You don’t need to downcast to interface, io.Reader is already an interface, and a type assertion on an interface (“if this io.Reader is just a byteslice and cursor, then use the byteslice”) is strictly safer than an untagged union and equally safe with a tagged union.

I wish Go had Rust-like enums as well, but they don’t make anything safer in this case (except for the nil interface concern which isn’t the point you’re raising).