StateMachineBoxing.cs

public struct AsyncTaskMethodBuilder
{
    private IAsyncStateMachine _stateMachine;
    private Action _moveNextRunner;

    public void AwaitUnsafeOnCompleted<TAwaiter, TStateMachine>(ref TAwaiter awaiter, ref TStateMachine stateMachine)
        where TAwaiter : ICriticalNotifyCompletion
        where TStateMachine : IAsyncStateMachine
    {
        if (_stateMachine == null)
        {
            IAsyncStateMachine boxed = stateMachine;
            boxed.SetStateMachine(boxed);
            _moveNextRunner = () => boxed.MoveNext();
        }

        awaiter.UnsafeOnCompleted(_moveNextRunner);
    }

    public void SetStateMachine(IAsyncStateMachine stateMachine) => _stateMachine = stateMachine;
}

public interface IAsyncStateMachine
{
    public void MoveNext();

    public void SetStateMachine(IAsyncStateMachine stateMachine);
}

public struct StateMachine : IAsyncStateMachine
{
    public AsyncTaskMethodBuilder _methodBuilder; 

    public void MoveNext()
    {
        // Implementation
    }

    public void SetStateMachine(IAsyncStateMachine stateMachine)
    {
        _methodBuilder.SetStateMachine(stateMachine);
    }
}