Add support for emitting RequiredMembersAttribute in source

Adding `required` to a member now results in the type having a `RequiredMembersAttribute` emitted with the name of that member as the contents. Reading this data from metadata is not yet supported, nor is adding the requisite `ObsoleteAttribute` to constructors that depend on such contracts. The rules for when required is allowed and when it is disallowed are documented in dotnet/csharplang#5566, pending LDM review.

Test plan: dotnet#57046

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6944,4 +6944,16 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="IDS_FeatureRequiredMembers" xml:space="preserve">
     <value>required members</value>
   </data>
+  <data name="ERR_OverrideMustHaveRequired" xml:space="preserve">
+    <value>'{0}': cannot remove 'required' from '{1}' when overriding</value>
+  </data>
+  <data name="ERR_RequiredMembersCannotBeHidden" xml:space="preserve">
+    <value>Required member '{0}' cannot be hidden by '{1}'.</value>
+  </data>
+  <data name="ERR_RequiredMembersCannotBeLessVisibleThanContainingType" xml:space="preserve">
+    <value>Required member '{0}' cannot be less visible than the containing type '{1}'.</value>
+  </data>
+  <data name="ERR_ExplicitRequiredMembers" xml:space="preserve">
+    <value>Do not use 'System.Runtime.CompilerSerives.RequiredMembersAttribute'. Use the 'required' keyword on required fields and properties instead.</value>
+  </data>
 </root>

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

@@ -2032,5 +2032,9 @@ internal enum ErrorCode
         // PROTOTYPE(req): Move above the comment and condense before merge
 
         ERR_RequiredNameDisallowed = 9500,
+        ERR_OverrideMustHaveRequired = 9501,
+        ERR_RequiredMembersCannotBeHidden = 9502,
+        ERR_RequiredMembersCannotBeLessVisibleThanContainingType = 9503,
+        ERR_ExplicitRequiredMembers = 9504,
     }
 }

src/Compilers/CSharp/Portable/Lowering/StateMachineRewriter/SynthesizedStateMachineProperty.cs

@@ -151,6 +151,8 @@ public override bool IsExtern
             get { return false; }
         }
 
+        internal override bool IsRequired => false;
+
         internal override ObsoleteAttributeData ObsoleteAttributeData
         {
             get { return null; }

src/Compilers/CSharp/Portable/Symbols/AnonymousTypes/SynthesizedSymbols/AnonymousType.FieldSymbol.cs

@@ -150,6 +150,8 @@ internal override void AddSynthesizedAttributes(PEModuleBuilder moduleBuilder, r
                     ImmutableArray.Create(
                         new TypedConstant(manager.System_Diagnostics_DebuggerBrowsableState, TypedConstantKind.Enum, DebuggerBrowsableState.Never))));
             }
+
+            internal override bool IsRequired => false;
         }
     }
 }

src/Compilers/CSharp/Portable/Symbols/AnonymousTypes/SynthesizedSymbols/AnonymousType.PropertySymbol.cs

@@ -129,6 +129,8 @@ public override bool IsAbstract
                 get { return false; }
             }
 
+            internal override bool IsRequired => false;
+
             internal sealed override ObsoleteAttributeData ObsoleteAttributeData
             {
                 get { return null; }

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

@@ -77,6 +77,8 @@ public ErrorPropertySymbol(Symbol containingSymbol, TypeSymbol type, string name
 
         public override bool IsExtern { get { return false; } }
 
+        internal override bool IsRequired => false;
+
         internal sealed override ObsoleteAttributeData ObsoleteAttributeData { get { return null; } }
 
         public override ImmutableArray<ParameterSymbol> Parameters { get { return ImmutableArray<ParameterSymbol>.Empty; } }

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

@@ -325,6 +325,11 @@ internal virtual FieldSymbol AsMember(NamedTypeSymbol newOwner)
             return newOwner.IsDefinition ? this : new SubstitutedFieldSymbol(newOwner as SubstitutedNamedTypeSymbol, this);
         }
 
+        /// <summary>
+        /// Returns true if this field is required to be set in an object initializer on object creation.
+        /// </summary>
+        internal abstract bool IsRequired { get; }
+
         #region Use-Site Diagnostics
 
         internal override UseSiteInfo<AssemblySymbol> GetUseSiteInfo()

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEFieldSymbol.cs

@@ -587,5 +587,8 @@ internal override ObsoleteAttributeData ObsoleteAttributeData
         {
             get { return null; }
         }
+
+        // PROTOTYPE(req): Implement
+        internal override bool IsRequired => false;
     }
 }

