Records: allow positional members to be implemented with fields

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6419,7 +6419,7 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
     <value>Only records may inherit from records.</value>
   </data>
   <data name="ERR_BadRecordMemberForPositionalParameter" xml:space="preserve">
-    <value>Record member '{0}' must be a readable instance property of type '{1}' to match positional parameter '{2}'.</value>
+    <value>Record member '{0}' must be a readable instance property or field of type '{1}' to match positional parameter '{2}'.</value>
   </data>
   <data name="ERR_NoCopyConstructorInBaseType" xml:space="preserve">
     <value>No accessible copy constructor found in base type '{0}'.</value>
@@ -6570,6 +6570,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="IDS_FeatureWithOnStructs" xml:space="preserve">
     <value>with on structs</value>
   </data>
+  <data name="IDS_FeaturePositionalFieldsInRecords" xml:space="preserve">
+    <value>positional fields in records</value>
+  </data>
   <data name="IDS_FeatureVarianceSafetyForStaticInterfaceMembers" xml:space="preserve">
     <value>variance safety for static interface members</value>
   </data>

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

@@ -218,6 +218,7 @@ internal enum MessageID
         IDS_FeatureMixedDeclarationsAndExpressionsInDeconstruction = MessageBase + 12793,
         IDS_FeatureRecordStructs = MessageBase + 12794,
         IDS_FeatureWithOnStructs = MessageBase + 12795,
+        IDS_FeaturePositionalFieldsInRecords = MessageBase + 12796,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -328,6 +329,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureRecordStructs:
                 case MessageID.IDS_FeatureWithOnStructs: // semantic check
                 case MessageID.IDS_FeatureMixedDeclarationsAndExpressionsInDeconstruction: // semantic check
+                case MessageID.IDS_FeaturePositionalFieldsInRecords: // semantic check
                     return LanguageVersion.Preview;
                 // C# 9.0 features.
                 case MessageID.IDS_FeatureLambdaDiscardParameters: // semantic check

src/Compilers/CSharp/Portable/Symbols/SignatureOnlyFieldSymbol.cs

@@ -0,0 +1,75 @@
+// 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 Roslyn.Utilities;
+
+namespace Microsoft.CodeAnalysis.CSharp.Symbols
+{
+    /// <summary>
+    /// A representation of a field symbol that is intended only to be used for comparison purposes
+    /// (esp in MemberSignatureComparer).
+    /// </summary>
+    internal sealed class SignatureOnlyFieldSymbol : FieldSymbol
+    {
+        private readonly string _name;
+        private readonly TypeSymbol _containingType;
+        private readonly TypeWithAnnotations _type;
+
+        public SignatureOnlyFieldSymbol(
+            string name,
+            TypeSymbol containingType,
+            TypeWithAnnotations type)
+        {
+            _type = type;
+            _containingType = containingType;
+            _name = name;
+        }
+
+        public override string Name => _name;
+
+        internal override TypeWithAnnotations GetFieldType(ConsList<FieldSymbol> fieldsBeingBound) => _type;
+
+        public override Symbol ContainingSymbol => _containingType;
+
+        #region Not used by MemberSignatureComparer
+        public override bool IsReadOnly => throw ExceptionUtilities.Unreachable;
+
+        public override bool IsStatic => throw ExceptionUtilities.Unreachable;
+
+        internal override bool HasSpecialName => throw ExceptionUtilities.Unreachable;
+
+        public override ImmutableArray<Location> Locations => throw ExceptionUtilities.Unreachable;
+
+        public override ImmutableArray<SyntaxReference> DeclaringSyntaxReferences => throw ExceptionUtilities.Unreachable;
+
+        public override Accessibility DeclaredAccessibility => throw ExceptionUtilities.Unreachable;
+
+        internal override ObsoleteAttributeData ObsoleteAttributeData => throw ExceptionUtilities.Unreachable;
+
+        public override AssemblySymbol ContainingAssembly => throw ExceptionUtilities.Unreachable;
+
+        internal override ModuleSymbol ContainingModule => throw ExceptionUtilities.Unreachable;
+
+        public override FlowAnalysisAnnotations FlowAnalysisAnnotations => throw ExceptionUtilities.Unreachable;
+
+        public override Symbol AssociatedSymbol => throw ExceptionUtilities.Unreachable;
+
+        public override bool IsVolatile => throw ExceptionUtilities.Unreachable;
+
+        public override bool IsConst => throw ExceptionUtilities.Unreachable;
+
+        internal override bool HasRuntimeSpecialName => throw ExceptionUtilities.Unreachable;
+
+        internal override bool IsNotSerialized => throw ExceptionUtilities.Unreachable;
+
+        internal override MarshalPseudoCustomAttributeData MarshallingInformation => throw ExceptionUtilities.Unreachable;
+
+        internal override int? TypeLayoutOffset => throw ExceptionUtilities.Unreachable;
+
+        internal override ConstantValue GetConstantValue(ConstantFieldsInProgress inProgress, bool earlyDecodingWellKnownAttributes) => throw new System.NotImplementedException();
+
+        #endregion Not used by PropertySignatureComparer
+    }
+}

