Ensure that partially assigned structures are properly hoisted by async rewriter.

Fixes #1942.

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

@@ -130,17 +130,33 @@ private void MarkLocalsUnassigned()
             for (int i = 0; i < nextVariableSlot; i++)
             {
                 var symbol = variableBySlot[i].Symbol;
-                var local = symbol as LocalSymbol;
-                if ((object)local != null && !local.IsConst)
-                {
-                    SetSlotState(i, false);
-                    continue;
-                }
 
-                var parameter = symbol as ParameterSymbol;
-                if ((object)parameter != null)
+                if ((object)symbol != null)
                 {
-                    SetSlotState(i, false);
+                    switch (symbol.Kind)
+                    {
+                        case SymbolKind.Local:
+                            if (!((LocalSymbol)symbol).IsConst)
+                            {
+                                SetSlotState(i, false);
+                            }
+                            break;
+
+                        case SymbolKind.Parameter:
+                            SetSlotState(i, false);
+                            break;
+
+                        case SymbolKind.Field:
+                            if (!((FieldSymbol)symbol).IsConst)
+                            {
+                                SetSlotState(i, false);
+                            }
+                            break;
+
+                        default:
+                            Debug.Assert(false);
+                            break;
+                    }
                 }
             }
         }

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

@@ -3288,5 +3288,45 @@ .maxstack  3
 }",
             sequencePoints: "Test+<F>d__2.MoveNext");
         }
+
+        [Fact, WorkItem(1942, "https://github.com/dotnet/roslyn/issues/1942")]
+        public void HoistStructure()
+        {
+            var source = @"
+using System;
+using System.Threading.Tasks;
+namespace ConsoleApp
+{
+    struct TestStruct
+    {
+        public long i;
+        public long j;
+    }
+    class Program
+    {
+        static async Task TestAsync()
+        {
+            TestStruct t;
+            t.i = 12;
+            Console.WriteLine(""Before {0}"", t.i); // emits ""Before 12"" 
+            await Task.Delay(100);
+            Console.WriteLine(""After {0}"", t.i); // emits ""After 0"" expecting ""After 12"" 
+        }
+        static void Main(string[] args)
+        {
+            TestAsync().Wait();
+        }
+    }
+}";
+
+            var expectedOutput = @"Before 12
+After 12";
+
+            var comp = CreateCompilation(source, options: TestOptions.DebugExe);
+
+            CompileAndVerify(comp, expectedOutput: expectedOutput);
+
+            CompileAndVerify(comp.WithOptions(TestOptions.ReleaseExe), expectedOutput: expectedOutput);
+        }
     }
 }

src/Compilers/VisualBasic/Test/Emit/CodeGen/CodeGenAsyncTests.vb

@@ -8386,6 +8386,43 @@ End Class
    at C.VB$StateMachine_2_M.MoveNext()")
         End Sub
 
+        <Fact, WorkItem(1942, "https://github.com/dotnet/roslyn/issues/1942")>
+        Public Sub HoistStructure()
+            Dim source =
+<compilation name="Async">
+    <file name="a.vb">
+Imports System
+Imports System.Threading.Tasks
+
+Structure TestStruct
+    Public i As Long
+    Public j As Long
+End Structure
+
+Class Program
+    Shared Async Function TestAsync() As Task
+        Dim t As TestStruct
+        t.i = 12
+        Console.WriteLine("Before {0}", t.i)
+        Await Task.Delay(100)
+        Console.WriteLine("After {0}", t.i)
+    End Function
+
+    Shared Sub Main()
+        TestAsync().Wait()
+    End Sub
+End Class
+    </file>
+</compilation>
+
+            Dim expectedOutput = <![CDATA[Before 12
+After 12]]>
+
+            Dim compilation = CompilationUtils.CreateCompilationWithReferences(source, references:=Me.LatestReferences, options:=TestOptions.DebugExe)
+            CompileAndVerify(compilation, expectedOutput:=expectedOutput)
+
+            CompileAndVerify(compilation.WithOptions(TestOptions.ReleaseExe), expectedOutput:=expectedOutput)
+        End Sub
     End Class
 End Namespace