Currently the async state machine is a struct. Struct is better for...

 Currently the async state machine is a struct. Struct is better for performance (no allocation) but prevents us to modify local variables in the async method in EnC. The CLR doesn't support adding fields to a struct.

     This change switches the state machine to a class when emitting unoptimized debug code (/debug:full /optimize-). It keeps it struct otherwise. (changeset 1251785)

Src/Compilers/CSharp/Source/Lowering/AsyncRewriter/AsyncMethodToStateMachineRewriter.cs

@@ -11,7 +11,7 @@
 
 namespace Microsoft.CodeAnalysis.CSharp
 {
-    sealed internal class AsyncMethodToStateMachineRewriter : MethodToStateMachineRewriter
+    internal sealed class AsyncMethodToStateMachineRewriter : MethodToStateMachineRewriter
     {
         /// <summary>
         /// The method being rewritten.
@@ -63,8 +63,8 @@ sealed internal class AsyncMethodToStateMachineRewriter : MethodToStateMachineRe
             DiagnosticBag diagnostics,
             bool generateDebugInfo)
             : base(F, method, state, variablesCaptured, initialProxies, diagnostics,
-                    useFinalizerBookkeeping: false,
-                    generateDebugInfo: generateDebugInfo)
+                   useFinalizerBookkeeping: false,
+                   generateDebugInfo: generateDebugInfo)
         {
             this.method = method;
             this.asyncMethodBuilderMemberCollection = asyncMethodBuilderMemberCollection;
@@ -87,6 +87,7 @@ private FieldSymbol GetAwaiterField(TypeSymbol awaiterType)
                 result = F.StateMachineField(awaiterType, GeneratedNames.AsyncAwaiterFieldName(CompilationState.GenerateTempNumber()), isPublic: true);
                 awaiterFields.Add(awaiterType, result);
             }
+
             return result;
         }
 
@@ -116,12 +117,12 @@ internal void GenerateMoveNext(BoundStatement body, MethodSymbol moveNextMethod)
                 F.Try(
                     F.Block(
                         ImmutableArray<LocalSymbol>.Empty,
-                        // switch (state) ...
-                        F.HiddenSequencePoint(),
+                            // switch (state) ...
+                            F.HiddenSequencePoint(),
                         Dispatch(),
                         F.Label(initialLabel),
-                        // [body]
-                        rewrittenBody
+                            // [body]
+                            rewrittenBody
                     ),
                     F.CatchBlocks(
                         F.Catch(
@@ -129,10 +130,10 @@ internal void GenerateMoveNext(BoundStatement body, MethodSymbol moveNextMethod)
                             F.Block(
                                 F.NoOp(method.ReturnsVoid ? NoOpStatementFlavor.AsyncMethodCatchHandler : NoOpStatementFlavor.Default),
                                 F.HiddenSequencePoint(),
-                                // this.state = finishedState
-                                F.Assignment(F.Field(F.This(), stateField), F.Literal(StateMachineStates.FinishedStateMachine)),
-                                // builder.SetException(ex)
-                                F.ExpressionStatement(
+                                    // this.state = finishedState
+                                    F.Assignment(F.Field(F.This(), stateField), F.Literal(StateMachineStates.FinishedStateMachine)),
+                                    // builder.SetException(ex)
+                                    F.ExpressionStatement(
                                     F.Call(
                                         F.Field(F.This(), asyncMethodBuilderField),
                                         asyncMethodBuilderMemberCollection.SetException,
@@ -262,13 +263,13 @@ private BoundBlock VisitAwaitExpression(BoundAwaitExpression node, BoundExpressi
             }
 
             var awaitIfIncomplete = F.Block(
-                // temp $awaiterTemp = <expr>.GetAwaiter();
-                F.Assignment(
+                    // temp $awaiterTemp = <expr>.GetAwaiter();
+                    F.Assignment(
                     F.Local(awaiterTemp),
                     MakeCallMaybeDynamic(expression, getAwaiter, WellKnownMemberNames.GetAwaiter)),
 
-                // if(!($awaiterTemp.IsCompleted)) { ... }
-                F.If(
+                    // if(!($awaiterTemp.IsCompleted)) { ... }
+                    F.If(
                     condition: F.Not(GenerateGetIsCompleted(awaiterTemp, isCompletedMethod)),
                     thenClause: GenerateAwaitForIncompleteTask(awaiterTemp)));
 
@@ -286,7 +287,7 @@ private BoundBlock VisitAwaitExpression(BoundAwaitExpression node, BoundExpressi
                 // $resultTemp
                 LocalSymbol resultTemp = F.SynthesizedLocal(type);
                 return F.Block(
-                    ImmutableArray.Create(awaiterTemp, resultTemp),
+                ImmutableArray.Create(awaiterTemp, resultTemp),
                     awaitIfIncomplete,
                     F.Assignment(F.Local(resultTemp), getResultCall),
                     F.ExpressionStatement(nullAwaiter),
@@ -297,7 +298,7 @@ private BoundBlock VisitAwaitExpression(BoundAwaitExpression node, BoundExpressi
                 // $awaiterTemp.GetResult();
                 // $awaiterTemp = null;
                 return F.Block(
-                    ImmutableArray.Create(awaiterTemp),
+                ImmutableArray.Create(awaiterTemp),
                     awaitIfIncomplete,
                     F.ExpressionStatement(getResultCall),
                     F.ExpressionStatement(nullAwaiter));
@@ -307,7 +308,7 @@ private BoundBlock VisitAwaitExpression(BoundAwaitExpression node, BoundExpressi
         private BoundExpression MakeCallMaybeDynamic(
             BoundExpression receiver,
             MethodSymbol methodSymbol = null,
-            string methodName = null,
+        string methodName = null,
             bool resultsDiscarded = false)
         {
             if ((object)methodSymbol != null)
@@ -327,7 +328,7 @@ private BoundBlock VisitAwaitExpression(BoundAwaitExpression node, BoundExpressi
                 receiver,
                 typeArguments: ImmutableArray<TypeSymbol>.Empty,
                 loweredArguments: ImmutableArray<BoundExpression>.Empty,
-                argumentNames: ImmutableArray<string>.Empty,
+            argumentNames: ImmutableArray<string>.Empty,
                 refKinds: ImmutableArray<RefKind>.Empty,
                 hasImplicitReceiver: false,
                 resultDiscarded: resultsDiscarded).ToExpression();
@@ -366,16 +367,16 @@ private BoundBlock GenerateAwaitForIncompleteTask(LocalSymbol awaiterTemp)
             var blockBuilder = ArrayBuilder<BoundStatement>.GetInstance();
 
