> So it is surprising to me that it doesn’t blow up somewhere when that invariant doesn’t hold.

The final program may be broken in various manners because you don't respect the language's prescribed semantics, in about the same way they do in C and C++. From the compiler's perspective the borrow checker validates that rules it assumes are upheld are actually upheld.

mrustc already compiles rust code without having a borrow checker (well IIRC recent-ish versions of mrustc have some borrow checking bits, but for the most part it still assumes that somebody else has done all the borrow checking).