Add a code fix provider for invalid DuckType null checks

tracer/src/Datadog.Trace.Tools.Analyzers/DuckTypeAnalyzer/DuckDiagnostics.cs

@@ -0,0 +1,27 @@
+// <copyright file="DuckDiagnostics.cs" company="Datadog">
+// Unless explicitly stated otherwise all files in this repository are licensed under the Apache 2 License.
+// This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2017 Datadog, Inc.
+// </copyright>
+
+using Microsoft.CodeAnalysis;
+
+namespace Datadog.Trace.Tools.Analyzers.DuckTypeAnalyzer;
+
+/// <summary>
+/// Helper class for holding various diagnostics on ducks.
+/// </summary>
+public class DuckDiagnostics
+{
+    /// <summary>
+    /// The DiagnosticID for duck type null check rule.
+    /// </summary>
+    public const string DuckTypeNullCheckDiagnosticId = "DDDUCK001";
+
+    internal static readonly DiagnosticDescriptor ADuckIsNeverNullRule = new(
+        DuckTypeNullCheckDiagnosticId,
+        title: "Checking IDuckType for null",
+        messageFormat: "The IDuckType is almost always non-null, check the Instance for null to ensure we have access to the duck typed object",
+        category: "CodeQuality",
+        defaultSeverity: DiagnosticSeverity.Error,
+        isEnabledByDefault: true);
+}

tracer/src/Datadog.Trace.Tools.Analyzers/DuckTypeAnalyzer/DuckTypeNullCheckAnalyzer.cs