             blockBuilder.Add(
-                // this.state = cachedState = stateForLabel
-                F.Assignment(F.Field(F.This(), stateField), F.AssignmentExpression(F.Local(cachedState), F.Literal(stateNumber))));
+                    // this.state = cachedState = stateForLabel
+                    F.Assignment(F.Field(F.This(), stateField), F.AssignmentExpression(F.Local(cachedState), F.Literal(stateNumber))));
 
             blockBuilder.Add(
-                // Emit await yield point to be injected into PDB
-                F.NoOp(NoOpStatementFlavor.AwaitYieldPoint));
+                    // Emit await yield point to be injected into PDB
+                    F.NoOp(NoOpStatementFlavor.AwaitYieldPoint));
 
             blockBuilder.Add(
-                // this.<>t__awaiter = $awaiterTemp
-                F.Assignment(
+                    // this.<>t__awaiter = $awaiterTemp
+                    F.Assignment(
                     F.Field(F.This(), awaiterField),
                     (awaiterField.Type == awaiterTemp.Type)
                         ? F.Local(awaiterTemp)
@@ -392,13 +393,13 @@ private BoundBlock GenerateAwaitForIncompleteTask(LocalSymbol awaiterTemp)
                 F.Label(resumeLabel));
 
             blockBuilder.Add(
-                // Emit await resume point to be injected into PDB
-                F.NoOp(NoOpStatementFlavor.AwaitResumePoint));
+                    // Emit await resume point to be injected into PDB
+                    F.NoOp(NoOpStatementFlavor.AwaitResumePoint));
 
             blockBuilder.Add(
-                // $awaiterTemp = this.<>t__awaiter   or   $awaiterTemp = (AwaiterType)this.<>t__awaiter
-                // $this.<>t__awaiter = null;
-                F.Assignment(
+                    // $awaiterTemp = this.<>t__awaiter   or   $awaiterTemp = (AwaiterType)this.<>t__awaiter
+                    // $this.<>t__awaiter = null;
+                    F.Assignment(
                     F.Local(awaiterTemp),
                     awaiterTemp.Type == awaiterField.Type
                         ? F.Field(F.This(), awaiterField)
@@ -408,8 +409,8 @@ private BoundBlock GenerateAwaitForIncompleteTask(LocalSymbol awaiterTemp)
                 F.Assignment(F.Field(F.This(), awaiterField), F.NullOrDefault(awaiterField.Type)));
 
             blockBuilder.Add(
-                // this.state = cachedState = NotStartedStateMachine
-                F.Assignment(F.Field(F.This(), stateField), F.AssignmentExpression(F.Local(cachedState), F.Literal(StateMachineStates.NotStartedStateMachine))));
+                    // this.state = cachedState = NotStartedStateMachine
+                    F.Assignment(F.Field(F.This(), stateField), F.AssignmentExpression(F.Local(cachedState), F.Literal(StateMachineStates.NotStartedStateMachine))));
 
             return F.Block(blockBuilder.ToImmutableAndFree());
         }
@@ -439,14 +440,22 @@ private BoundStatement GenerateAwaitOnCompletedDynamic(LocalSymbol awaiterTemp)
                 F.WellKnownType(WellKnownType.System_Runtime_CompilerServices_INotifyCompletion),
                 null);
 
-            return F.Block(
-                ImmutableArray.Create(criticalNotifyCompletedTemp),
+            LocalSymbol thisTemp = (F.CurrentClass.TypeKind == TypeKind.Class) ? F.SynthesizedLocal(F.CurrentClass) : null;
+
+            var blockBuilder = ArrayBuilder<BoundStatement>.GetInstance();
 
+            blockBuilder.Add(
                 F.Assignment(
                     F.Local(criticalNotifyCompletedTemp),
-                    // Use reference conversion rather than dynamic conversion:
-                    F.As(F.Local(awaiterTemp), criticalNotifyCompletedTemp.Type)),
+                        // Use reference conversion rather than dynamic conversion:
+                        F.As(F.Local(awaiterTemp), criticalNotifyCompletedTemp.Type)));
 
+            if (thisTemp != null)
+            {
+                blockBuilder.Add(F.Assignment(F.Local(thisTemp), F.This()));
+            }
+
+            blockBuilder.Add(
                 F.If(
                     condition: F.ObjectEqual(F.Local(criticalNotifyCompletedTemp), F.Null(criticalNotifyCompletedTemp.Type)),
 
@@ -454,15 +463,15 @@ private BoundStatement GenerateAwaitOnCompletedDynamic(LocalSymbol awaiterTemp)
                         ImmutableArray.Create(notifyCompletionTemp),
                         F.Assignment(
                             F.Local(notifyCompletionTemp),
-                            // Use reference conversion rather than dynamic conversion:
-                            F.Convert(notifyCompletionTemp.Type, F.Local(awaiterTemp), ConversionKind.ExplicitReference)),
+                                // Use reference conversion rather than dynamic conversion:
+                                F.Convert(notifyCompletionTemp.Type, F.Local(awaiterTemp), ConversionKind.ExplicitReference)),
                         F.ExpressionStatement(
                             F.Call(
                                 F.Field(F.This(), asyncMethodBuilderField),
                                 asyncMethodBuilderMemberCollection.AwaitOnCompleted.Construct(
                                     notifyCompletionTemp.Type,
                                     F.This().Type),
-                                F.Local(notifyCompletionTemp), F.This())),
+                                F.Local(notifyCompletionTemp), F.This(thisTemp))),
                         F.Assignment(
                             F.Local(notifyCompletionTemp),
                             F.NullOrDefault(notifyCompletionTemp.Type))),
@@ -474,32 +483,70 @@ private BoundStatement GenerateAwaitOnCompletedDynamic(LocalSymbol awaiterTemp)
                                 asyncMethodBuilderMemberCollection.AwaitUnsafeOnCompleted.Construct(
                                     criticalNotifyCompletedTemp.Type,
                                     F.This().Type),
-                                F.Local(criticalNotifyCompletedTemp), F.This())))),
+                                F.Local(criticalNotifyCompletedTemp), F.This(thisTemp))))));
 
