The idea of levels or areas is always applicable. A vast open game like Star Citizen is partitioned into area spaces, usually in a cubic array. When you move out of range of one of the cubes, past the point where you can see it, then all resources in that cube can be safely freed.

I don't know where Rust came from, since the discussion was about C++. But feel free to write an engine in Rust. It seems like a promising approach.