Bind and emit lambda attributes

src/Compilers/CSharp/Portable/Binder/Binder.QueryUnboundLambdaState.cs

@@ -7,6 +7,7 @@
 using System;
 using System.Collections.Immutable;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis.CSharp
@@ -31,6 +32,7 @@ public QueryUnboundLambdaState(Binder binder, RangeVariableMap rangeVariableMap,
 
             public override string ParameterName(int index) { return _parameters[index].Name; }
             public override bool ParameterIsDiscard(int index) { return false; }
+            public override SyntaxList<AttributeListSyntax> ParameterAttributes(int index) => default;
             public override bool HasNames { get { return true; } }
             public override bool HasSignature { get { return true; } }
             public override bool HasExplicitlyTypedParameterList { get { return false; } }

src/Compilers/CSharp/Portable/Binder/Binder_Lambda.cs

@@ -7,7 +7,6 @@
 using System.Diagnostics;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
-using Microsoft.CodeAnalysis.Collections;
 using Microsoft.CodeAnalysis.PooledObjects;
 
 namespace Microsoft.CodeAnalysis.CSharp
@@ -42,12 +41,18 @@ private UnboundLambda AnalyzeAnonymousFunction(
             var names = default(ImmutableArray<string>);
             var refKinds = default(ImmutableArray<RefKind>);
             var types = default(ImmutableArray<TypeWithAnnotations>);
+            var attributes = default(ImmutableArray<SyntaxList<AttributeListSyntax>>);
 
             var namesBuilder = ArrayBuilder<string>.GetInstance();
             ImmutableArray<bool> discardsOpt = default;
             SeparatedSyntaxList<ParameterSyntax>? parameterSyntaxList = null;
             bool hasSignature;
 
+            if (syntax is LambdaExpressionSyntax lambdaSyntax)
+            {
+                checkAttributes(syntax, lambdaSyntax.AttributeLists, diagnostics);
+            }
+
             switch (syntax.Kind())
             {
                 default:
@@ -87,6 +92,7 @@ private UnboundLambda AnalyzeAnonymousFunction(
 
                 var typesBuilder = ArrayBuilder<TypeWithAnnotations>.GetInstance();
                 var refKindsBuilder = ArrayBuilder<RefKind>.GetInstance();
+                var attributesBuilder = ArrayBuilder<SyntaxList<AttributeListSyntax>>.GetInstance();
 
                 // In the batch compiler case we probably should have given a syntax error if the
                 // user did something like (int x, y)=>x+y -- but in the IDE scenario we might be in
@@ -104,10 +110,7 @@ private UnboundLambda AnalyzeAnonymousFunction(
                         underscoresCount++;
                     }
 
-                    foreach (var attributeList in p.AttributeLists)
-                    {
-                        Error(diagnostics, ErrorCode.ERR_AttributesNotAllowed, attributeList);
-                    }
+                    checkAttributes(syntax, p.AttributeLists, diagnostics);
 
                     if (p.Default != null)
                     {
@@ -167,6 +170,7 @@ private UnboundLambda AnalyzeAnonymousFunction(
                     namesBuilder.Add(p.Identifier.ValueText);
                     typesBuilder.Add(type);
                     refKindsBuilder.Add(refKind);
+                    attributesBuilder.Add(syntax.Kind() == SyntaxKind.ParenthesizedLambdaExpression ? p.AttributeLists : default);
                 }
 
                 discardsOpt = computeDiscards(parameterSyntaxList.Value, underscoresCount);
@@ -181,8 +185,14 @@ private UnboundLambda AnalyzeAnonymousFunction(
                     refKinds = refKindsBuilder.ToImmutable();
                 }
 
+                if (attributesBuilder.Any(a => a.Count > 0))
+                {
+                    attributes = attributesBuilder.ToImmutable();
+                }
+
                 typesBuilder.Free();
                 refKindsBuilder.Free();
+                attributesBuilder.Free();
             }
 
             if (hasSignature)
@@ -192,7 +202,7 @@ private UnboundLambda AnalyzeAnonymousFunction(
 
             namesBuilder.Free();
 
