Add analyzer "Lookup Must Return Nullable Type"

src/HotChocolate/Core/src/Types.Analyzers/Errors.cs

@@ -256,4 +256,13 @@ public static class Errors
             category: "TypeSystem",
             DiagnosticSeverity.Error,
             isEnabledByDefault: true);
+
+    public static readonly DiagnosticDescriptor LookupReturnsNonNullableType =
+        new(
+            id: "HC0113",
+            title: "Lookup Must Return Nullable Type",
+            messageFormat: "A method or property with the [Lookup] attribute must return a nullable type",
+            category: "TypeSystem",
+            DiagnosticSeverity.Warning,
+            isEnabledByDefault: true);
 }

src/HotChocolate/Core/src/Types.Analyzers/LookupReturnsNonNullableTypeAnalyzer.cs

@@ -0,0 +1,135 @@
+using System.Collections.Immutable;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace HotChocolate.Types.Analyzers;
+
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public sealed class LookupReturnsNonNullableTypeAnalyzer : DiagnosticAnalyzer
+{
+    public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } =
+        [Errors.LookupReturnsNonNullableType];
+
+    public override void Initialize(AnalysisContext context)
+    {
+        context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
+        context.EnableConcurrentExecution();
+        context.RegisterSyntaxNodeAction(AnalyzeMethodDeclaration, SyntaxKind.MethodDeclaration);
+        context.RegisterSyntaxNodeAction(AnalyzePropertyDeclaration, SyntaxKind.PropertyDeclaration);
+    }
+
+    private static void AnalyzeMethodDeclaration(SyntaxNodeAnalysisContext context)
+    {
+        var methodDeclaration = (MethodDeclarationSyntax)context.Node;
+
+        if (!HasLookupAttribute(context, methodDeclaration.AttributeLists))
+        {
+            return;
+        }
+
+        var methodSymbol = context.SemanticModel.GetDeclaredSymbol(methodDeclaration);
+        if (methodSymbol is null)
+        {
+            return;
+        }
+
+        var returnType = UnwrapTaskType(methodSymbol.ReturnType);
+
+        if (IsNullableType(returnType))
+        {
+            return;
+        }
+
+        var diagnostic = Diagnostic.Create(
+            Errors.LookupReturnsNonNullableType,
+            methodDeclaration.ReturnType.GetLocation());
+
+        context.ReportDiagnostic(diagnostic);
+    }
+
+    private static void AnalyzePropertyDeclaration(SyntaxNodeAnalysisContext context)
+    {
+        var propertyDeclaration = (PropertyDeclarationSyntax)context.Node;
+
+        if (!HasLookupAttribute(context, propertyDeclaration.AttributeLists))
+        {
+            return;
+        }
+
+        var propertySymbol = context.SemanticModel.GetDeclaredSymbol(propertyDeclaration);
+        if (propertySymbol is null)
+        {
+            return;
+        }
+
+        var propertyType = UnwrapTaskType(propertySymbol.Type);
+
+        if (IsNullableType(propertyType))
+        {
+            return;
+        }
+
+        var diagnostic = Diagnostic.Create(
+            Errors.LookupReturnsNonNullableType,
+            propertyDeclaration.Type.GetLocation());
+
+        context.ReportDiagnostic(diagnostic);
+    }
+
+    private static bool HasLookupAttribute(
+        SyntaxNodeAnalysisContext context,
+        SyntaxList<AttributeListSyntax> attributeLists)
+    {
+        var semanticModel = context.SemanticModel;
+
+        foreach (var attributeList in attributeLists)
+        {
+            foreach (var attribute in attributeList.Attributes)
+            {
+                var symbolInfo = semanticModel.GetSymbolInfo(attribute);
+                if (symbolInfo.Symbol is not IMethodSymbol attributeSymbol)
+                {
+                    continue;
+                }
+
+                var attributeType = attributeSymbol.ContainingType;
+                if (attributeType.ToDisplayString() == WellKnownAttributes.LookupAttribute)
+                {
+                    return true;
+                }
+            }
+        }
+
+        return false;
+    }
+
+    private static ITypeSymbol UnwrapTaskType(ITypeSymbol typeSymbol)
+    {
+        if (typeSymbol is INamedTypeSymbol namedType
+            && namedType.TypeArguments.Length == 1
+            && namedType.Name is nameof(Task) or nameof(ValueTask))
+        {
+            return namedType.TypeArguments[0];
+        }
+
+        return typeSymbol;
+    }
+
+    private static bool IsNullableType(ITypeSymbol typeSymbol)
+    {
+        if (typeSymbol.NullableAnnotation == NullableAnnotation.Annotated)
+        {
+            return true;
+        }
+
+        if (typeSymbol is INamedTypeSymbol namedType
+            && namedType.OriginalDefinition.SpecialType == SpecialType.System_Nullable_T)
+        {
+            return true;
+        }
+
+        return false;
+    }
+}

src/HotChocolate/Core/src/Types.Analyzers/LookupReturnsNonNullableTypeCodeFixProvider.cs