src/Compilers/CSharp/Portable/Symbols/Source/SourceMemberContainerSymbol.cs

@@ -3466,7 +3466,6 @@ private void AddSynthesizedRecordMembersIfNecessary(MembersAndInitializersBuilde
             {
                 switch (member)
                 {
-                    case FieldSymbol:
                     case EventSymbol:
                     case MethodSymbol { MethodKind: not (MethodKind.Ordinary or MethodKind.Constructor) }:
                         continue;
@@ -3542,8 +3541,10 @@ private void AddSynthesizedRecordMembersIfNecessary(MembersAndInitializersBuilde
 
             return;
 
-            void addDeconstruct(SynthesizedRecordConstructor ctor, ImmutableArray<PropertySymbol> properties)
+            void addDeconstruct(SynthesizedRecordConstructor ctor, ImmutableArray<Symbol> positionalMembers)
             {
+                Debug.Assert(positionalMembers.All(p => p is PropertySymbol or FieldSymbol));
+
                 var targetMethod = new SignatureOnlyMethodSymbol(
                     WellKnownMemberNames.DeconstructMethodName,
                     this,
@@ -3563,7 +3564,7 @@ void addDeconstruct(SynthesizedRecordConstructor ctor, ImmutableArray<PropertySy
 
                 if (!memberSignatures.TryGetValue(targetMethod, out Symbol? existingDeconstructMethod))
                 {
-                    members.Add(new SynthesizedRecordDeconstruct(this, ctor, properties, memberOffset: members.Count, diagnostics));
+                    members.Add(new SynthesizedRecordDeconstruct(this, ctor, positionalMembers, memberOffset: members.Count, diagnostics));
                 }
                 else
                 {
@@ -3722,9 +3723,9 @@ void addToStringMethod(MethodSymbol printMethod)
                 }
             }
 
-            ImmutableArray<PropertySymbol> addProperties(ImmutableArray<ParameterSymbol> recordParameters)
+            ImmutableArray<Symbol> addProperties(ImmutableArray<ParameterSymbol> recordParameters)
             {
-                var existingOrAddedMembers = ArrayBuilder<PropertySymbol>.GetInstance(recordParameters.Length);
+                var existingOrAddedMembers = ArrayBuilder<Symbol>.GetInstance(recordParameters.Length);
                 int addedCount = 0;
                 foreach (ParameterSymbol param in recordParameters)
                 {
@@ -3743,12 +3744,42 @@ ImmutableArray<PropertySymbol> addProperties(ImmutableArray<ParameterSymbol> rec
                     if (!memberSignatures.TryGetValue(targetProperty, out var existingMember))
                     {
                         existingMember = OverriddenOrHiddenMembersHelpers.FindFirstHiddenMemberIfAny(targetProperty, memberIsFromSomeCompilation: true);
-                        isInherited = true;
+                        if (existingMember is FieldSymbol)
+                        {
+                            // field from base type should not be considered before we look at fields from current type
+                            existingMember = null;
+                        }
+                        else
+                        {
+                            isInherited = true;
+                        }
+                    }
+
+                    if (existingMember is null)
+                    {
+                        var targetField = new SignatureOnlyFieldSymbol(param.Name,
+                                                                       this,
+                                                                       param.TypeWithAnnotations);
+
+                        if (!memberSignatures.TryGetValue(targetField, out existingMember))
+                        {
+                            existingMember = OverriddenOrHiddenMembersHelpers.FindFirstHiddenMemberIfAny(targetField, memberIsFromSomeCompilation: true);
+                            if (existingMember is not FieldSymbol)
+                            {
+                                existingMember = null;
+                            }
+                        }
                     }
+
                     if (existingMember is null)
                     {
                         addProperty(new SynthesizedRecordPropertySymbol(this, syntax, param, isOverride: false, diagnostics));
                     }
+                    else if (existingMember is FieldSymbol { IsStatic: false } field)
+                    {
+                        Binder.CheckFeatureAvailability(syntax, MessageID.IDS_FeaturePositionalFieldsInRecords, diagnostics);
+                        existingOrAddedMembers.Add(field);
+                    }
                     else if (existingMember is PropertySymbol { IsStatic: false, GetMethod: { } } prop
                         && prop.TypeWithAnnotations.Equals(param.TypeWithAnnotations, TypeCompareKind.AllIgnoreOptions))
                     {

src/Compilers/CSharp/Portable/Symbols/Synthesized/Records/SynthesizedRecordDeconstruct.cs

