Could collections hypothetically store keys and values inline?

In my experience, having a large number of small hash maps is very rare—in part because a hash map is a bad implementation for tiny sets/maps anyway (faster to do linear search over an array than hash stuff).

By comparison, "a bunch of short strings" is a common use case. I think it is reasonable to say that if the Rust standard library is willing to give up small string optimizations for pointer stability guarantees, giving up a purely hypothetical[1] small hashmap optimization is also worth it.

Even if you take the position that making this choice with String was a mistake (which I find to be a relatively compelling case), that choice has been made and having the same consistency and guarantees in other places at negligible sacrifice seems to make sense to me.

1. To my knowledge, nobody has bothered to implement this: compact_str exists, compact_map does not, to my knowledge. This is not an optimization C++ bothers to make, nor is it an optimization that Google's C++ hashmap implementation which Hashbrown is based off makes afaik. If you are aware of any case—in any language—where somebody actually does a small hashmap optimization like this, I would be curious to see it. ↩︎