Static Allocation, Constant Work

Static Allocation, Constant Work Sep 2, 2026 In reply to this email: Memory Safety’s Hardest Problem named something I’d hit but couldn’t articulate. Your case is a pointer into one union variant surviving a write of a different variant, so live typed pointers end up reading bytes that belong to something else now. Last year I wrote a limit-order matching engine and shipped a use-after-free: a cancelled order was released back to the pool while it was still linked into its price level, so the next allocation handed that memory to a new order and the stale link kept resolving. I’d filed it under “I was careless with lifetimes.” After your post I’m not sure that’s what it was. A recycling pool looks like a tagged union where the tag is “which generation of object currently lives in this slot,” and nothing in the type system tracks it. Is that a fair reading, or does the pool case stay genuinely easier because generational indices actually solve it and the union case has no equivalent?…