The trouble I’ve had as a beginner is crafting error types for those “Union of multiple existing error types”. E.g., myfunc() can return an IO error or a Parse error. The boilerplate for creating a new error type (absent macros) is significant, and while I’m aware that there are macro crates available which automate this, it’s not clear to me which of these if any are “most standard” or otherwise how to distinguish between them.