Add 'PropertyNameCollisionObservablePropertyAttributeAnalyzer'

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

@@ -40,6 +40,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)ComponentModel\TransitiveMembersGenerator.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)ComponentModel\TransitiveMembersGenerator.Execute.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\AsyncVoidReturningRelayCommandMethodAnalyzer.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\PropertyNameCollisionObservablePropertyAttributeAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\InvalidTargetObservablePropertyAttributeAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\InvalidClassLevelNotifyDataErrorInfoAttributeAnalyzer.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Diagnostics\Analyzers\AutoPropertyWithFieldTargetedObservablePropertyAttributeAnalyzer.cs" />

src/CommunityToolkit.Mvvm.SourceGenerators/ComponentModel/ObservablePropertyGenerator.Execute.cs

@@ -166,12 +166,6 @@ public static bool TryGetInfo(
             // Check for name collisions (only for fields)
             if (fieldName == propertyName && memberSyntax.IsKind(SyntaxKind.FieldDeclaration))
             {
-                builder.Add(
-                    ObservablePropertyNameCollisionError,
-                    memberSymbol,
-                    memberSymbol.ContainingType,
-                    memberSymbol.Name);
-
                 propertyInfo = null;
                 diagnostics = builder.ToImmutable();
 

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

@@ -46,7 +46,7 @@ public override void Initialize(AnalysisContext context)
                 }
 
                 // Ensure we do have the [ObservableProperty] attribute
-                if (!context.Symbol.TryGetAttributeWithType(observablePropertySymbol, out AttributeData? attributeDataobservablePropertyAttribute))
+                if (!context.Symbol.HasAttributeWithType(observablePropertySymbol))
                 {
                     return;
                 }

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

@@ -0,0 +1,63 @@
+// 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 System.Linq;
+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 a generated property from <c>[ObservableProperty]</c> would collide with the field name.
+/// </summary>
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public sealed class PropertyNameCollisionObservablePropertyAttributeAnalyzer : DiagnosticAnalyzer
+{
+    /// <inheritdoc/>
+    public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } = ImmutableArray.Create(ObservablePropertyNameCollisionError);
+
+    /// <inheritdoc/>
+    public override void Initialize(AnalysisContext context)
+    {
+        context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
+        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 =>
+            {
+                // Ensure we do have a valid field
+                if (context.Symbol is not IFieldSymbol fieldSymbol)
+                {
+                    return;
+                }
+
+                // We only care if the field has [ObservableProperty]
+                if (!fieldSymbol.HasAttributeWithType(observablePropertySymbol))
+                {
+                    return;
+                }
+
+                // Emit the diagnostic if there is a name collision
+                if (fieldSymbol.Name == ObservablePropertyGenerator.Execute.GetGeneratedPropertyName(fieldSymbol))
+                {
+                    context.ReportDiagnostic(Diagnostic.Create(
+                        ObservablePropertyNameCollisionError,
+                        fieldSymbol.Locations.FirstOrDefault(),
+                        fieldSymbol.ContainingType,
+                        fieldSymbol.Name));
+                }
+            }, SymbolKind.Field);
+        });
+    }
+}

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

@@ -664,7 +664,7 @@ private async Task GreetUserAsync(User user)
     }
 
     [TestMethod]
-    public void NameCollisionForGeneratedObservableProperty()
+    public async Task NameCollisionForGeneratedObservableProperty_PascalCaseField_Warns()
     {
         string source = """
             using CommunityToolkit.Mvvm.ComponentModel;
@@ -674,12 +674,51 @@ namespace MyApp
                 public partial class SampleViewModel : ObservableObject
                 {
                     [ObservableProperty]
-                    private string Name;
+                    private string {|MVVMTK0014:Name|};
                 }
             }
             """;
 
-        VerifyGeneratedDiagnostics<ObservablePropertyGenerator>(source, "MVVMTK0014");
+        await VerifyAnalyzerDiagnosticsAndSuccessfulGeneration<PropertyNameCollisionObservablePropertyAttributeAnalyzer>(source, LanguageVersion.CSharp8);
+    }
+
+    [TestMethod]
+    public async Task NameCollisionForGeneratedObservableProperty_CamelCaseField_DoesNotWarn()
+    {
+        string source = """
+            using CommunityToolkit.Mvvm.ComponentModel;
+
+            namespace MyApp
+            {
+                public partial class SampleViewModel : ObservableObject
+                {
+                    [ObservableProperty]
+                    private string name;
+                }
+            }
+            """;
+
+        // Using C# 9 here because the generated code will emit [MemberNotNull] on the property setter, which requires C# 9
+        await VerifyAnalyzerDiagnosticsAndSuccessfulGeneration<PropertyNameCollisionObservablePropertyAttributeAnalyzer>(source, LanguageVersion.CSharp9);
+    }
+
+    [TestMethod]
+    public async Task NameCollisionForGeneratedObservableProperty_PascalCaseProperty_DoesNotWarn()
+    {
+        string source = """
+            using CommunityToolkit.Mvvm.ComponentModel;
+
+            namespace MyApp
+            {
+                public partial class SampleViewModel : ObservableObject
+                {
+                    [ObservableProperty]
+                    private string Name { get; set; }
+                }
+            }
+            """;
+
+        await VerifyAnalyzerDiagnosticsAndSuccessfulGeneration<PropertyNameCollisionObservablePropertyAttributeAnalyzer>(source, LanguageVersion.CSharp8);
     }
 
     [TestMethod]