+            blockBuilder.Add(
                 F.Assignment(
                     F.Local(criticalNotifyCompletedTemp),
                     F.NullOrDefault(criticalNotifyCompletedTemp.Type)));
+
+            return F.Block(
+                SingletonOrPair(criticalNotifyCompletedTemp, thisTemp), 
+                blockBuilder.ToImmutableAndFree());
         }
 
-        private BoundExpressionStatement GenerateAwaitOnCompleted(TypeSymbol loweredAwaiterType, LocalSymbol awaiterTemp)
+        private BoundStatement GenerateAwaitOnCompleted(TypeSymbol loweredAwaiterType, LocalSymbol awaiterTemp)
         {
             // this.builder.AwaitOnCompleted<TAwaiter,TSM>(ref $awaiterTemp, ref this)
             //    or
             // this.builder.AwaitOnCompleted<TAwaiter,TSM>(ref $awaiterArrayTemp[0], ref this)
 
+            LocalSymbol thisTemp = (F.CurrentClass.TypeKind == TypeKind.Class) ? F.SynthesizedLocal(F.CurrentClass) : null;
+
             HashSet<DiagnosticInfo> useSiteDiagnostics = null;
             var useUnsafeOnCompleted = F.Compilation.Conversions.ClassifyImplicitConversion(
                 loweredAwaiterType,
                 F.Compilation.GetWellKnownType(WellKnownType.System_Runtime_CompilerServices_ICriticalNotifyCompletion),
                 ref useSiteDiagnostics).IsImplicit;
+
             var onCompleted = (useUnsafeOnCompleted ?
                 asyncMethodBuilderMemberCollection.AwaitUnsafeOnCompleted :
                 asyncMethodBuilderMemberCollection.AwaitOnCompleted).Construct(loweredAwaiterType, F.This().Type);
-            return F.ExpressionStatement(
+
+            BoundExpression result = 
                 F.Call(
                     F.Field(F.This(), asyncMethodBuilderField),
                     onCompleted,
-                    F.Local(awaiterTemp), F.This()));
+                    F.Local(awaiterTemp), F.This(thisTemp));
+
+            if (thisTemp != null)
+            {
+                result = F.Sequence(
+                    ImmutableArray.Create(thisTemp), 
+                    ImmutableArray.Create<BoundExpression>(F.AssignmentExpression(F.Local(thisTemp), F.This())), 
+                    result); 
+            }
+
+            return F.ExpressionStatement(result);
+        }
+
+        private static ImmutableArray<LocalSymbol> SingletonOrPair(LocalSymbol var1, LocalSymbol var2 = null)
+        {
+            return (var2 == null) ? ImmutableArray.Create(var1) : ImmutableArray.Create(var1, var2);
+        }
+
+        private void MakeThisExpression(out LocalSymbol thisTemp, out BoundExpression @this)
+        {
+            if (F.CurrentClass.TypeKind == TypeKind.Class)
+            {
+                thisTemp = F.SynthesizedLocal(F.CurrentClass);
+                @this = F.Local(thisTemp);
+            }
+            else
+            {
+                thisTemp = null;
+                @this = F.This();
+            }
         }
 
         public override BoundNode VisitReturnStatement(BoundReturnStatement node)
@@ -517,4 +564,4 @@ public override BoundNode VisitReturnStatement(BoundReturnStatement node)
 
         #endregion Visitors
     }
-}
\ No newline at end of file
+}

Src/Compilers/CSharp/Source/Lowering/AsyncRewriter/AsyncRewriter.cs

@@ -31,9 +31,12 @@ internal partial class AsyncRewriter : StateMachineRewriter
                 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);
+            stateMachineType = new AsyncStateMachine(method, typeKind);
             compilationState.ModuleBuilderOpt.CompilationState.SetStateMachineType(method, stateMachineType);
             var rewriter = new AsyncRewriter(bodyWithAwaitLifted, method, stateMachineType, compilationState, diagnostics, generateDebugInfo);
             if (!rewriter.constructedSuccessfully)
@@ -85,23 +88,28 @@ protected override void GenerateMethodImplementations()
             var IAsyncStateMachine_SetStateMachine = F.WellKnownMethod(WellKnownMember.System_Runtime_CompilerServices_IAsyncStateMachine_SetStateMachine);
 
             // Add IAsyncStateMachine.MoveNext()
-            {
-                var moveNextMethod = OpenMethodImplementation(IAsyncStateMachine_MoveNext, "MoveNext", asyncKickoffMethod: this.method, hasMethodBodyDependency: true);
-                GenerateMoveNext(moveNextMethod);
-            }
+            
+            var moveNextMethod = OpenMethodImplementation(IAsyncStateMachine_MoveNext, "MoveNext", asyncKickoffMethod: this.method, hasMethodBodyDependency: true);
+            GenerateMoveNext(moveNextMethod);
 
