/// <summary>
        /// Rewrite an iterator method into a state machine class.
        /// </summary>
        /// <param name="body">The original body of the method</param>
        /// <param name="method">The method's identity</param>
        /// <param name="compilationState">The collection of generated methods that result from this transformation and which must be emitted</param>
        /// <param name="diagnostics">Diagnostic bag for diagnostics.</param>
        /// <param name="generateDebugInfo"></param>
        internal static BoundStatement Rewrite(
            BoundStatement body,
            MethodSymbol method,
            TypeCompilationState compilationState,
            DiagnosticBag diagnostics,
            bool generateDebugInfo)
        {
            TypeSymbol elementType = method.IteratorElementType;
            if ((object)elementType == null)
            {
                return body;
            }

            // Figure out what kind of iterator we are generating.
            bool isEnumerable;
            switch (method.ReturnType.OriginalDefinition.SpecialType)
            {
                case SpecialType.System_Collections_IEnumerable:
                case SpecialType.System_Collections_Generic_IEnumerable_T:
                    isEnumerable = true;
                    break;

                case SpecialType.System_Collections_IEnumerator:
                case SpecialType.System_Collections_Generic_IEnumerator_T:
                    isEnumerable = false;
                    break;

                default:
                    throw ExceptionUtilities.UnexpectedValue(method.ReturnType.OriginalDefinition.SpecialType);
            }

            var iteratorClass = new IteratorStateMachine(method, isEnumerable, elementType, compilationState);
            return new IteratorRewriter(body, method, isEnumerable, iteratorClass, compilationState, diagnostics, generateDebugInfo).Rewrite();
        }

        /// <summary>
        /// Rewrite an async method into a state machine class.
        /// </summary>
        /// <param name="body">The original body of the method</param>
        /// <param name="method">The method's identity</param>
        /// <param name="compilationState">The collection of generated methods that result from this transformation and which must be emitted</param>
        /// <param name="diagnostics">Diagnostic bag for diagnostics.</param>
        /// <param name="generateDebugInfo"></param>
        /// <param name="stateMachineType"></param>
        internal static BoundStatement Rewrite(
            BoundStatement body,
            MethodSymbol method,
            TypeCompilationState compilationState,
            DiagnosticBag diagnostics,
            bool generateDebugInfo,
            out AsyncStateMachine stateMachineType)
        {
            if (!method.IsAsync)
            {
                stateMachineType = null;
                return body;
            }

            // The CLR doesn't support adding fields to structs, so in order to enable EnC in an async method we need to generate a class.
            var typeKind = compilationState.Compilation.Options.EnableEditAndContinue ? TypeKind.Class : TypeKind.Struct;

            var bodyWithAwaitLifted = AwaitLiftingRewriter.Rewrite(body, method, compilationState, diagnostics);

            stateMachineType = new AsyncStateMachine(method, typeKind);
            compilationState.ModuleBuilderOpt.CompilationState.SetStateMachineType(method, stateMachineType);
            var rewriter = new AsyncRewriter(bodyWithAwaitLifted, method, stateMachineType, compilationState, diagnostics, generateDebugInfo);
            if (!rewriter.constructedSuccessfully)
            {
                return body;
            }

            return rewriter.Rewrite();
        }

            internal SynthesizedLambdaMethod(NamedTypeSymbol containingType, MethodSymbol topLevelMethod, BoundLambda node, bool isStatic, TypeCompilationState compilationState)
                : base(containingType,
                       node.Symbol,
                       null,
                       node.SyntaxTree.GetReference(node.Body.Syntax),
                       node.Syntax.GetLocation(),
                       GeneratedNames.MakeLambdaMethodName(topLevelMethod.Name, compilationState.GenerateTempNumber()),
                       (containingType is LambdaFrame ? DeclarationModifiers.Internal : DeclarationModifiers.Private)
                            | (isStatic ? DeclarationModifiers.Static : 0)
                            | (node.Symbol.IsAsync ? DeclarationModifiers.Async : 0))
            {
                TypeMap typeMap;
                ImmutableArray<TypeParameterSymbol> typeParameters;
                LambdaFrame lambdaFrame;

                if (!topLevelMethod.IsGenericMethod)
                {
                    typeMap = TypeMap.Empty;
                    typeParameters = ImmutableArray<TypeParameterSymbol>.Empty;
                }
                else if ((object)(lambdaFrame = this.ContainingType as LambdaFrame) != null)
                {
                    typeMap = lambdaFrame.TypeMap;
                    typeParameters = ImmutableArray<TypeParameterSymbol>.Empty;
                }
                else
                {
                    typeMap = TypeMap.Empty.WithAlphaRename(topLevelMethod, this, out typeParameters);
                }

