[_bxg1]

Right. The problem is that functional purity always comes with overhead; data structures like Clojure's help a whole lot, but there's still a cost.

But one of the primary motivators for functional purity is to avoid unintended side-effects.

In pure FP, You never even have to bother your mind with unexpected side-effects because there are no side-effects.

Rust instead gives you the vocabulary to carefully articulate intended side-effects, preventing all other, unexpected side-effects at the same time. So you can colonize the wilderness, instead of avoiding it altogether, and for that you get to skip the overhead of treating everything as immutable. Not that this is an indisputable improvement for every use-case, but it's a novel tradeoff and one that is definitely preferable in many domains.