-            return new UnboundLambda(syntax, this, diagnostics.AccumulatesDependencies, refKinds, types, names, discardsOpt, isAsync, isStatic);
+            return new UnboundLambda(syntax, this, diagnostics.AccumulatesDependencies, attributes, refKinds, types, names, discardsOpt, isAsync, isStatic);
 
             static ImmutableArray<bool> computeDiscards(SeparatedSyntaxList<ParameterSyntax> parameters, int underscoresCount)
             {
@@ -210,9 +220,24 @@ static ImmutableArray<bool> computeDiscards(SeparatedSyntaxList<ParameterSyntax>
 
                 return discardsBuilder.ToImmutableAndFree();
             }
+
+            static void checkAttributes(AnonymousFunctionExpressionSyntax syntax, SyntaxList<AttributeListSyntax> attributeLists, BindingDiagnosticBag diagnostics)
+            {
+                foreach (var attributeList in attributeLists)
+                {
+                    if (syntax.Kind() == SyntaxKind.ParenthesizedLambdaExpression)
+                    {
+                        MessageID.IDS_FeatureLambdaAttributes.CheckFeatureAvailability(diagnostics, attributeList);
+                    }
+                    else
+                    {
+                        Error(diagnostics, syntax.Kind() == SyntaxKind.SimpleLambdaExpression ? ErrorCode.ERR_AttributesRequireParenthesizedLambdaExpression : ErrorCode.ERR_AttributesNotAllowed, attributeList);
+                    }
+                }
+            }
         }
 
-        private void CheckParenthesizedLambdaParameters(
+        private static void CheckParenthesizedLambdaParameters(
             SeparatedSyntaxList<ParameterSyntax> parameterSyntaxList, BindingDiagnosticBag diagnostics)
         {
             if (parameterSyntaxList.Count > 0)

src/Compilers/CSharp/Portable/BoundTree/UnboundLambda.cs

@@ -344,13 +344,14 @@ public UnboundLambda(
             CSharpSyntaxNode syntax,
             Binder binder,
             bool withDependencies,
+            ImmutableArray<SyntaxList<AttributeListSyntax>> attributes,
             ImmutableArray<RefKind> refKinds,
             ImmutableArray<TypeWithAnnotations> types,
             ImmutableArray<string> names,
             ImmutableArray<bool> discardsOpt,
             bool isAsync,
             bool isStatic)
-            : this(syntax, new PlainUnboundLambdaState(binder, names, discardsOpt, types, refKinds, isAsync, isStatic, includeCache: true), withDependencies, !types.IsDefault && types.Any(t => t.Type?.Kind == SymbolKind.ErrorType))
+            : this(syntax, new PlainUnboundLambdaState(binder, attributes, names, discardsOpt, types, refKinds, isAsync, isStatic, includeCache: true), withDependencies, !types.IsDefault && types.Any(t => t.Type?.Kind == SymbolKind.ErrorType))
         {
             Debug.Assert(binder != null);
             Debug.Assert(syntax.IsAnonymousFunction());
@@ -409,6 +410,7 @@ public TypeWithAnnotations InferReturnType(ConversionsBase conversions, NamedTyp
         public bool GenerateSummaryErrors(BindingDiagnosticBag diagnostics) { return Data.GenerateSummaryErrors(diagnostics); }
         public bool IsAsync { get { return Data.IsAsync; } }
         public bool IsStatic => Data.IsStatic;
+        public SyntaxList<AttributeListSyntax> ParameterAttributes(int index) { return Data.ParameterAttributes(index); }
         public TypeWithAnnotations ParameterTypeWithAnnotations(int index) { return Data.ParameterTypeWithAnnotations(index); }
         public TypeSymbol ParameterType(int index) { return ParameterTypeWithAnnotations(index).Type; }
         public Location ParameterLocation(int index) { return Data.ParameterLocation(index); }
@@ -472,6 +474,7 @@ internal UnboundLambdaState WithCaching(bool includeCache)
         public abstract MessageID MessageID { get; }
         public abstract string ParameterName(int index);
         public abstract bool ParameterIsDiscard(int index);
+        public abstract SyntaxList<AttributeListSyntax> ParameterAttributes(int index);
         public abstract bool HasSignature { get; }
         public abstract bool HasExplicitlyTypedParameterList { get; }
         public abstract int ParameterCount { get; }
@@ -602,6 +605,9 @@ private BoundLambda ReallyBind(NamedTypeSymbol delegateType)
                 block = BindLambdaBody(lambdaSymbol, lambdaBodyBinder, diagnostics);
             }
 