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEPropertySymbol.cs

@@ -462,6 +462,15 @@ public override bool IsStatic
             }
         }
 
+        internal override bool IsRequired
+        {
+            get
+            {
+                // PROTOTYPE(req): Implement
+                return false;
+            }
+        }
+
         public override ImmutableArray<ParameterSymbol> Parameters
         {
             get { return _parameters; }

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

@@ -182,6 +182,11 @@ public bool IsWriteOnly
             }
         }
 
+        /// <summary>
+        /// Returns true if this property is required to be set in an object initializer on object creation.
+        /// </summary>
+        internal abstract bool IsRequired { get; }
+
         /// <summary>
         /// True if the property itself is excluded from code coverage instrumentation.
         /// True for source properties marked with <see cref="AttributeDescription.ExcludeFromCodeCoverageAttribute"/>.

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

@@ -85,6 +85,8 @@ internal sealed class SignatureOnlyPropertySymbol : PropertySymbol
 
         public override bool IsExtern { get { throw ExceptionUtilities.Unreachable; } }
 
+        internal override bool IsRequired => throw ExceptionUtilities.Unreachable;
+
         internal override ObsoleteAttributeData ObsoleteAttributeData { get { throw ExceptionUtilities.Unreachable; } }
 
         public override AssemblySymbol ContainingAssembly { get { throw ExceptionUtilities.Unreachable; } }

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

@@ -422,6 +422,14 @@ internal static CSDiagnosticInfo CheckAccessibility(DeclarationModifiers modifie
                 }
             }
 
+            if ((modifiers & DeclarationModifiers.Required) != 0
+                && symbol.Kind is SymbolKind.Property or SymbolKind.Field
+                && symbol.DeclaredAccessibility < symbol.ContainingType.DeclaredAccessibility)
+            {
+                // Required member '{0}' cannot be less visible than the containing type '{1}'.
+                return new CSDiagnosticInfo(ErrorCode.ERR_RequiredMembersCannotBeLessVisibleThanContainingType, symbol, symbol.ContainingType);
+            }
+
             return null;
         }
 

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

@@ -154,6 +154,8 @@ internal sealed override bool HasRuntimeSpecialName
                 return this.Name == WellKnownMemberNames.EnumBackingFieldName;
             }
         }
+
+        internal override bool IsRequired => (Modifiers & DeclarationModifiers.Required) != 0;
     }
 
     internal abstract class SourceFieldSymbolWithSyntaxReference : SourceFieldSymbol

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

@@ -1622,6 +1622,7 @@ protected void AfterMembersChecks(BindingDiagnosticBag diagnostics)
             CheckSequentialOnPartialType(diagnostics);
             CheckForProtectedInStaticClass(diagnostics);
             CheckForUnmatchedOperators(diagnostics);
+            CheckForRequiredMembers(diagnostics);
 
             var location = Locations[0];
             var compilation = DeclaringCompilation;
@@ -2355,6 +2356,15 @@ private void CheckForEqualityAndGetHashCode(BindingDiagnosticBag diagnostics)
             }
         }
 
+        private void CheckForRequiredMembers(BindingDiagnosticBag diagnostics)
+        {
+            if (GetMembersUnordered().Any(SymbolExtensions.IsRequired))
+            {
+                // Ensure that an error is reported if the required constructor isn't present.
+                _ = Binder.GetWellKnownTypeMember(DeclaringCompilation, WellKnownMember.System_Runtime_CompilerServices_RequiredMembersAttribute__ctor, diagnostics, Locations[0]);
+            }
+        }
+
         private bool TypeOverridesObjectMethod(string name)
         {
             foreach (var method in this.GetMembers(name).OfType<MethodSymbol>())

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

@@ -712,6 +712,12 @@ private void CheckNewModifier(Symbol symbol, bool isNew, BindingDiagnosticBag di
 
                         AddHidingAbstractDiagnostic(symbol, symbolLocation, hiddenMember, diagnostics, ref unused);
 
