This wouldn't be a perfect solution. Due to conditional compilation, any given variable in the source code might not exist on certain platforms, or it might even have a different type depending on the platform or configuration. So the question of "what type does this variable have" doesn't always have a single answer to record in a lockfile. And macros make things even harder.

But even as an imperfect solution, I think it still has the potential to avert most real-world cases of breakage caused by Rust upgrades (and even dependency upgrades). As a bonus, the same mechanism could potentially also be used to detect breaking API changes.

Would that not be "take the outcome of said type inference, write it into the LHS as a type declaration or into the RHS as a generic parameter", aka make the inferred type permanently declared? We're not totally powerless for addressing the practical implication here, and I don't know how we could have stable yet still implicit type inference across compiler versions unless that type inference logic never changed ever again.