+            // PROTOTYPE: Test that we're reporting diagnostics even in the case where we re-used the lambda above.
+            lambdaSymbol.GetDeclarationDiagnostics(diagnostics);
+
             if (lambdaSymbol.RefKind == CodeAnalysis.RefKind.RefReadOnly)
             {
                 compilation.EnsureIsReadOnlyAttributeExists(diagnostics, lambdaSymbol.DiagnosticLocation, modifyCompilation: false);
@@ -676,6 +682,7 @@ internal LambdaSymbol CreateLambdaSymbol(
             ImmutableArray<RefKind> parameterRefKinds,
             RefKind refKind)
             => new LambdaSymbol(
+                Binder,
                 Binder.Compilation,
                 containingSymbol,
                 _unboundLambda,
@@ -1159,6 +1166,7 @@ private static int CanonicallyCompareDiagnostics(Diagnostic x, Diagnostic y)
 
     internal sealed class PlainUnboundLambdaState : UnboundLambdaState
     {
+        private readonly ImmutableArray<SyntaxList<AttributeListSyntax>> _parameterAttributes;
         private readonly ImmutableArray<string> _parameterNames;
         private readonly ImmutableArray<bool> _parameterIsDiscardOpt;
         private readonly ImmutableArray<TypeWithAnnotations> _parameterTypesWithAnnotations;
@@ -1168,6 +1176,7 @@ internal sealed class PlainUnboundLambdaState : UnboundLambdaState
 
         internal PlainUnboundLambdaState(
             Binder binder,
+            ImmutableArray<SyntaxList<AttributeListSyntax>> parameterAttributes,
             ImmutableArray<string> parameterNames,
             ImmutableArray<bool> parameterIsDiscardOpt,
             ImmutableArray<TypeWithAnnotations> parameterTypesWithAnnotations,
@@ -1177,6 +1186,7 @@ internal PlainUnboundLambdaState(
             bool includeCache)
             : base(binder, includeCache)
         {
+            _parameterAttributes = parameterAttributes;
             _parameterNames = parameterNames;
             _parameterIsDiscardOpt = parameterIsDiscardOpt;
             _parameterTypesWithAnnotations = parameterTypesWithAnnotations;
@@ -1225,6 +1235,11 @@ public override Location ParameterLocation(int index)
 
         private bool IsExpressionLambda { get { return Body.Kind() != SyntaxKind.Block; } }
 
+        public override SyntaxList<AttributeListSyntax> ParameterAttributes(int index)
+        {
+            return _parameterAttributes.IsDefault ? default : _parameterAttributes[index];
+        }
+
         public override string ParameterName(int index)
         {
             Debug.Assert(!_parameterNames.IsDefault && 0 <= index && index < _parameterNames.Length);
@@ -1251,7 +1266,7 @@ public override TypeWithAnnotations ParameterTypeWithAnnotations(int index)
 
         protected override UnboundLambdaState WithCachingCore(bool includeCache)
         {
-            return new PlainUnboundLambdaState(Binder, _parameterNames, _parameterIsDiscardOpt, _parameterTypesWithAnnotations, _parameterRefKinds, _isAsync, _isStatic, includeCache);
+            return new PlainUnboundLambdaState(Binder, _parameterAttributes, _parameterNames, _parameterIsDiscardOpt, _parameterTypesWithAnnotations, _parameterRefKinds, _isAsync, _isStatic, includeCache);
         }
 
         protected override BoundExpression GetLambdaExpressionBody(BoundBlock body)

