Describe the situation

The struct itself is big, and we also generate some loops over struct members, it leads to very large functions, large stack frames, too many local variables in a function, and too many symbols for anonymous lambda functions in the code.

Proposed solution

If we replace settings' access from, e.g., settings.max_threads (a struct member) to settings[Settings::max_threads] (a map indexed by an integer), we can get rid of these problems, and keep the ergonomics (Settings::max_threads will still be auto-completed and validated at compile time).