I looked at trying to implement -fbounds-safety and -Wunsafe-buffer on a reasonably large codebase (4,000 C and C++ files), and it's basically impossible.

You have to instrument every single file. It can be done in stages though. Just turn the flag on one-by-one for each file. The xnu kernel is _mostly_ instrumented with -fbounds-safety.

There is GWPAsan that has lower overhead than asan but still is not super popular.