             // Add IAsyncStateMachine.SetStateMachine()
+            
+            OpenMethodImplementation(IAsyncStateMachine_SetStateMachine, "SetStateMachine", debuggerHidden: true, hasMethodBodyDependency: false);
+            F.CloseMethod(
+                F.Block(
+                    // this.builderField.SetStateMachine(sm)
+                    F.ExpressionStatement(
+                        F.Call(
+                            F.Field(F.This(), builderField),
+                            asyncMethodBuilderMemberCollection.SetStateMachine,
+                            new BoundExpression[] { F.Parameter(F.CurrentMethod.Parameters[0]) })),
+                    F.Return()));
+
+            // Constructor
+            if (stateMachineClass.TypeKind == TypeKind.Class)
             {
-                OpenMethodImplementation(IAsyncStateMachine_SetStateMachine, "SetStateMachine", debuggerHidden: true, hasMethodBodyDependency: false);
-                F.CloseMethod(
-                    F.Block(
-                        // this.builderField.SetStateMachine(sm)
-                        F.ExpressionStatement(
-                            F.Call(
-                                F.Field(F.This(), builderField),
-                                asyncMethodBuilderMemberCollection.SetStateMachine,
-                                new BoundExpression[] { F.Parameter(F.CurrentMethod.Parameters[0]) })),
-                        F.Return()));
+                F.CurrentMethod = stateMachineClass.Constructor;
+                F.CloseMethod(F.Block(ImmutableArray.Create(F.BaseInitialization(), F.Return())));
             }
         }
 

Src/Compilers/CSharp/Source/Lowering/AsyncRewriter/AsyncStateMachine.cs

@@ -11,14 +11,16 @@ namespace Microsoft.CodeAnalysis.CSharp
     /// </summary>
     internal sealed class AsyncStateMachine : SynthesizedContainer, ISynthesizedMethodBodyImplementationSymbol
     {
+        private readonly TypeKind typeKind;
         private readonly MethodSymbol constructor;
         private readonly MethodSymbol asyncMethod;
         private readonly ImmutableArray<NamedTypeSymbol> interfaces;
 
-        public AsyncStateMachine(MethodSymbol asyncMethod)
+        public AsyncStateMachine(MethodSymbol asyncMethod, TypeKind typeKind)
             : base(GeneratedNames.MakeIteratorOrAsyncDisplayClassName(asyncMethod.Name, SequenceNumber(asyncMethod)), asyncMethod)
         {
             // TODO: report use-site errors on these types
+            this.typeKind = typeKind;
             this.asyncMethod = asyncMethod;
             this.interfaces = ImmutableArray.Create(asyncMethod.DeclaringCompilation.GetWellKnownType(WellKnownType.System_Runtime_CompilerServices_IAsyncStateMachine));
             this.constructor = new AsyncConstructor(this);
@@ -42,7 +44,7 @@ private static int SequenceNumber(MethodSymbol method)
 
         public override TypeKind TypeKind
         {
-            get { return TypeKind.Struct; }
+            get { return typeKind; }
         }
 
         internal override MethodSymbol Constructor

Src/Compilers/CSharp/Source/Lowering/SyntheticBoundNodeFactory.cs

@@ -174,6 +174,11 @@ public BoundThisReference This()
             return new BoundThisReference(Syntax, CurrentMethod.ThisParameter.Type) { WasCompilerGenerated = true };
         }
 
+        public BoundExpression This(LocalSymbol thisTempOpt)
+        {
+            return (thisTempOpt != null) ? Local(thisTempOpt) : (BoundExpression)This();
+        }
+
         public BoundBaseReference Base()
         {
             Debug.Assert((object)CurrentMethod != null && !CurrentMethod.IsStatic);

Src/Compilers/CSharp/Source/Symbols/Compilation_WellKnownMembers.cs

@@ -392,11 +392,11 @@ internal SynthesizedAttributeData SynthesizeDebuggableAttribute()
             if (optimizationsDisabled)
             {
                 constantVal |= disableOptsDebuggingMode.GetConstantValue(ConstantFieldsInProgress.Empty, earlyDecodingWellKnownAttributes: false).Int32Value;
+            }
 
-                if (emittingFullDebugInfo)
-                {
-                    constantVal |= enableENCDebuggingMode.GetConstantValue(ConstantFieldsInProgress.Empty, earlyDecodingWellKnownAttributes: false).Int32Value;
-                }
+            if (options.EnableEditAndContinue)
+            {
+                constantVal |= enableENCDebuggingMode.GetConstantValue(ConstantFieldsInProgress.Empty, earlyDecodingWellKnownAttributes: false).Int32Value;
             }
 
             var typedConstantDebugMode = new TypedConstant(debuggingModesType, TypedConstantKind.Enum, constantVal);

Src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenAsyncTests.cs

@@ -34,6 +34,70 @@ private CompilationVerifier CompileAndVerify(string source, string expectedOutpu
             return base.CompileAndVerify(compilation, expectedOutput: expectedOutput, emitOptions: emitOptions);
         }
 
