Could collections hypothetically store keys and values inline?

robofinch:

Or is there some reason I'm missing that already prevents std::collections data structures from storing values and (if applicable) keys inline, even hypothetically? In other words, can unsafe code stably and soundly use e.g. a HashMap as backing data for self-references to keys and values?

That's a libs-api question, not a UCG question (so you put this in the wrong section of this forum). And the answer is no, you can't rely on anything that's not explicitly documented.