41,6 +41,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)ComponentModel\TransitiveMembersGenerator.Execute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\AsyncVoidReturningRelayCommandMethodAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\InvalidClassLevelNotifyDataErrorInfoAttributeAnalyzer.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\AutoPropertyWithFieldTargetedObservablePropertyAttributeAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\InvalidClassLevelNotifyPropertyChangedRecipientsAttributeAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\ClassUsingAttributeInsteadOfInheritanceAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\FieldWithOrphanedDependentObservablePropertyAttributesAnalyzer.cs" />

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

@@ -0,0 +1,74 @@
+// 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 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 an error when an auto-property is using <c>[field: ObservableProperty]</c>.
+/// </summary>
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public sealed class AutoPropertyWithFieldTargetedObservablePropertyAttributeAnalyzer : DiagnosticAnalyzer
+{
+    /// <inheritdoc/>
+    public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } = ImmutableArray.Create(AutoPropertyBackingFieldObservableProperty);
+
+    /// <inheritdoc/>
+    public override void Initialize(AnalysisContext context)
+    {
+        context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.Analyze | GeneratedCodeAnalysisFlags.ReportDiagnostics);
+        context.EnableConcurrentExecution();
+
+        context.RegisterCompilationStartAction(static context =>
+        {
+            // Get the symbol for [ObservableProperty]
+            if (context.Compilation.GetTypeByMetadataName("CommunityToolkit.Mvvm.ComponentModel.ObservablePropertyAttribute") is not INamedTypeSymbol observablePropertySymbol)
+            {
+                return;
+            }
+
+            context.RegisterSymbolAction(context =>
+            {
+                // Get the property symbol and the type symbol for the containing type
+                if (context.Symbol is not IPropertySymbol { ContainingType: INamedTypeSymbol typeSymbol } propertySymbol)
+                {
+                    return;
+                }
+
+                foreach (ISymbol memberSymbol in typeSymbol.GetMembers())
+                {
+                    // We're only looking for fields with an associated property
+                    if (memberSymbol is not IFieldSymbol { AssociatedSymbol: IPropertySymbol associatedPropertySymbol })
+                    {
+                        continue;
+                    }
+
+                    // Check that this field is in fact the backing field for the target auto-property
+                    if (!SymbolEqualityComparer.Default.Equals(associatedPropertySymbol, propertySymbol))
+                    {
+                        continue;
+                    }
+
+                    // If the field isn't using [ObservableProperty], this analyzer isn't applicable
+                    if (!memberSymbol.TryGetAttributeWithType(observablePropertySymbol, out AttributeData? attributeData))
+                    {
+                        return;
+                    }
+
+                    // Report the diagnostic on the attribute location
+                    context.ReportDiagnostic(Diagnostic.Create(
+                        AutoPropertyBackingFieldObservableProperty,
+                        attributeData.GetLocation(),
+                        typeSymbol,
+                        propertySymbol));
+                }
+            }, SymbolKind.Property);
+        });
+    }
+}

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

@@ -658,4 +658,20 @@ internal static class DiagnosticDescriptors
         isEnabledByDefault: true,
         description: "All asynchronous methods annotated with [RelayCommand] should return a Task type, to benefit from the additional support provided by AsyncRelayCommand and AsyncRelayCommand<T>.",
         helpLinkUri: "https://aka.ms/mvvmtoolkit/errors/mvvmtk0039");
+
+    /// <summary>
+    /// Gets a <see cref="DiagnosticDescriptor"/> indicating when <c>[ObservableProperty]</c> is used on a generated field of an auto-property.
+    /// <para>
+    /// Format: <c>"The backing field for property {0}.{1} cannot be annotated with [ObservableProperty] (the attribute can only be used directly on fields, and the generator will then handle generating the corresponding property)"</c>.
+    /// </para>
+    /// </summary>
+    public static readonly DiagnosticDescriptor AutoPropertyBackingFieldObservableProperty = new DiagnosticDescriptor(
+        id: "MVVMTK0040",
+        title: "[ObservableProperty] on auto-property backing field",
+        messageFormat: "The backing field for property {0}.{1} cannot be annotated with [ObservableProperty] (the attribute can only be used directly on fields, and the generator will then handle generating the corresponding property)",
+        category: typeof(ObservablePropertyGenerator).FullName,
+        defaultSeverity: DiagnosticSeverity.Error,
+        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");
 }

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

@@ -38,6 +38,21 @@ properties.Value.Value is T argumentValue &&
         return false;
     }
 
+    /// <summary>
+    /// Tries to get the location of the input <see cref="AttributeData"/> instance.
+    /// </summary>
+    /// <param name="attributeData">The input <see cref="AttributeData"/> instance to get the location for.</param>
+    /// <returns>The resulting location for <paramref name="attributeData"/>, if a syntax reference is available.</returns>
+    public static Location? GetLocation(this AttributeData attributeData)
+    {
+        if (attributeData.ApplicationSyntaxReference is { } syntaxReference)
+        {
+            return syntaxReference.SyntaxTree.GetLocation(syntaxReference.Span);
+        }
+
+        return null;
+    }
+
     /// <summary>
     /// Gets a given named argument value from an <see cref="AttributeData"/> instance, or a fallback value.
     /// </summary>