+        [Fact]
+        public void StructVsClass()
+        {
+            var source = @"
+using System.Threading;
+using System.Threading.Tasks;
+
+class Test
+{
+    public static async void F()
+    {
+        await Task.Factory.StartNew(() => {});
+    }
+}";
+            var c = CreateCompilationWithMscorlib45(source);
+
+            CompilationOptions options;
+
+            options = TestOptions.OptimizedDll;
+            Assert.False(options.EnableEditAndContinue);
+
+            CompileAndVerify(c.WithOptions(options), symbolValidator: module =>
+            {
+                var stateMachine = module.GlobalNamespace.GetMember<NamedTypeSymbol>("Test").GetMember<NamedTypeSymbol>("<F>d__1");
+                Assert.Equal(TypeKind.Struct, stateMachine.TypeKind);
+            });
+
+            options = TestOptions.UnoptimizedDll;
+            Assert.False(options.EnableEditAndContinue);
+
+            CompileAndVerify(c.WithOptions(options), symbolValidator: module =>
+            {
+                var stateMachine = module.GlobalNamespace.GetMember<NamedTypeSymbol>("Test").GetMember<NamedTypeSymbol>("<F>d__1");
+                Assert.Equal(TypeKind.Struct, stateMachine.TypeKind);
+            });
+
+            options = TestOptions.OptimizedDll.WithDebugInformationKind(DebugInformationKind.Full);
+            Assert.False(options.EnableEditAndContinue);
+
+            CompileAndVerify(c.WithOptions(options), symbolValidator: module =>
+            {
+                var stateMachine = module.GlobalNamespace.GetMember<NamedTypeSymbol>("Test").GetMember<NamedTypeSymbol>("<F>d__1");
+                Assert.Equal(TypeKind.Struct, stateMachine.TypeKind);
+            });
+
+            options = TestOptions.UnoptimizedDll.WithDebugInformationKind(DebugInformationKind.PDBOnly);
+            Assert.False(options.EnableEditAndContinue);
+
+            CompileAndVerify(c.WithOptions(options), symbolValidator: module =>
+            {
+                var stateMachine = module.GlobalNamespace.GetMember<NamedTypeSymbol>("Test").GetMember<NamedTypeSymbol>("<F>d__1");
+                Assert.Equal(TypeKind.Struct, stateMachine.TypeKind);
+            });
+
+            options = TestOptions.UnoptimizedDll.WithDebugInformationKind(DebugInformationKind.Full);
+            Assert.True(options.EnableEditAndContinue);
+
+            CompileAndVerify(c.WithOptions(options), symbolValidator: module =>
+            {
+                var stateMachine = module.GlobalNamespace.GetMember<NamedTypeSymbol>("Test").GetMember<NamedTypeSymbol>("<F>d__1");
+                Assert.Equal(TypeKind.Class, stateMachine.TypeKind);
+            });
+        }
+
         [Fact]
         public void VoidReturningAsync()
         {
@@ -6709,21 +6773,19 @@ async void M()
 
             // CONSIDER: It would be nice if we didn't squiggle the whole method body, but this is a corner case.
             comp.VerifyEmitDiagnostics(
-    // (4,16): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
-    //     async void M()
-    Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "M").WithLocation(4, 16),
-    // (5,5): error CS0518: Predefined type 'System.Runtime.CompilerServices.AsyncVoidMethodBuilder' is not defined or imported
-    //     { 
-    Diagnostic(ErrorCode.ERR_PredefinedTypeNotFound, @"{ 
-    }").WithArguments("System.Runtime.CompilerServices.AsyncVoidMethodBuilder").WithLocation(5, 5),
-    // (5,5): error CS0656: Missing compiler required member 'System.Runtime.CompilerServices.AsyncVoidMethodBuilder.SetException'
-    //     { 
-    Diagnostic(ErrorCode.ERR_MissingPredefinedMember, @"{ 
-    }").WithArguments("System.Runtime.CompilerServices.AsyncVoidMethodBuilder", "SetException").WithLocation(5, 5)
-                );
+                // (4,16): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
+                //     async void M()
+                Diagnostic(ErrorCode.WRN_AsyncLacksAwaits, "M").WithLocation(4, 16),
+                // (5,5): error CS0518: Predefined type 'System.Runtime.CompilerServices.AsyncVoidMethodBuilder' is not defined or imported
+                //     { 
+                Diagnostic(ErrorCode.ERR_PredefinedTypeNotFound, @"{ 
+                }").WithArguments("System.Runtime.CompilerServices.AsyncVoidMethodBuilder").WithLocation(5, 5),
+                // (5,5): error CS0656: Missing compiler required member 'System.Runtime.CompilerServices.AsyncVoidMethodBuilder.SetException'
+                //     { 
+                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, @"{ 
+                }").WithArguments("System.Runtime.CompilerServices.AsyncVoidMethodBuilder", "SetException").WithLocation(5, 5));
         }
 
-
         [Fact]
         [WorkItem(868822, "DevDiv")]
         public void AsyncDelegates()

