Rust's Rayon could probably have a mode with additional heuristics, so it wouldn't multi-thread if it guessed the memory usage increase or overhead would be too

Rust's Rayon could probably have a mode with additional heuristics, so it wouldn't multi-thread if it guessed the memory usage increase or overhead would be too much. Not really automatic, but for some iterator chains you can just slap an adapter on. Currently you have to think and benchmark, but for some scripting type applications the increased overhead of the heuristics might be justified, as long as it was a new mode.