It means that if you have a library A providing a trait and a library B providing a type, either A has to add optional support for B or B has to add optional support for A, or someone has to hack around that with a newtype wrapper. Usually, whichever library is less popular ends up adding optional support for the more popular library. This is, for instance, one reason why it's really really hard to write a replacement for serde: you'd have to get every crate currently providing optional serde support to provide optional support for your library as well.
In other ecosystems, you'd either add quick-and-dirty support in your application, or you'd write (and perhaps publish) an A-B crate that implements support for using A and B together. This should be possible in Rust.
