React to new compiler nullability warnings

Commit migrated from dotnet/coreclr@b5ccdc3

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs

@@ -570,13 +570,13 @@ private static void ExecutionContextCallback(object? s)
             {
                 Debug.Assert(s is AsyncStateMachineBox<TStateMachine>);
                 // Only used privately to pass directly to EC.Run
-                Unsafe.As<AsyncStateMachineBox<TStateMachine>>(s).StateMachine.MoveNext();
+                Unsafe.As<AsyncStateMachineBox<TStateMachine>>(s).StateMachine!.MoveNext();
             }
 
             /// <summary>A delegate to the <see cref="MoveNext()"/> method.</summary>
             private Action? _moveNextAction;
             /// <summary>The state machine itself.</summary>
-            [AllowNull, MaybeNull] public TStateMachine StateMachine = default!; // mutable struct; do not make this readonly. SOS DumpAsync command depends on this name.  // TODO-NULLABLE: Remove ! when nullable attributes are respected
+            [AllowNull, MaybeNull] public TStateMachine StateMachine = default; // mutable struct; do not make this readonly. SOS DumpAsync command depends on this name.
             /// <summary>Captured ExecutionContext with which to invoke <see cref="MoveNextAction"/>; may be null.</summary>
             public ExecutionContext? Context;
 
@@ -601,7 +601,8 @@ private void MoveNext(Thread? threadPoolThread)
                 ExecutionContext? context = Context;
                 if (context == null)
                 {
-                    StateMachine.MoveNext();
+                    Debug.Assert(StateMachine != null);
+                    StateMachine!.MoveNext(); // TODO-NULLABLE: remove ! when Debug.Assert on fields is respected (https://github.com/dotnet/roslyn/issues/36830)
                 }
                 else
                 {
@@ -620,7 +621,7 @@ private void MoveNext(Thread? threadPoolThread)
                     // Clear out state now that the async method has completed.
                     // This avoids keeping arbitrary state referenced by lifted locals
                     // if this Task / state machine box is held onto.
-                    StateMachine = default!; // TODO-NULLABLE: Remove ! when nullable attributes are respected
+                    StateMachine = default;
                     Context = default;
 
 #if !CORERT
@@ -641,7 +642,7 @@ private void MoveNext(Thread? threadPoolThread)
             }
 
             /// <summary>Gets the state machine as a boxed object.  This should only be used for debugging purposes.</summary>
-            IAsyncStateMachine IAsyncStateMachineBox.GetStateMachineObject() => StateMachine; // likely boxes, only use for debugging
+            IAsyncStateMachine IAsyncStateMachineBox.GetStateMachineObject() => StateMachine!; // likely boxes, only use for debugging
         }
 
         /// <summary>Gets the <see cref="System.Threading.Tasks.Task{TResult}"/> for this builder.</summary>

src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/FutureFactory.cs

@@ -1260,14 +1260,15 @@ internal static void CompleteFromAsyncResult(IAsyncResult asyncResult)
                 // Grab the relevant state and then null it out so that the task doesn't hold onto the state unnecessarily
                 var thisRef = promise!.m_thisRef; // TODO-NULLABLE: Remove ! when [DoesNotReturn] respected
                 var endMethod = promise.m_endMethod;
-                promise.m_thisRef = default!; // TODO-NULLABLE: Remove ! when nullable attributes are respected
+                promise.m_thisRef = default;
                 promise.m_endMethod = null;
                 if (endMethod == null) ThrowHelper.ThrowArgumentException(ExceptionResource.InvalidOperation_WrongAsyncResultOrEndCalledMultiple, ExceptionArgument.asyncResult);
 
                 // Complete the promise.  If the IAsyncResult completed synchronously,
                 // we'll instead complete the promise at the call site.
                 if (!asyncResult.CompletedSynchronously)
                 {
+                    Debug.Assert(thisRef != null);
                     promise.Complete(thisRef, endMethod!, asyncResult, requiresSynchronization: true); // TODO-NULLABLE: Remove ! when [DoesNotReturn] respected
                 }
             }