@@ -0,0 +1,114 @@
+using System.Collections.Immutable;
+using System.Composition;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CodeActions;
+using Microsoft.CodeAnalysis.CodeFixes;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+
+namespace HotChocolate.Types.Analyzers;
+
+[Shared]
+[ExportCodeFixProvider(LanguageNames.CSharp, Name = nameof(LookupReturnsNonNullableTypeCodeFixProvider))]
+public sealed class LookupReturnsNonNullableTypeCodeFixProvider : CodeFixProvider
+{
+    public override ImmutableArray<string> FixableDiagnosticIds { get; } = ["HC0113"];
+
+    public override FixAllProvider GetFixAllProvider()
+        => WellKnownFixAllProviders.BatchFixer;
+
+    public override async Task RegisterCodeFixesAsync(CodeFixContext context)
+    {
+        var root = await context.Document.GetSyntaxRootAsync(context.CancellationToken).ConfigureAwait(false);
+        if (root is null)
+        {
+            return;
+        }
+
+        var diagnostic = context.Diagnostics[0];
+        var diagnosticSpan = diagnostic.Location.SourceSpan;
+
+        var node = root.FindNode(diagnosticSpan);
+
+        // Determine the type syntax to make nullable.
+        TypeSyntax? typeSyntax = null;
+
+        var methodDeclaration = node.AncestorsAndSelf().OfType<MethodDeclarationSyntax>().FirstOrDefault();
+        if (methodDeclaration is not null)
+        {
+            typeSyntax = methodDeclaration.ReturnType;
+        }
+        else
+        {
+            var propertyDeclaration = node.AncestorsAndSelf().OfType<PropertyDeclarationSyntax>().FirstOrDefault();
+            if (propertyDeclaration is not null)
+            {
+                typeSyntax = propertyDeclaration.Type;
+            }
+        }
+
+        if (typeSyntax is null)
+        {
+            return;
+        }
+
+        const string title = "Make return type nullable";
+
+        context.RegisterCodeFix(
+            CodeAction.Create(
+                title: title,
+                createChangedDocument: c => MakeReturnTypeNullableAsync(
+                    context.Document,
+                    typeSyntax,
+                    c),
+                equivalenceKey: title),
+            diagnostic);
+    }
+
+    private static async Task<Document> MakeReturnTypeNullableAsync(
+        Document document,
+        TypeSyntax typeSyntax,
+        CancellationToken cancellationToken)
+    {
+        var root = await document.GetSyntaxRootAsync(cancellationToken).ConfigureAwait(false);
+        if (root is null)
+        {
+            return document;
+        }
+
+        TypeSyntax newTypeSyntax;
+
+        // If the type is Task<T> or ValueTask<T>, wrap the inner type argument.
+        if (typeSyntax is GenericNameSyntax genericName
+            && genericName.TypeArgumentList.Arguments.Count == 1
+            && genericName.Identifier.Text is nameof(Task) or nameof(ValueTask))
+        {
+            var innerType = genericName.TypeArgumentList.Arguments[0];
+
+            // Guard against double-wrapping.
+            if (innerType is NullableTypeSyntax)
+            {
+                return document;
+            }
+
+            var nullableInnerType = SyntaxFactory.NullableType(innerType);
+            var newTypeArgumentList = genericName.TypeArgumentList.WithArguments(
+                SyntaxFactory.SingletonSeparatedList<TypeSyntax>(nullableInnerType));
+            newTypeSyntax = genericName.WithTypeArgumentList(newTypeArgumentList);
+        }
+        else
+        {
+            // Guard against double-wrapping.
+            if (typeSyntax is NullableTypeSyntax)
+            {
+                return document;
+            }
+
+            newTypeSyntax = SyntaxFactory.NullableType(typeSyntax);
+        }
+
+        newTypeSyntax = newTypeSyntax.WithTriviaFrom(typeSyntax);
+        var newRoot = root.ReplaceNode(typeSyntax, newTypeSyntax);
+        return document.WithSyntaxRoot(newRoot);
+    }
+}