- do I really need threads when I can share non-mutable messages or copies of same? Maybe (speed, throughput). Maybe not. Depends. What's clear is I need choice.

- how do I get correctness? Rust maybe checks for index bounds. But in C++ I just relegate that to a library and voila std::string/vector.

I am concerned for Rust: placing too much emphasis on correctness through language alone seems to be a weak point. Correctness through libraries and augmenting with formal tools seems like a much stronger solution.