                AssignTypeMapAndTypeParameters(typeMap, typeParameters);
            }

        protected StateMachineRewriter(
            BoundStatement body,
            MethodSymbol method,
            SynthesizedContainer stateMachineType,
            VariableSlotAllocator slotAllocatorOpt,
            TypeCompilationState compilationState,
            DiagnosticBag diagnostics)
        {
            Debug.Assert(body != null);
            Debug.Assert(method != null);
            Debug.Assert(stateMachineType != null);
            Debug.Assert(compilationState != null);
            Debug.Assert(diagnostics != null);

            this.body = body;
            this.method = method;
            this.stateMachineType = stateMachineType;
            this.slotAllocatorOpt = slotAllocatorOpt;
            this.synthesizedLocalOrdinals = new SynthesizedLocalOrdinalsDispenser();
            this.diagnostics = diagnostics;

            this.F = new SyntheticBoundNodeFactory(method, body.Syntax, compilationState, diagnostics);
            Debug.Assert(F.CurrentType == method.ContainingType);
            Debug.Assert(F.Syntax == body.Syntax);
        }

        /// <summary>
        /// Rewrite an async method into a state machine type.
        /// </summary>
        internal static BoundStatement Rewrite(
            BoundStatement body,
            MethodSymbol method,
            int methodOrdinal,
            VariableSlotAllocator slotAllocatorOpt,
            TypeCompilationState compilationState,
            DiagnosticBag diagnostics,
            out AsyncStateMachine stateMachineType)
        {
            if (!method.IsAsync)
            {
                stateMachineType = null;
                return body;
            }

            // The CLR doesn't support adding fields to structs, so in order to enable EnC in an async method we need to generate a class.
            var typeKind = compilationState.Compilation.Options.EnableEditAndContinue ? TypeKind.Class : TypeKind.Struct;

            var bodyWithAwaitLifted = AwaitExpressionSpiller.Rewrite(body, method, compilationState, diagnostics);

            stateMachineType = new AsyncStateMachine(slotAllocatorOpt, compilationState, method, methodOrdinal, typeKind);
            compilationState.ModuleBuilderOpt.CompilationState.SetStateMachineType(method, stateMachineType);
            var rewriter = new AsyncRewriter(bodyWithAwaitLifted, method, methodOrdinal, stateMachineType, slotAllocatorOpt, compilationState, diagnostics);

            if (!rewriter.VerifyPresenceOfRequiredAPIs())
            {
                return body;
            }

            return rewriter.Rewrite();
        }

        internal static BoundBlock Rewrite(SourceMethodSymbol sourceMethodSymbol, MethodContractSyntax contract, BoundBlock body, TypeCompilationState compilationState, DiagnosticBag diagsForCurrentMethod)
        {
            var binder = compilationState.Compilation.GetBinderFactory(sourceMethodSymbol.SyntaxTree)
                                     .GetBinder(body.Syntax);
            SyntheticBoundNodeFactory factory = new SyntheticBoundNodeFactory(sourceMethodSymbol, sourceMethodSymbol.SyntaxNode, compilationState, diagsForCurrentMethod);
            var contractType = compilationState.Compilation.GetTypeByReflectionType(typeof(System.Diagnostics.Contracts.Contract), diagsForCurrentMethod);

            var contractStatements = ArrayBuilder<BoundStatement>.GetInstance(contract.Requires.Count);
            foreach (var requires in contract.Requires)
            {
                var condition = binder.BindExpression(requires.Condition, diagsForCurrentMethod);
                