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -1725,7 +1725,7 @@ If such a class is used as a base class and if the deriving class defines a dest
     <value>Alias '{0}' conflicts with {1} definition</value>
   </data>
   <data name="ERR_ConditionalOnSpecialMethod" xml:space="preserve">
-    <value>The Conditional attribute is not valid on '{0}' because it is a constructor, destructor, operator, or explicit interface implementation</value>
+    <value>The Conditional attribute is not valid on '{0}' because it is a constructor, destructor, operator, lambda expression, or explicit interface implementation</value>
   </data>
   <data name="ERR_ConditionalMustReturnVoid" xml:space="preserve">
     <value>The Conditional attribute is not valid on '{0}' because its return type is not void</value>
@@ -3941,6 +3941,9 @@ You should consider suppressing the warning only if you're sure that you don't w
   <data name="ERR_AttributesNotAllowed" xml:space="preserve">
     <value>Attributes are not valid in this context.</value>
   </data>
+  <data name="ERR_AttributesRequireParenthesizedLambdaExpression" xml:space="preserve">
+    <value>Attributes on lambda expressions require a parenthesized parameter list.</value>
+  </data>
   <data name="ERR_ExternAliasNotAllowed" xml:space="preserve">
     <value>'extern alias' is not valid in this context</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2,8 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-#nullable disable
-
 namespace Microsoft.CodeAnalysis.CSharp
 {
     internal enum ErrorCode
@@ -1932,6 +1930,8 @@ internal enum ErrorCode
 
         #endregion diagnostics introduced for C# 9.0
 
+        ERR_AttributesRequireParenthesizedLambdaExpression = 8916,
+
         // Note: you will need to re-generate compiler code after adding warnings (eng\generate-compiler-code.cmd)
     }
 }

src/Compilers/CSharp/Portable/Errors/MessageID.cs

@@ -325,7 +325,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
             {
                 // C# preview features.
                 case MessageID.IDS_FeatureMixedDeclarationsAndExpressionsInDeconstruction:
-                case MessageID.IDS_FeatureLambdaAttributes:
+                case MessageID.IDS_FeatureLambdaAttributes: // semantic check
                     return LanguageVersion.Preview;
 
                 // C# 9.0 features.

src/Compilers/CSharp/Portable/Generated/CSharpSyntaxGenerator/CSharpSyntaxGenerator.SourceGenerator/Syntax.xml.Internal.Generated.cs

@@ -6247,6 +6247,8 @@ protected LambdaExpressionSyntax(ObjectReader reader)
         {
         }
 
+        public abstract Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<AttributeListSyntax> AttributeLists { get; }
+
         /// <summary>SyntaxToken representing equals greater than.</summary>
         public abstract SyntaxToken ArrowToken { get; }
     }
@@ -6352,7 +6354,7 @@ internal SimpleLambdaExpressionSyntax(SyntaxKind kind, GreenNode? attributeLists
             }
         }
 
-        public Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<AttributeListSyntax> AttributeLists => new Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<AttributeListSyntax>(this.attributeLists);
+        public override Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<AttributeListSyntax> AttributeLists => new Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<AttributeListSyntax>(this.attributeLists);
         public override Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<SyntaxToken> Modifiers => new Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<SyntaxToken>(this.modifiers);
         /// <summary>ParameterSyntax node representing the parameter of the lambda expression.</summary>
         public ParameterSyntax Parameter => this.parameter;
@@ -6663,7 +6665,7 @@ internal ParenthesizedLambdaExpressionSyntax(SyntaxKind kind, GreenNode? attribu
             }
         }
 
-        public Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<AttributeListSyntax> AttributeLists => new Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<AttributeListSyntax>(this.attributeLists);
+        public override Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<AttributeListSyntax> AttributeLists => new Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<AttributeListSyntax>(this.attributeLists);
         public override Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<SyntaxToken> Modifiers => new Microsoft.CodeAnalysis.Syntax.InternalSyntax.SyntaxList<SyntaxToken>(this.modifiers);
         /// <summary>ParameterListSyntax node representing the list of parameters for the lambda expression.</summary>
         public ParameterListSyntax ParameterList => this.parameterList;