+                        if (hiddenMember.IsRequired())
+                        {
+                            // Required member '{0}' cannot be hidden by '{1}'.
+                            diagnostics.Add(ErrorCode.ERR_RequiredMembersCannotBeHidden, symbolLocation, hiddenMember, symbol);
+                        }
+
                         return;
                     }
                 }
@@ -900,6 +906,11 @@ void checkSingleOverriddenMember(Symbol overridingMember, Symbol overriddenMembe
                     // it is ok to override with no tuple names, for compatibility with C# 6, but otherwise names should match
                     diagnostics.Add(ErrorCode.ERR_CantChangeTupleNamesOnOverride, overridingMemberLocation, overridingMember, overriddenMember);
                 }
+                else if (overriddenMember is PropertySymbol { IsRequired: true } && overridingMember is PropertySymbol { IsRequired: false })
+                {
+                    // '{0}': cannot remove 'required' from '{1}' when overriding
+                    diagnostics.Add(ErrorCode.ERR_OverrideMustHaveRequired, overridingMemberLocation, overridingMember, overriddenMember);
+                }
                 else
                 {
                     // As in dev11, we don't compare obsoleteness to the immediately-overridden member,
@@ -1409,6 +1420,12 @@ TypeWithAnnotations getNotNullIfNotNullOutputType(TypeWithAnnotations outputType
                             diagnostics.Add(ErrorCode.WRN_NewOrOverrideExpected, hidingMemberLocation, hidingMember, hiddenMember);
                             diagnosticAdded = true;
                         }
+                        else if (hiddenMember.IsRequired())
+                        {
+                            // Required member '{0}' cannot be hidden by '{1}'.
+                            diagnostics.Add(ErrorCode.ERR_RequiredMembersCannotBeHidden, hidingMemberLocation, hiddenMember, hidingMember);
+                            diagnosticAdded = true;
+                        }
 
                         if (diagnosticAdded)
                         {

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

@@ -1110,7 +1110,16 @@ internal sealed override void DecodeWellKnownAttribute(ref DecodeWellKnownAttrib
                 diagnostics.Add(ErrorCode.ERR_CantUseRequiredAttribute, arguments.AttributeSyntaxOpt.Name.Location);
             }
             else if (ReportExplicitUseOfReservedAttributes(in arguments,
-                ReservedAttributes.DynamicAttribute | ReservedAttributes.IsReadOnlyAttribute | ReservedAttributes.IsUnmanagedAttribute | ReservedAttributes.IsByRefLikeAttribute | ReservedAttributes.TupleElementNamesAttribute | ReservedAttributes.NullableAttribute | ReservedAttributes.NullableContextAttribute | ReservedAttributes.NativeIntegerAttribute | ReservedAttributes.CaseSensitiveExtensionAttribute))
+                ReservedAttributes.DynamicAttribute
+                | ReservedAttributes.IsReadOnlyAttribute
+                | ReservedAttributes.IsUnmanagedAttribute
+                | ReservedAttributes.IsByRefLikeAttribute
+                | ReservedAttributes.TupleElementNamesAttribute
+                | ReservedAttributes.NullableAttribute
+                | ReservedAttributes.NullableContextAttribute
+                | ReservedAttributes.NativeIntegerAttribute
+                | ReservedAttributes.CaseSensitiveExtensionAttribute
+                | ReservedAttributes.RequiredMemberAttribute))
             {
             }
             else if (attribute.IsTargetAttribute(this, AttributeDescription.SecurityCriticalAttribute)
@@ -1583,6 +1592,39 @@ internal override void AddSynthesizedAttributes(PEModuleBuilder moduleBuilder, r
                 AddSynthesizedAttribute(ref attributes,
                     this.DeclaringCompilation.TrySynthesizeAttribute(WellKnownMember.System_Runtime_CompilerServices_CompilerGeneratedAttribute__ctor));
             }
+
+            var requiredMembers = ArrayBuilder<Symbol>.GetInstance();
+            foreach (var member in GetMembers())
+            {
+                switch (member)
+                {
+                    case SourceFieldSymbol { IsRequired: true }:
+                    case SourcePropertySymbol { IsRequired: true, IsOverride: false }:
+                    case SourcePropertySymbol { IsRequired: true, IsOverride: true, OverriddenProperty.IsRequired: false }:
+                        requiredMembers.Add(member);
+                        break;
+                }
+            }
+
+            if (requiredMembers.Any())
+            {
+                var stringType = compilation.GetSpecialType(SpecialType.System_String);
+                // Because GetMembers() is already sorted in lexical order, we don't need to do
+                // any additional sorting here.
+                var nameConstants = requiredMembers.SelectAsArray(
+                    static (member, stringType) => new TypedConstant(stringType, TypedConstantKind.Primitive, member.Name),
+                    stringType);
+                var stringArrayType = ArrayTypeSymbol.CreateSZArray(stringType.ContainingAssembly, TypeWithAnnotations.Create(stringType));
+
+                AddSynthesizedAttribute(
+                    ref attributes,
+                    compilation.TrySynthesizeAttribute(WellKnownMember.System_Runtime_CompilerServices_RequiredMembersAttribute__ctor,
+                    ImmutableArray.Create(new TypedConstant(stringArrayType, nameConstants))));
+
+                // PROTOTYPE(req): Add obsolete marker to constructors if required members and Obsolete hasn't already been emitted
+            }
+
+            requiredMembers.Free();
         }
 