                var methodCall = factory.StaticCall(contractType, "Requires", condition);
                var statement = factory.ExpressionStatement(methodCall);

                contractStatements.Add(statement);
            }
            foreach (var requires in contract.Ensures)
            {
                var condition = binder.BindExpression(requires.Condition, diagsForCurrentMethod);
                var methodCall = factory.StaticCall(contractType, "Ensures", condition);
                var statement = factory.ExpressionStatement(methodCall);

                contractStatements.Add(statement);
            }

            return body.Update(body.Locals, body.Statements.InsertRange(0, contractStatements.ToImmutableAndFree()));
        }

 internal SpillFieldAllocator(SyntheticBoundNodeFactory F, TypeCompilationState CompilationState)
 {
     allocatedFields = new KeyedStack<TypeSymbol, FieldSymbol>();
     realizedSpills = new Dictionary<BoundSpillTemp, FieldSymbol>();
     this.F = F;
     this.CompilationState = CompilationState;
 }

            public AsyncStruct(MethodSymbol method, TypeCompilationState compilationState)
                : base(method, GeneratedNames.MakeIteratorOrAsyncDisplayClassName(method.Name, compilationState.GenerateTempNumber()), TypeKind.Struct)
            {
                this.interfaces = ReadOnlyArray<NamedTypeSymbol>.CreateFrom(
                    compilationState.EmitModule.Compilation.GetWellKnownType(WellKnownType.System_Runtime_CompilerServices_IAsyncStateMachine));

                this.constructor = new SynthesizedInstanceConstructor(this);
            }

 public AsyncStateMachine(VariableSlotAllocator variableAllocatorOpt, TypeCompilationState compilationState, MethodSymbol asyncMethod, int asyncMethodOrdinal, TypeKind typeKind)
     : base(variableAllocatorOpt, compilationState, asyncMethod, asyncMethodOrdinal)
 {
     // TODO: report use-site errors on these types
     _typeKind = typeKind;
     _interfaces = ImmutableArray.Create(asyncMethod.DeclaringCompilation.GetWellKnownType(WellKnownType.System_Runtime_CompilerServices_IAsyncStateMachine));
     _constructor = new AsyncConstructor(this);
 }

 private ExpressionLambdaRewriter(TypeCompilationState compilationState, CSharpSyntaxNode node, DiagnosticBag diagnostics)
 {
     Bound = new SyntheticBoundNodeFactory((NamedTypeSymbol)null, node, compilationState, diagnostics);
     Int32Type = Bound.SpecialType(SpecialType.System_Int32);
     ObjectType = Bound.SpecialType(SpecialType.System_Object);
     NullableType = Bound.SpecialType(SpecialType.System_Nullable_T);
     IEnumerableType = Bound.SpecialType(SpecialType.System_Collections_Generic_IEnumerable_T);
 }

        private IteratorRewriter(
            BoundStatement body,
            MethodSymbol method,
            bool isEnumerable,
            IteratorStateMachine iteratorClass,
            TypeCompilationState compilationState,
            DiagnosticBag diagnostics)
            : base(body, method, iteratorClass, compilationState, diagnostics)
        {
            // the element type may contain method type parameters, which are now alpha-renamed into type parameters of the generated class
            this.elementType = iteratorClass.ElementType;

            this.isEnumerable = isEnumerable;
        }

 private AsyncRewriter(
     BoundStatement body,
     MethodSymbol method,
     int methodOrdinal,
     AsyncStateMachine stateMachineType,
     VariableSlotAllocator slotAllocatorOpt,
     TypeCompilationState compilationState,
     DiagnosticBag diagnostics)
     : base(body, method, stateMachineType, slotAllocatorOpt, compilationState, diagnostics)
 {
     _constructedSuccessfully = AsyncMethodBuilderMemberCollection.TryCreate(F, method, this.stateMachineType.TypeMap, out _asyncMethodBuilderMemberCollection);
     _methodOrdinal = methodOrdinal;
     _ignoreAccessibility = compilationState.ModuleBuilderOpt.IgnoreAccessibility;
 }

        private IteratorRewriter(
            BoundStatement body,
            MethodSymbol method,
            bool isEnumerable,
            IteratorStateMachine stateMachineType,
            VariableSlotAllocator slotAllocatorOpt,
            TypeCompilationState compilationState,
            DiagnosticBag diagnostics)
            : base(body, method, stateMachineType, slotAllocatorOpt, compilationState, diagnostics)
        {
            // the element type may contain method type parameters, which are now alpha-renamed into type parameters of the generated class
            _elementType = stateMachineType.ElementType;