@@ -0,0 +1,275 @@
+// <copyright file="DuckTypeNullCheckAnalyzer.cs" company="Datadog">
+// Unless explicitly stated otherwise all files in this repository are licensed under the Apache 2 License.
+// This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2017 Datadog, Inc.
+// </copyright>
+
+#nullable enable
+
+using System.Collections.Immutable;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Microsoft.CodeAnalysis.Operations;
+
+namespace Datadog.Trace.Tools.Analyzers.DuckTypeAnalyzer
+{
+    /// <summary>
+    /// Checks fo r null checks against IDuckType instances.
+    /// </summary>
+    [DiagnosticAnalyzer(LanguageNames.CSharp)]
+    public sealed class DuckTypeNullCheckAnalyzer : DiagnosticAnalyzer
+    {
+        private const string DatadogIDuckTypeInterface = "Datadog.Trace.DuckTyping.IDuckType";
+
+        /// <summary>
+        /// We exclude certain namespaces from this rule:
+        /// - Activity because we do a log of "as" pattern matching against various runtime implementations and expect null for some
+        /// </summary>
+        private static readonly ImmutableArray<string> ExcludedNamespacePrefixes =
+            ImmutableArray.Create(
+                "Datadog.Trace.Activity");
+
+        /// <inheritdoc/>
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics
+            => ImmutableArray.Create(DuckDiagnostics.ADuckIsNeverNullRule);
+
+        /// <inheritdoc/>
+        public override void Initialize(AnalysisContext context)
+        {
+            // not checking any generated code
+            context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
+            context.EnableConcurrentExecution();
+
+            context.RegisterCompilationStartAction(static compilationContext =>
+            {
+                var duckType = compilationContext.Compilation.GetTypeByMetadataName(DatadogIDuckTypeInterface);
+                if (duckType is null)
+                {
+                    return;
+                }
+
+                compilationContext.RegisterOperationAction(
+                    ctx => AnalyzeBinaryNullCheck(ctx, duckType),
+                    OperationKind.BinaryOperator);
+
+                compilationContext.RegisterOperationAction(
+                    ctx => AnalyzeIsPatternNullCheck(ctx, duckType),
+                    OperationKind.IsPattern);
+            });
+        }
+
+        private static void AnalyzeBinaryNullCheck(OperationAnalysisContext context, INamedTypeSymbol duckType)
+        {
+            // if (duckType == null) or if (duckType != null) is what we looking for (operands can be swapped)
+            // both are technically incorrect
+            var bin = (IBinaryOperation)context.Operation;
+
+            // make sure it is == or !=
+            if (bin.OperatorKind != BinaryOperatorKind.Equals &&
+                bin.OperatorKind != BinaryOperatorKind.NotEquals)
+            {
+                return;
+            }
+
+            // find which side we need, really unsure if anyone has ever written null == duckType :)
+            var leftIsNull = IsNullLiteral(bin.LeftOperand);
+            var rightIsNull = IsNullLiteral(bin.RightOperand);
+            if (!leftIsNull && !rightIsNull)
+            {
+                return;
+            }
+
+            // Look at the non-null side and unwrap casts/boxing to object/dynamic
+            // candidate here is: ConversionOperation Type: object
+            // it is an implicit cast / box to object (unlike the `is null` pattern)
+            // so we need to undo that before we can check if it is a IDuckType
+            var candidate = leftIsNull ? bin.RightOperand : bin.LeftOperand;
+
+            // When we have == or != null it seems that the candidate.Type is just an Object, we need to get the DuckType from it
+            // we can query the SemanticModel to get the actual type, but that proved to be very slow
+            // so we can unwrap it instead
+            var type = UnwrapForType(candidate);
+
+            if (type is null || !ImplementsDuckType(type, duckType) || IsExcluded(type))
+            {
+                return;
+            }
+
+            Report(context);
+        }
+
+        private static void AnalyzeIsPatternNullCheck(OperationAnalysisContext context, INamedTypeSymbol duckType)
+        {
+            var isPattern = (IIsPatternOperation)context.Operation;
+
+            if (!IsNullPattern(isPattern.Pattern))
+            {
+                return;
+            }
+
+            // this just falls through to the default op case as it isn't boxed / casted to object
+            var type = UnwrapForType(isPattern.Value);
+            if (type is null || !ImplementsDuckType(type, duckType) || IsExcluded(type))
+            {
+                return;
+            }
+
+            Report(context);
+        }
+
+        private static void Report(OperationAnalysisContext context)
+        {
+            var diagnostic = Diagnostic.Create(
+                DuckDiagnostics.ADuckIsNeverNullRule,
+                context.Operation.Syntax.GetLocation());
+            context.ReportDiagnostic(diagnostic);
+        }
+
+        private static ITypeSymbol? UnwrapForType(IOperation op)
+        {
+            op = Unwrap(op);
+
+            // The type here will be the actual type like Datadog.Trace.DuckTyping.IDuckType
+            return op.Type;
+        }
+
+        private static IOperation Unwrap(IOperation op)
+        {
+            while (true)
+            {
+                switch (op)
+                {
+                    case IConversionOperation c when c.IsImplicit:
+                        // implicit meaning that there wasn't a (object) cast
+                        // this happens automatically in the `==` and `!=` operators
+                        // c.Operand here is something like => ParameterReferenceOperation Type: Datadog.Trace.DuckTyping.IDuckType
+                        op = c.Operand;
+                        continue;
+
+                    case IConversionOperation c
+                        when c.Type is { SpecialType: SpecialType.System_Object } ||
+                             c.Type is { TypeKind: TypeKind.Dynamic }:
+                        // Explicit cast/as to object or dynamic — unwrap so we can see the original type
+                        op = c.Operand;
+                        continue;
+
+                    default:
+                        return op;
+                }
+            }
+        }
+
+        private static bool ImplementsDuckType(ITypeSymbol type, INamedTypeSymbol duckType)
+        {
+            // where T : IDuckType
+            // where T : IFoo, IDuckType
+            // where T : IFoo
+            // IFoo : IDuckType (in different file)
+            if (type is ITypeParameterSymbol tp)
+            {
+                foreach (var c in tp.ConstraintTypes)
+                {
+                    if (ImplementsDuckType(c, duckType))
+                    {
+                        return true;
+                    }
+                }
+
+                return false;
+            }
+
+            // NOTE: IDuckType? == IDuckType which surprised me
+            if (SymbolEqualityComparer.Default.Equals(type, duckType))
+            {
+                return true;
+            }
+
+            // IFoo : IBar
+            // IBar : IDuckType
+            // NOTE: IFoo : IBar (where IBar implements IDuckType) is handled by the AllInterfaces check below
+            foreach (var i in type.AllInterfaces)
+            {
+                if (SymbolEqualityComparer.Default.Equals(i, duckType))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        private static bool IsExcluded(ITypeSymbol type)
+        {
+            var ns = GetNamespace(type);
+            if (ns is null)
+            {
+                return false;
+            }
+
+            foreach (var prefix in ExcludedNamespacePrefixes)
+            {
+                if (ns.StartsWith(prefix, System.StringComparison.Ordinal))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        private static string? GetNamespace(ITypeSymbol type)
+        {
+            if (type is ITypeParameterSymbol tp)
+            {
+                foreach (var c in tp.ConstraintTypes)
+                {
+                    var nam = GetNamespace(c);
+                    if (nam is not null)
+                    {
+                        return nam;
+                    }
+                }
+
+                return null;
+            }
+
+            // Avoid allocations from "global::"
+            var ns = type.ContainingNamespace?.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat);
+            if (ns is null)
+            {
+                return null;
+            }
+
+            return ns.StartsWith("global::", System.StringComparison.Ordinal) ? ns.Substring(8) : ns;
+        }
+
+        private static bool IsNullLiteral(IOperation operation)
+        {
+            var op = Unwrap(operation);
+            if (op is ILiteralOperation lit && lit.ConstantValue.HasValue)
+            {
+                return lit.ConstantValue.Value is null;
+            }
+
+            return false;
+        }
+
+        private static bool IsNullPattern(IPatternOperation pattern)
+        {
+            // is null
+            if (pattern is IConstantPatternOperation cp && IsNullLiteral(cp.Value))
+            {
+                return true;
+            }
+
+            // is not null => Negated(Constant(null))
+            if (pattern is INegatedPatternOperation neg &&
+                neg.Pattern is IConstantPatternOperation cp2 &&
+                IsNullLiteral(cp2.Value))
+            {
+                return true;
+            }
+
+            return false;
+        }
+    }
+}