The compat issue has always been associated types on std traits.

For example, should Iterator::Item be Move or ?Move

If you leave it as Move, you can't create any iterators over !Move types. If you change it to ?Move, then functions using generic iterators can't assume that the elements of an Iterator are always moveable. Which is a breaking change compared to now.

The most critical trait is probably Deref. Using !Move types without `Deref::Target: ?Move` is painful, because calling any method on boxed types relies on Deref.