Src/Compilers/CSharp/Test/Emit/PDB/PDBAsyncTests.cs

@@ -378,28 +378,28 @@ static async void Await(dynamic d)
         <entry il_offset=""0x7"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
         <entry il_offset=""0x19"" start_row=""5"" start_column=""5"" end_row=""5"" end_column=""6"" file_ref=""0"" />
         <entry il_offset=""0x1a"" start_row=""6"" start_column=""9"" end_row=""6"" end_column=""27"" file_ref=""0"" />
-        <entry il_offset=""0x230"" start_row=""7"" start_column=""5"" end_row=""7"" end_column=""6"" file_ref=""0"" />
-        <entry il_offset=""0x232"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
-        <entry il_offset=""0x234"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
-        <entry il_offset=""0x24c"" start_row=""7"" start_column=""5"" end_row=""7"" end_column=""6"" file_ref=""0"" />
-        <entry il_offset=""0x254"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0x235"" start_row=""7"" start_column=""5"" end_row=""7"" end_column=""6"" file_ref=""0"" />
+        <entry il_offset=""0x237"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0x239"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0x251"" start_row=""7"" start_column=""5"" end_row=""7"" end_column=""6"" file_ref=""0"" />
+        <entry il_offset=""0x259"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
       </sequencepoints>
       <locals>
-        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x261"" attributes=""0"" />
-        <local name=""rez"" il_index=""1"" il_start=""0x19"" il_end=""0x232"" attributes=""0"" />
-        <local name=""&lt;&gt;t__ex"" il_index=""8"" il_start=""0x232"" il_end=""0x24c"" attributes=""0"" />
+        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x266"" attributes=""0"" />
+        <local name=""rez"" il_index=""1"" il_start=""0x19"" il_end=""0x237"" attributes=""0"" />
+        <local name=""&lt;&gt;t__ex"" il_index=""9"" il_start=""0x237"" il_end=""0x251"" attributes=""0"" />
       </locals>
-      <scope startOffset=""0x0"" endOffset=""0x261"">
-        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x261"" attributes=""0"" />
-        <scope startOffset=""0x19"" endOffset=""0x232"">
-          <local name=""rez"" il_index=""1"" il_start=""0x19"" il_end=""0x232"" attributes=""0"" />
+      <scope startOffset=""0x0"" endOffset=""0x266"">
+        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x266"" attributes=""0"" />
+        <scope startOffset=""0x19"" endOffset=""0x237"">
+          <local name=""rez"" il_index=""1"" il_start=""0x19"" il_end=""0x237"" attributes=""0"" />
         </scope>
-        <scope startOffset=""0x232"" endOffset=""0x24c"">
-          <local name=""&lt;&gt;t__ex"" il_index=""8"" il_start=""0x232"" il_end=""0x24c"" attributes=""0"" />
+        <scope startOffset=""0x237"" endOffset=""0x251"">
+          <local name=""&lt;&gt;t__ex"" il_index=""9"" il_start=""0x237"" il_end=""0x251"" attributes=""0"" />
         </scope>
       </scope>
-      <async-info kickoff-method=""100663297"" catch-IL-offset=""564"">
-        <await yield=""340"" resume=""417"" method=""100663299"" />
+      <async-info kickoff-method=""100663297"" catch-IL-offset=""569"">
+        <await yield=""340"" resume=""422"" method=""100663300"" />
       </async-info>
     </method>
   </methods>
@@ -451,7 +451,7 @@ static async Task M(byte b)
       <customDebugInfo version=""4"" count=""2"">
         <forward version=""4"" kind=""ForwardInfo"" size=""12"" declaringType=""C+&lt;&gt;c__DisplayClass0"" methodName=""&lt;M&gt;b__2"" parameterNames="""" />
         <iteratorLocals version=""4"" kind=""IteratorLocals"" size=""20"" bucketCount=""1"">
-          <bucket startOffset=""0x30"" endOffset=""0x1CF"" />
+          <bucket startOffset=""0x30"" endOffset=""0x1D8"" />
         </iteratorLocals>
       </customDebugInfo>
       <sequencepoints total=""16"">
@@ -464,28 +464,28 @@ static async Task M(byte b)
         <entry il_offset=""0x54"" start_row=""11"" start_column=""9"" end_row=""11"" end_column=""21"" file_ref=""0"" />
         <entry il_offset=""0x60"" start_row=""13"" start_column=""9"" end_row=""13"" end_column=""47"" file_ref=""0"" />
         <entry il_offset=""0x77"" start_row=""15"" start_column=""9"" end_row=""15"" end_column=""21"" file_ref=""0"" />
-        <entry il_offset=""0xea"" start_row=""16"" start_column=""9"" end_row=""16"" end_column=""21"" file_ref=""0"" />
-        <entry il_offset=""0x15d"" start_row=""17"" start_column=""9"" end_row=""17"" end_column=""21"" file_ref=""0"" />
-        <entry il_offset=""0x1cd"" start_row=""18"" start_column=""5"" end_row=""18"" end_column=""6"" file_ref=""0"" />
-        <entry il_offset=""0x1cf"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
-        <entry il_offset=""0x1d1"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
-        <entry il_offset=""0x1e8"" start_row=""18"" start_column=""5"" end_row=""18"" end_column=""6"" file_ref=""0"" />
-        <entry il_offset=""0x1f0"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0xed"" start_row=""16"" start_column=""9"" end_row=""16"" end_column=""21"" file_ref=""0"" />
+        <entry il_offset=""0x163"" start_row=""17"" start_column=""9"" end_row=""17"" end_column=""21"" file_ref=""0"" />
+        <entry il_offset=""0x1d6"" start_row=""18"" start_column=""5"" end_row=""18"" end_column=""6"" file_ref=""0"" />
+        <entry il_offset=""0x1d8"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0x1db"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0x1f3"" start_row=""18"" start_column=""5"" end_row=""18"" end_column=""6"" file_ref=""0"" />
+        <entry il_offset=""0x1fb"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
       </sequencepoints>
       <locals>
-        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x1fd"" attributes=""0"" />
-        <local name=""&lt;&gt;t__ex"" il_index=""3"" il_start=""0x1cf"" il_end=""0x1e8"" attributes=""0"" />
+        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x208"" attributes=""0"" />
+        <local name=""&lt;&gt;t__ex"" il_index=""4"" il_start=""0x1d8"" il_end=""0x1f3"" attributes=""0"" />
       </locals>
-      <scope startOffset=""0x0"" endOffset=""0x1fd"">
-        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x1fd"" attributes=""0"" />
-        <scope startOffset=""0x1cf"" endOffset=""0x1e8"">
-          <local name=""&lt;&gt;t__ex"" il_index=""3"" il_start=""0x1cf"" il_end=""0x1e8"" attributes=""0"" />
+      <scope startOffset=""0x0"" endOffset=""0x208"">
+        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x208"" attributes=""0"" />
+        <scope startOffset=""0x1d8"" endOffset=""0x1f3"">
+          <local name=""&lt;&gt;t__ex"" il_index=""4"" il_start=""0x1d8"" il_end=""0x1f3"" attributes=""0"" />
         </scope>
       </scope>
       <async-info kickoff-method=""100663297"">
-        <await yield=""161"" resume=""188"" method=""100663301"" />
-        <await yield=""276"" resume=""303"" method=""100663301"" />
-        <await yield=""391"" resume=""415"" method=""100663301"" />
+        <await yield=""161"" resume=""191"" method=""100663302"" />
+        <await yield=""279"" resume=""309"" method=""100663302"" />
+        <await yield=""397"" resume=""424"" method=""100663302"" />
       </async-info>
     </method>
   </methods>
@@ -549,28 +549,28 @@ static async Task M(byte b)
         <entry il_offset=""0x2b"" start_row=""11"" start_column=""9"" end_row=""11"" end_column=""21"" file_ref=""0"" />
         <entry il_offset=""0x32"" start_row=""13"" start_column=""9"" end_row=""13"" end_column=""47"" file_ref=""0"" />
         <entry il_offset=""0x44"" start_row=""15"" start_column=""9"" end_row=""15"" end_column=""47"" file_ref=""0"" />
-        <entry il_offset=""0xb0"" start_row=""16"" start_column=""5"" end_row=""16"" end_column=""6"" file_ref=""0"" />
-        <entry il_offset=""0xb2"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
-        <entry il_offset=""0xb5"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
-        <entry il_offset=""0xcd"" start_row=""16"" start_column=""5"" end_row=""16"" end_column=""6"" file_ref=""0"" />
-        <entry il_offset=""0xd5"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0xb4"" start_row=""16"" start_column=""5"" end_row=""16"" end_column=""6"" file_ref=""0"" />
+        <entry il_offset=""0xb6"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0xb9"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0xd1"" start_row=""16"" start_column=""5"" end_row=""16"" end_column=""6"" file_ref=""0"" />
+        <entry il_offset=""0xd9"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
       </sequencepoints>
       <locals>