            _isEnumerable = isEnumerable;
        }

 protected StateMachineRewriter(
     BoundStatement body,
     MethodSymbol method,
     SynthesizedContainer stateMachineClass,
     TypeCompilationState compilationState,
     DiagnosticBag diagnostics)
 {
     this.body = body;
     this.method = method;
     this.stateMachineClass = stateMachineClass;
     this.compilationState = compilationState;
     this.diagnostics = diagnostics;
     this.F = new SyntheticBoundNodeFactory(method, body.Syntax, compilationState, diagnostics);
     Debug.Assert(F.CurrentClass == method.ContainingType);
     Debug.Assert(F.Syntax == body.Syntax);
 }

 private AsyncRewriter(
     BoundStatement body,
     MethodSymbol method,
     AsyncStateMachine stateMachineClass,
     TypeCompilationState compilationState,
     DiagnosticBag diagnostics)
     : base(body, method, stateMachineClass, compilationState, diagnostics)
 {
     try
     {
         constructedSuccessfully = AsyncMethodBuilderMemberCollection.TryCreate(F, method, this.stateMachineClass.TypeMap, out this.asyncMethodBuilderMemberCollection);
     }
     catch (SyntheticBoundNodeFactory.MissingPredefinedMember ex)
     {
         diagnostics.Add(ex.Diagnostic);
         constructedSuccessfully = false;
     }
 }

            public IteratorClass(MethodSymbol method, bool isEnumerable, TypeSymbol elementType, TypeCompilationState compilationState)
                : base(method, GeneratedNames.MakeIteratorOrAsyncDisplayClassName(method.Name, compilationState.GenerateTempNumber()), TypeKind.Class)
            {
                this.ElementType = TypeMap.SubstituteType(elementType);
                var interfaces = ArrayBuilder<NamedTypeSymbol>.GetInstance();
                if (isEnumerable)
                {
                    interfaces.Add(ContainingAssembly.GetSpecialType(SpecialType.System_Collections_Generic_IEnumerable_T).Construct(ElementType));
                    interfaces.Add(ContainingAssembly.GetSpecialType(SpecialType.System_Collections_IEnumerable));
                }

                interfaces.Add(ContainingAssembly.GetSpecialType(SpecialType.System_Collections_Generic_IEnumerator_T).Construct(ElementType));
                interfaces.Add(ContainingAssembly.GetSpecialType(SpecialType.System_IDisposable));
                interfaces.Add(ContainingAssembly.GetSpecialType(SpecialType.System_Collections_IEnumerator));
                this.interfaces = interfaces.ToImmutableAndFree();

                this.constructor = new IteratorConstructor(this);
            }

        /// <summary>
        /// Rewrite an iterator method into a state machine class.
        /// </summary>
        internal static BoundStatement Rewrite(
            BoundStatement body,
            MethodSymbol method,
            int methodOrdinal,
            VariableSlotAllocator slotAllocatorOpt,
            TypeCompilationState compilationState,
            DiagnosticBag diagnostics,
            out IteratorStateMachine stateMachineType)
        {
            TypeSymbol elementType = method.IteratorElementType;
            if ((object)elementType == null)
            {
                stateMachineType = null;
                return body;
            }

