I can only imagine that

1. it’s to performance sensitive

Or

2. The os is so darn large it’s hard to recompile everything

I worked at Apple for a long time. The OS gets fully recompiled regularly.

A simultaneous total world build is relatively rare (is that needed here?), but it does happen. Sometimes new compiler versions or features need this.

Hm that leaves more questions for me. Why does this path not have bounds checking, is think perhaps a limit of the clang flag or is it more simply a mistake of omission on apples part. Either way it seems like a bad look. I wish we’d get a post mortem

I dunno if that's sensitive information, but how long did a build usually take?

I wasn't close enough to that to know how long a world build took. Didn't seem like it was too crazy though. Incremental (non-world) builds of the OS come out every day.