Libc++ aims to preserve a stable ABI to avoid subtle bugs when code built under the old ABI is linked with code built under the new ABI. At the same time, libc++ wants to make ABI-breaking improvements and bugfixes in scenarios where the user doesn't mind ABI breaks.
To support both cases, libc++ allows specifying an ABI version at build time. The version is defined with CMake option LIBCXX_ABI_VERSION. Currently supported values are 1 (the stable default) and 2 (the unstable "next" version). At some point "ABI version 2" will be frozen and new ABI-breaking changes will start being applied to version 3; but this has not happened yet.
To always use the most cutting-edge, most unstable ABI (which is currently 2 but at some point will become 3), set the CMake option LIBCXX_ABI_UNSTABLE.
Internally, each ABI-changing feature is placed under its own C++ macro, _LIBCPP_ABI_XXX. These macros' definitions are controlled by the C++ macro _LIBCPP_ABI_VERSION, which is controlled by the LIBCXX_ABI_VERSION set at build time. Libc++ does not intend users to interact with these C++ macros directly.
The exception to this is MSVC environments. Libc++ does not currently have users that require a stable ABI in MSVC environments, so MSVC-only changes may be applied unconditionally.
