As somebody else wrote, what's really important is to stabilize a high level parallel iterator API that would probably be common even between Rayon 1 and 2. Of course I see some new APIs, like setting the task splitting policy, but some things, like parallel iteration over a vector and mapping through it should still work.

Java had to change the collection API when it added generics, but it showed that it's possible to split the API contract while improving on the underlying implementation over time. The low level Rayon primitives of course shouldn't be in the standard library name space.