         #endregion

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

@@ -89,6 +89,7 @@ private enum Flags : byte
         {
             Debug.Assert(!isExpressionBodied || !isAutoProperty);
             Debug.Assert(!isExpressionBodied || !hasInitializer);
+            Debug.Assert((modifiers & DeclarationModifiers.Required) == 0 || this is SourcePropertySymbol);
 
             _syntaxRef = syntax.GetReference();
             Location = location;
@@ -518,6 +519,8 @@ public override bool IsVirtual
             get { return (_modifiers & DeclarationModifiers.Virtual) != 0; }
         }
 
+        internal sealed override bool IsRequired => (_modifiers & DeclarationModifiers.Required) != 0;
+
         internal bool IsNew
         {
             get { return (_modifiers & DeclarationModifiers.New) != 0; }

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

@@ -1367,6 +1367,7 @@ internal enum ReservedAttributes
             NullablePublicOnlyAttribute = 1 << 8,
             NativeIntegerAttribute = 1 << 9,
             CaseSensitiveExtensionAttribute = 1 << 10,
+            RequiredMemberAttribute = 1 << 11,
         }
 
         internal bool ReportExplicitUseOfReservedAttributes(in DecodeWellKnownAttributeArguments<AttributeSyntax, CSharpAttributeData, AttributeLocation> arguments, ReservedAttributes reserved)
@@ -1421,6 +1422,12 @@ internal bool ReportExplicitUseOfReservedAttributes(in DecodeWellKnownAttributeA
                 // ExtensionAttribute should not be set explicitly.
                 diagnostics.Add(ErrorCode.ERR_ExplicitExtension, arguments.AttributeSyntaxOpt.Location);
             }
+            else if ((reserved & ReservedAttributes.RequiredMemberAttribute) != 0 &&
+                attribute.IsTargetAttribute(this, AttributeDescription.RequiredMembersAttribute))
+            {
+                // Do not use 'System.Runtime.CompilerSerives.RequiredMembersAttribute'. Use the 'required' keyword on required fields and properties instead.
+                diagnostics.Add(ErrorCode.ERR_ExplicitRequiredMembers, arguments.AttributeSyntaxOpt.Location);
+            }
             else
             {
                 return false;

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

@@ -839,5 +839,12 @@ internal static bool HasAsyncMethodBuilderAttribute(this Symbol symbol, [NotNull
             builderArgument = null;
             return false;
         }
+
+        internal static bool IsRequired(this Symbol symbol)
+            => symbol switch
+            {
+                FieldSymbol { IsRequired: true } or PropertySymbol { IsRequired: true } => true,
+                _ => false
+            };
     }
 }

src/Compilers/CSharp/Portable/Symbols/Synthesized/SynthesizedBackingFieldSymbol.cs

@@ -151,5 +151,7 @@ private void CheckForFieldTargetedAttribute(BindingDiagnosticBag diagnostics)
                 }
             }
         }
+
+        internal override bool IsRequired => false;
     }
 }

src/Compilers/CSharp/Portable/Symbols/Synthesized/SynthesizedFieldSymbolBase.cs

@@ -192,5 +192,7 @@ public override bool IsImplicitlyDeclared
         {
             get { return true; }
         }
+
+        internal override bool IsRequired => false;
     }
 }

src/Compilers/CSharp/Portable/Symbols/Wrapped/WrappedFieldSymbol.cs

@@ -204,5 +204,7 @@ public override bool IsStatic
                 return _underlyingField.IsStatic;
             }
         }
+
+        internal override bool IsRequired => _underlyingField.IsRequired;
     }
 }

src/Compilers/CSharp/Portable/Symbols/Wrapped/WrappedPropertySymbol.cs

@@ -161,6 +161,8 @@ public override bool IsExtern
             }
         }
 
+        internal override bool IsRequired => _underlyingProperty.IsRequired;
+
         internal override ObsoleteAttributeData ObsoleteAttributeData
         {
             get