Custom awaitables for dummies

Why would you want a custom awaiter?

You can see the compiler’s interpretation of await here. Essentially:

var temp = e.GetAwaiter();
if (!temp.IsCompleted)
{
  SAVE_STATE()
  temp.OnCompleted(&cont);
  return;

cont:
  RESTORE_STATE()
}
var i = temp.GetResult();

Edit from comments: OnCompleted should schedule its argument as a continuation of the asynchronous operation.