-        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0xe2"" attributes=""0"" />
-        <local name=""CS$&lt;&gt;8__locals1"" il_index=""1"" il_start=""0x16"" il_end=""0xb2"" attributes=""0"" />
-        <local name=""&lt;&gt;t__ex"" il_index=""4"" il_start=""0xb2"" il_end=""0xcd"" attributes=""0"" />
+        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0xe6"" attributes=""0"" />
+        <local name=""CS$&lt;&gt;8__locals1"" il_index=""1"" il_start=""0x16"" il_end=""0xb6"" attributes=""0"" />
+        <local name=""&lt;&gt;t__ex"" il_index=""5"" il_start=""0xb6"" il_end=""0xd1"" attributes=""0"" />
       </locals>
-      <scope startOffset=""0x0"" endOffset=""0xe2"">
-        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0xe2"" attributes=""0"" />
-        <scope startOffset=""0x16"" endOffset=""0xb2"">
-          <local name=""CS$&lt;&gt;8__locals1"" il_index=""1"" il_start=""0x16"" il_end=""0xb2"" attributes=""0"" />
+      <scope startOffset=""0x0"" endOffset=""0xe6"">
+        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0xe6"" attributes=""0"" />
+        <scope startOffset=""0x16"" endOffset=""0xb6"">
+          <local name=""CS$&lt;&gt;8__locals1"" il_index=""1"" il_start=""0x16"" il_end=""0xb6"" attributes=""0"" />
         </scope>
-        <scope startOffset=""0xb2"" endOffset=""0xcd"">
-          <local name=""&lt;&gt;t__ex"" il_index=""4"" il_start=""0xb2"" il_end=""0xcd"" attributes=""0"" />
+        <scope startOffset=""0xb6"" endOffset=""0xd1"">
+          <local name=""&lt;&gt;t__ex"" il_index=""5"" il_start=""0xb6"" il_end=""0xd1"" attributes=""0"" />
         </scope>
       </scope>
       <async-info kickoff-method=""100663297"">
-        <await yield=""106"" resume=""130"" method=""100663301"" />
+        <await yield=""106"" resume=""134"" method=""100663302"" />
       </async-info>
     </method>
   </methods>
@@ -622,7 +622,7 @@ static async Task M(object o)
           <namespace usingCount=""2"" />
         </using>
         <iteratorLocals version=""4"" kind=""IteratorLocals"" size=""20"" bucketCount=""1"">
-          <bucket startOffset=""0x19"" endOffset=""0x288"" />
+          <bucket startOffset=""0x19"" endOffset=""0x28D"" />
         </iteratorLocals>
       </customDebugInfo>
       <sequencepoints total=""11"">
@@ -631,27 +631,27 @@ static async Task M(object o)
         <entry il_offset=""0x19"" start_row=""8"" start_column=""5"" end_row=""8"" end_column=""6"" file_ref=""0"" />
         <entry il_offset=""0x1a"" start_row=""9"" start_column=""9"" end_row=""9"" end_column=""23"" file_ref=""0"" />
         <entry il_offset=""0x26"" start_row=""10"" start_column=""9"" end_row=""10"" end_column=""20"" file_ref=""0"" />
-        <entry il_offset=""0x22e"" start_row=""11"" start_column=""9"" end_row=""11"" end_column=""22"" file_ref=""0"" />
-        <entry il_offset=""0x286"" start_row=""12"" start_column=""5"" end_row=""12"" end_column=""6"" file_ref=""0"" />
-        <entry il_offset=""0x288"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
-        <entry il_offset=""0x28b"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
-        <entry il_offset=""0x2a3"" start_row=""12"" start_column=""5"" end_row=""12"" end_column=""6"" file_ref=""0"" />
-        <entry il_offset=""0x2ab"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0x233"" start_row=""11"" start_column=""9"" end_row=""11"" end_column=""22"" file_ref=""0"" />
+        <entry il_offset=""0x28b"" start_row=""12"" start_column=""5"" end_row=""12"" end_column=""6"" file_ref=""0"" />
+        <entry il_offset=""0x28d"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0x290"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0x2a8"" start_row=""12"" start_column=""5"" end_row=""12"" end_column=""6"" file_ref=""0"" />
+        <entry il_offset=""0x2b0"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
       </sequencepoints>
       <locals>
-        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x2b8"" attributes=""0"" />
-        <local name=""&lt;&gt;t__ex"" il_index=""5"" il_start=""0x288"" il_end=""0x2a3"" attributes=""0"" />
+        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x2bd"" attributes=""0"" />
+        <local name=""&lt;&gt;t__ex"" il_index=""6"" il_start=""0x28d"" il_end=""0x2a8"" attributes=""0"" />
       </locals>
-      <scope startOffset=""0x0"" endOffset=""0x2b8"">
+      <scope startOffset=""0x0"" endOffset=""0x2bd"">
         <namespace name=""System"" />
         <namespace name=""System.Threading.Tasks"" />
-        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x2b8"" attributes=""0"" />
-        <scope startOffset=""0x288"" endOffset=""0x2a3"">
-          <local name=""&lt;&gt;t__ex"" il_index=""5"" il_start=""0x288"" il_end=""0x2a3"" attributes=""0"" />
+        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x2bd"" attributes=""0"" />
+        <scope startOffset=""0x28d"" endOffset=""0x2a8"">
+          <local name=""&lt;&gt;t__ex"" il_index=""6"" il_start=""0x28d"" il_end=""0x2a8"" attributes=""0"" />
         </scope>
       </scope>
       <async-info kickoff-method=""100663297"">
-        <await yield=""376"" resume=""448"" method=""100663299"" />
+        <await yield=""376"" resume=""453"" method=""100663300"" />
       </async-info>
     </method>
   </methods>
@@ -699,30 +699,30 @@ static async Task M(object o)
         <entry il_offset=""0x19"" start_row=""8"" start_column=""5"" end_row=""8"" end_column=""6"" file_ref=""0"" />
         <entry il_offset=""0x1a"" start_row=""9"" start_column=""9"" end_row=""9"" end_column=""23"" file_ref=""0"" />
         <entry il_offset=""0x21"" start_row=""10"" start_column=""9"" end_row=""10"" end_column=""20"" file_ref=""0"" />
-        <entry il_offset=""0x227"" start_row=""11"" start_column=""5"" end_row=""11"" end_column=""6"" file_ref=""0"" />
-        <entry il_offset=""0x229"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
-        <entry il_offset=""0x22c"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
-        <entry il_offset=""0x244"" start_row=""11"" start_column=""5"" end_row=""11"" end_column=""6"" file_ref=""0"" />
-        <entry il_offset=""0x24c"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0x22c"" start_row=""11"" start_column=""5"" end_row=""11"" end_column=""6"" file_ref=""0"" />
+        <entry il_offset=""0x22e"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0x231"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
+        <entry il_offset=""0x249"" start_row=""11"" start_column=""5"" end_row=""11"" end_column=""6"" file_ref=""0"" />
+        <entry il_offset=""0x251"" hidden=""true"" start_row=""16707566"" start_column=""0"" end_row=""16707566"" end_column=""0"" file_ref=""0"" />
       </sequencepoints>
       <locals>
-        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x259"" attributes=""0"" />
-        <local name=""d"" il_index=""1"" il_start=""0x19"" il_end=""0x229"" attributes=""0"" />
-        <local name=""&lt;&gt;t__ex"" il_index=""6"" il_start=""0x229"" il_end=""0x244"" attributes=""0"" />
+        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x25e"" attributes=""0"" />
+        <local name=""d"" il_index=""1"" il_start=""0x19"" il_end=""0x22e"" attributes=""0"" />
+        <local name=""&lt;&gt;t__ex"" il_index=""7"" il_start=""0x22e"" il_end=""0x249"" attributes=""0"" />
       </locals>
-      <scope startOffset=""0x0"" endOffset=""0x259"">
+      <scope startOffset=""0x0"" endOffset=""0x25e"">
         <namespace name=""System"" />
         <namespace name=""System.Threading.Tasks"" />
-        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x259"" attributes=""0"" />
-        <scope startOffset=""0x19"" endOffset=""0x229"">
-          <local name=""d"" il_index=""1"" il_start=""0x19"" il_end=""0x229"" attributes=""0"" />
+        <local name=""cachedState"" il_index=""0"" il_start=""0x0"" il_end=""0x25e"" attributes=""0"" />
+        <scope startOffset=""0x19"" endOffset=""0x22e"">
+          <local name=""d"" il_index=""1"" il_start=""0x19"" il_end=""0x22e"" attributes=""0"" />
         </scope>
-        <scope startOffset=""0x229"" endOffset=""0x244"">
-          <local name=""&lt;&gt;t__ex"" il_index=""6"" il_start=""0x229"" il_end=""0x244"" attributes=""0"" />
+        <scope startOffset=""0x22e"" endOffset=""0x249"">
+          <local name=""&lt;&gt;t__ex"" il_index=""7"" il_start=""0x22e"" il_end=""0x249"" attributes=""0"" />
         </scope>
       </scope>
       <async-info kickoff-method=""100663297"">
-        <await yield=""366"" resume=""441"" method=""100663299"" />
+        <await yield=""366"" resume=""446"" method=""100663300"" />
       </async-info>
     </method>
   </methods>

