Hah, I remember error_chain. One of my projects during an internship was upgrading a bunch of the old error handling libraries to the new things. I'm glad that corner of the ecosystem has stabilized now.

At that same job we hit a pretty nasty breaking change where mem::uninitialized() was deprecated and this turned out to cause a lot of critical async libraries to explode at runtime. But these sorts of things don't really happen anymore. The editions system is an excellent design and a big contributor to making the language and stdlib reliable.