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Being actually enforced, even in release.

Most folks don't use those #defines, and many still haven't leaned about them.

It's a great question (_LIBCPP_DEBUG was already a thing in libc++), and AFAIK the answer is supposedly "it used to be too costly to enable these in production with libc++, and it no longer is." I have no first-hand insight as to how accurate this perception is.

That's exactly right. We've had extra hardening enabled in tests, and that does catch many issues. But tests can't exercise every potential out-of-bounds issue, which is why enabling it prod enabled us to find & fix additional issues.

They turned those on and 1. checked that the software using it didn't break and 2. made sure it didn't tank performance.

Source: I worked on this apparently