Like Python 2 vs 3? That's what Editions were created to fix. With backwards incompatibility you will get ecosystem breakage and looming threat of future compilers not compiling your code.
The problem is that assuring no breaks ever with inference means you can't ever improve the inference algorithm nor update the stdlib. This is what triggered the time 0.35 breakage. I have a still incomplete/unmerged rustc lint to avoid the situation that caused that (a useless .into() that didn't get flagged because the clippy lint has false positives so it is not on by default) which should minimize the likelihood of that happening again (once I get off my ass and finish it, it just requires some side-quests to add more accurate tracking of cfg'd out items). This might be able to be mitigated by editions, but in practice crater helps to not need that (yet?).
This is not the only kind of breakage a project can experience. Trying to bring up an older project on a new platform will be a compile error (like building a project from 2018 on an Mx Mac), and updating the appropriate dependency to an appropriate version might become a chore. I have no idea how to improve the situation there to make that less painful, beyond having a simple database of crate-version+platform+rustc-version so that the toolchain could provide better/actionable messaging beyond "shit's broken".
