There have already been two sets of backwards-incompatible changes in Rust, and there's a third on the way in the 2024 Edition (unfortunately due out in early 2025). They tend to be relatively conservative, but that's because they want it to be easy to update your code, but there is stuff the team want to do that requires syntactic or semantic incompatibilities, so that's what that mechanism is for.

It's a bit restricted on how much you can do because they do promise compatibility with older crates, but it seems to be working out pretty well and that compatibility promise is part of why it does work.