@@ -2,12 +2,9 @@
 // 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.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
-using System.Reflection;
-using Microsoft.Cci;
 using Microsoft.CodeAnalysis.PooledObjects;
 using Roslyn.Utilities;
 
@@ -16,19 +13,19 @@ namespace Microsoft.CodeAnalysis.CSharp.Symbols
     internal sealed class SynthesizedRecordDeconstruct : SynthesizedRecordOrdinaryMethod
     {
         private readonly SynthesizedRecordConstructor _ctor;
-        private readonly ImmutableArray<PropertySymbol> _properties;
+        private readonly ImmutableArray<Symbol> _positionalMembers;
 
         public SynthesizedRecordDeconstruct(
             SourceMemberContainerTypeSymbol containingType,
             SynthesizedRecordConstructor ctor,
-            ImmutableArray<PropertySymbol> properties,
+            ImmutableArray<Symbol> positionalMembers,
             int memberOffset,
             BindingDiagnosticBag diagnostics)
             : base(containingType, WellKnownMemberNames.DeconstructMethodName, hasBody: true, memberOffset, diagnostics)
         {
-            Debug.Assert(properties.All(prop => prop.GetMethod is object));
+            Debug.Assert(positionalMembers.All(p => p is PropertySymbol { GetMethod: not null } or FieldSymbol));
             _ctor = ctor;
-            _properties = properties;
+            _positionalMembers = positionalMembers;
         }
 
         protected override DeclarationModifiers MakeDeclarationModifiers(DeclarationModifiers allowedModifiers, BindingDiagnosticBag diagnostics)
@@ -63,29 +60,45 @@ internal override void GenerateMethodBody(TypeCompilationState compilationState,
         {
             var F = new SyntheticBoundNodeFactory(this, ContainingType.GetNonNullSyntaxNode(), compilationState, diagnostics);
 
-            if (ParameterCount != _properties.Length)
+            if (ParameterCount != _positionalMembers.Length)
             {
                 // There is a mismatch, an error was reported elsewhere
                 F.CloseMethod(F.ThrowNull());
                 return;
             }
 
-            var statementsBuilder = ArrayBuilder<BoundStatement>.GetInstance(_properties.Length + 1);
-            for (int i = 0; i < _properties.Length; i++)
+            var statementsBuilder = ArrayBuilder<BoundStatement>.GetInstance(_positionalMembers.Length + 1);
+            for (int i = 0; i < _positionalMembers.Length; i++)
             {
                 var parameter = Parameters[i];
-                var property = _properties[i];
+                var positionalMember = _positionalMembers[i];
 
-                if (!parameter.Type.Equals(property.Type, TypeCompareKind.AllIgnoreOptions))
+                var type = positionalMember switch
+                {
+                    PropertySymbol property => property.Type,
+                    FieldSymbol field => field.Type,
+                    _ => throw ExceptionUtilities.Unreachable
+                };
+
+                if (!parameter.Type.Equals(type, TypeCompareKind.AllIgnoreOptions))
                 {
                     // There is a mismatch, an error was reported elsewhere
                     statementsBuilder.Free();
                     F.CloseMethod(F.ThrowNull());
                     return;
                 }
 
-                // parameter_i = property_i;
-                statementsBuilder.Add(F.Assignment(F.Parameter(parameter), F.Property(F.This(), property)));
+                switch (positionalMember)
+                {
+                    case PropertySymbol property:
+                        // parameter_i = property_i;
+                        statementsBuilder.Add(F.Assignment(F.Parameter(parameter), F.Property(F.This(), property)));
+                        break;
+                    case FieldSymbol field:
+                        // parameter_i = field_i;
+                        statementsBuilder.Add(F.Assignment(F.Parameter(parameter), F.Field(F.This(), field)));
+                        break;
+                }
             }
 
             statementsBuilder.Add(F.Return());