One of the problems with something like “serde” is that due to coherency rules, only the crate that creates a new trait may impl that trait for an arbitrary crate and only a crate that declares a struct may impl an arbitrary trait for that struct. 3rd party crates may not impl traits they did not define themselves for structs they did not themselves define.
I wonder if there could ever be a notion of “Friend” or “Blessed” impl crate to work around this restriction in a way that maintains soundness? What if a crate could declare “friend” crates and those “friend” crates would be permitted to provide impls for arbitrary 3rd party traits?