Src/Compilers/Core/CodeAnalysisTest/CommonCompilationOptionsTests.cs

@@ -26,6 +26,7 @@ public void TestFieldsForEqualsAndGetHashCode()
                 "CryptoKeyFile",
                 "DebugInformationKind",
                 "DelaySign",
+                "EnableEditAndContinue",
                 "Errors",
                 "Features",
                 "FileAlignment",

Src/Compilers/Core/Source/Compilation/CompilationOptions.cs

@@ -376,6 +376,14 @@ internal bool CanReuseCompilationReferenceManager(CompilationOptions other)
                 && object.Equals(this.AssemblyIdentityComparer, other.AssemblyIdentityComparer);
         }
 
+        internal bool EnableEditAndContinue
+        {
+            get
+            {
+                return DebugInformationKind == DebugInformationKind.Full && !Optimize;
+            }
+        }
+
         internal static bool IsValidFileAlignment(int value)
         {
             switch (value)

Src/Compilers/VisualBasic/Source/Symbols/Source/SourceAssemblySymbol.vb

@@ -1412,10 +1412,10 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
 
                     If Not options.Optimize Then
                         debuggingMode = debuggingMode Or DebuggableAttribute.DebuggingModes.DisableOptimizations
+                    End If
 
-                        If options.DebugInformationKind = DebugInformationKind.Full Then
-                            debuggingMode = debuggingMode Or DebuggableAttribute.DebuggingModes.EnableEditAndContinue
-                        End If
+                    If options.EnableEditAndContinue Then
+                        debuggingMode = debuggingMode Or DebuggableAttribute.DebuggingModes.EnableEditAndContinue
                     End If
 
                     Dim typedConstantDebugMode = New TypedConstant(int32Type, TypedConstantKind.Enum, CInt(debuggingMode))