            // Figure out what kind of iterator we are generating.
            bool isEnumerable;
            switch (method.ReturnType.OriginalDefinition.SpecialType)
            {
                case SpecialType.System_Collections_IEnumerable:
                case SpecialType.System_Collections_Generic_IEnumerable_T:
                    isEnumerable = true;
                    break;

                case SpecialType.System_Collections_IEnumerator:
                case SpecialType.System_Collections_Generic_IEnumerator_T:
                    isEnumerable = false;
                    break;

                default:
                    throw ExceptionUtilities.UnexpectedValue(method.ReturnType.OriginalDefinition.SpecialType);
            }

            stateMachineType = new IteratorStateMachine(slotAllocatorOpt, compilationState, method, methodOrdinal, isEnumerable, elementType);
            compilationState.ModuleBuilderOpt.CompilationState.SetStateMachineType(method, stateMachineType);
            var rewriter = new IteratorRewriter(body, method, isEnumerable, stateMachineType, slotAllocatorOpt, compilationState, diagnostics);
            if (!rewriter.VerifyPresenceOfRequiredAPIs())
            {
                return body;
            }

            return rewriter.Rewrite();
        }

        public IteratorStateMachine(VariableSlotAllocator slotAllocatorOpt, TypeCompilationState compilationState, MethodSymbol iteratorMethod, int iteratorMethodOrdinal, bool isEnumerable, TypeSymbol elementType)
            : base(slotAllocatorOpt, compilationState, iteratorMethod, iteratorMethodOrdinal)
        {
            this.ElementType = TypeMap.SubstituteType(elementType);

            var interfaces = ArrayBuilder<NamedTypeSymbol>.GetInstance();
            if (isEnumerable)
            {
                interfaces.Add(ContainingAssembly.GetSpecialType(SpecialType.System_Collections_Generic_IEnumerable_T).Construct(ElementType));
                interfaces.Add(ContainingAssembly.GetSpecialType(SpecialType.System_Collections_IEnumerable));
            }

            interfaces.Add(ContainingAssembly.GetSpecialType(SpecialType.System_Collections_Generic_IEnumerator_T).Construct(ElementType));
            interfaces.Add(ContainingAssembly.GetSpecialType(SpecialType.System_IDisposable));
            interfaces.Add(ContainingAssembly.GetSpecialType(SpecialType.System_Collections_IEnumerator));
            _interfaces = interfaces.ToImmutableAndFree();

            _constructor = new IteratorConstructor(this);
        }

        /// <summary>
        /// Rewrite an async method into a state machine class.
        /// </summary>
        /// <param name="body">The original body of the method</param>
        /// <param name="method">The method's identity</param>
        /// <param name="compilationState">The collection of generated methods that result from this transformation and which must be emitted</param>
        /// <param name="diagnostics">Diagnostic bag for diagnostics.</param>
        /// <param name="generateDebugInfo"></param>
        internal static BoundStatement Rewrite(
            BoundStatement body,
            MethodSymbol method,
            TypeCompilationState compilationState,
            DiagnosticBag diagnostics,
            bool generateDebugInfo)
        {
            if (!method.IsAsync)
            {
                return body;
            }

            var bodyWithAwaitLifted = AwaitLiftingRewriter.Rewrite(body, method, compilationState, diagnostics);
            var rewriter = new AsyncRewriter2(bodyWithAwaitLifted, method, ((SourceMethodSymbol)method).AsyncStateMachineType, compilationState, diagnostics, generateDebugInfo);
            if (!rewriter.constructedSuccessfully)
            {
                return body;
            }

            var bodyReplacement = rewriter.Rewrite();
            return bodyReplacement;
        }

        private AsyncRewriter(
            BoundStatement body,
            MethodSymbol method,
            int methodOrdinal,
            AsyncStateMachine stateMachineType,
            VariableSlotAllocator slotAllocatorOpt,
            TypeCompilationState compilationState,
            DiagnosticBag diagnostics)
            : base(body, method, stateMachineType, slotAllocatorOpt, compilationState, diagnostics)
        {
            try
            {
                _constructedSuccessfully = AsyncMethodBuilderMemberCollection.TryCreate(F, method, this.stateMachineType.TypeMap, out _asyncMethodBuilderMemberCollection);
            }
            catch (SyntheticBoundNodeFactory.MissingPredefinedMember ex)
            {
                diagnostics.Add(ex.Diagnostic);
                _constructedSuccessfully = false;
            }

            _methodOrdinal = methodOrdinal;
            _ignoreAccessibility = compilationState.ModuleBuilderOpt.IgnoreAccessibility;
        }