My maybe-wrong sense was more than the MIR could change from one compiler release to the next, but that the editions couldn't introduce language changes that wo

My maybe-wrong sense was more than the MIR could change from one compiler release to the next, but that the editions couldn't introduce language changes that would require any given release of the compiler to have to produce MIR of a different kind for code of one edition vs. another. You'll be able to compile a single binary relying on different libraries pinned to different editions, and I think the idea is that from the MIR stage of the build onward, whatever differences may exist (e.g., a reserved word in one edition might be a function name in another) have to be papered over so that whole-program optimization/compilation can occur.