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

@@ -2,9 +2,7 @@
 // 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_3_1_OR_GREATER
 using System.Diagnostics.CodeAnalysis;
-#endif
 using Microsoft.CodeAnalysis;
 
 namespace CommunityToolkit.Mvvm.SourceGenerators.Extensions;
@@ -65,21 +63,37 @@ public static bool HasAttributeWithFullyQualifiedMetadataName(this ISymbol symbo
     }
 
     /// <summary>
-    /// Checks whether or not a given symbol has an attribute with the specified fully qualified metadata name.
+    /// Checks whether or not a given symbol has an attribute with the specified type.
     /// </summary>
     /// <param name="symbol">The input <see cref="ISymbol"/> instance to check.</param>
     /// <param name="typeSymbol">The <see cref="ITypeSymbol"/> instance for the attribute type to look for.</param>
     /// <returns>Whether or not <paramref name="symbol"/> has an attribute with the specified type.</returns>
     public static bool HasAttributeWithType(this ISymbol symbol, ITypeSymbol typeSymbol)
+    {
+        return TryGetAttributeWithType(symbol, typeSymbol, out _);
+    }
+
+    /// <summary>
+    /// Tries to get an attribute with the specified type.
+    /// </summary>
+    /// <param name="symbol">The input <see cref="ISymbol"/> instance to check.</param>
+    /// <param name="typeSymbol">The <see cref="ITypeSymbol"/> instance for the attribute type to look for.</param>
+    /// <param name="attributeData">The resulting attribute, if it was found.</param>
+    /// <returns>Whether or not <paramref name="symbol"/> has an attribute with the specified type.</returns>
+    public static bool TryGetAttributeWithType(this ISymbol symbol, ITypeSymbol typeSymbol, [NotNullWhen(true)] out AttributeData? attributeData)
     {
         foreach (AttributeData attribute in symbol.GetAttributes())
         {
             if (SymbolEqualityComparer.Default.Equals(attribute.AttributeClass, typeSymbol))
             {
+                attributeData = attribute;
+
                 return true;
             }
         }
 
+        attributeData = null;
+
         return false;
     }
 

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

@@ -1820,6 +1820,66 @@ public partial class MyViewModel
         await VerifyAnalyzerDiagnosticsAndSuccessfulGeneration<AsyncVoidReturningRelayCommandMethodAnalyzer>(source, LanguageVersion.CSharp8);
     }
 
+    [TestMethod]
+    public async Task FieldTargetedObservablePropertyAttribute_InstanceAutoProperty()
+    {
+        string source = """
+            using CommunityToolkit.Mvvm.ComponentModel;
+
+            namespace MyApp
+            {
+                public partial class SampleViewModel : ObservableObject
+                {
+                    [field: {|MVVMTK0040:ObservableProperty|}]
+                    public string Name { get; set; }
+                }
+            }
+            """;
+
+        await VerifyAnalyzerDiagnosticsAndSuccessfulGeneration<AutoPropertyWithFieldTargetedObservablePropertyAttributeAnalyzer>(source, LanguageVersion.CSharp8);
+    }
+
+    [TestMethod]
+    public async Task FieldTargetedObservablePropertyAttribute_StaticAutoProperty()
+    {
+        string source = """
+            using CommunityToolkit.Mvvm.ComponentModel;
+
+            namespace MyApp
+            {
+                public partial class SampleViewModel : ObservableObject
+                {
+                    [field: {|MVVMTK0040:ObservableProperty|}]
+                    public static string Name { get; set; }
+                }
+            }
+            """;
+
+        await VerifyAnalyzerDiagnosticsAndSuccessfulGeneration<AutoPropertyWithFieldTargetedObservablePropertyAttributeAnalyzer>(source, LanguageVersion.CSharp8);
+    }
+
+    [TestMethod]
+    public async Task FieldTargetedObservablePropertyAttribute_RecordPrimaryConstructorParameter()
+    {
+        string source = """
+            using CommunityToolkit.Mvvm.ComponentModel;
+
+            namespace MyApp
+            {
+                public partial record SampleViewModel([field: {|MVVMTK0040:ObservableProperty|}] string Name);
+            }
+
+            namespace System.Runtime.CompilerServices
+            {
+                internal static class IsExternalInit
+                {
+                }
+            }
+            """;
+
+        await VerifyAnalyzerDiagnosticsAndSuccessfulGeneration<AutoPropertyWithFieldTargetedObservablePropertyAttributeAnalyzer>(source, LanguageVersion.CSharp9);
+    }
+
     /// <summary>
     /// Verifies the diagnostic errors for a given analyzer, and that all available source generators can run successfully with the input source (including subsequent compilation).
     /// </summary>