According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

Should we maybe keep version_id up front and read it first so that we can avoid format change issues?

It is a bit more nuanced than that, on one hand we have things like --skip-version-check, so we shouldn't blanket abandon cache if version_id is different, on other hand, we need to abandon cache if version of librt changed (incompatibly) even if mypy version is the same. So my idea is to have two first bytes in cache meta be: librt (low level) cache format version (that would be bumped each time cache layout changes, even if ABI stays the same), and mypy (high level) cache format version, that would be bumped each time we change e.g. type tags, or other high-level cache logic.

I am going to add something like this in a separate PR.