So if we replace Ounchecked with an option -unsafe-remove-checks (similar to -assume-single-threaded), as Johannes suggested, this is more like a “at your own risk” thing (regarding performance). For example, it might happen that users see perf regressions from one release to another, using this option.

I would suggest going further and removing the code for -unsafe-remove-checks altogether. Even untested code has a cost in terms of maintainability.