This is a real problem; i’m not sure rooting all mutable state on the stack will help in this case without language-level metadata restricting frees, passing references, and dereferencing.

It seems like the rust answer of making it difficult to share mutable state is a solid answer—you explicitly manage concurrent state, rather than implicitly allowing sharing. In addition rust has some interesting run-once constraints you can add to closures that work well with global initialization. I’ll admit I haven’t seen enough rust to see it work well in action, but the pieces are there.