- We previously compiled libcxx with _LIBCPP_DEBUG=0. In old libcxx
versions, this (surprisingly) enabled the basic debug mode [1].
- In libcxx 15 (the version we are currently using), _LIBCPP_DEBUG=0
does nothing [2], the replacement is _LIBCPP_ENABLE_DEBUG_MODE=1.
- The debug mode is only enabled in Debug builds. Their docs say the
extra check change complexity guarantees + the asserts crash which we
don't want in Release builds. The debug mode detects issues like in
[3]. Crashes look like this (for example)
```
/data/ch/contrib/llvm-project/libcxx/include/__iterator/wrap_iter.h:99: assertion ::std::__libcpp_is_constant_evaluated() || (__get_const_db()->__dereferenceable(this)) failed: Attempted to increment a non-incrementable iteratorAborted (core dumped)
```
- I had to mute some new clang-tidy warnings in places that deal with
container iterators. They got heavier and copy-by-value now yields a
warning, e.g.
for (auto it : iterators) /// <-- warning
[...]
[1] https://releases.llvm.org/12.0.0/projects/libcxx/docs/DesignDocs/DebugMode.html
[2] https://releases.llvm.org/15.0.0/projects/libcxx/docs/DesignDocs/DebugMode.html
[3] https://github.com/llvm/llvm-project/blob/main/libcxx/test/support/container_debug_tests.h
