Extern types have been blocked for an unreasonably long time on a fairly narrow, specialized question: Rust today divides all types into two categories — sized, whose size can be statically computed, and unsized, whose size can only be computed at runtime. But for external types what we really want is a third category, types whose size can never be known, even at runtime (in C, you can model this by defining structs with an unknown set of fields). The problem is that Rust’s ?Sized notatio...| smallcultfollowing.com
The Dark Arts of Advanced and Unsafe Rust Programming| doc.rust-lang.org