In this case, it means your actual error will be packed and boxed someplace behind a smart pointer, its type will be lost (so you wouldn't be able to match on i

In this case, it means your actual error will be packed and boxed someplace behind a smart pointer, its type will be lost (so you wouldn't be able to match on its specific variants downstream might you want to) but it will be still able to format itself to a string and will comply with std::error::Error trait.