Add docs for use-configurable libc++ features
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@286784 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/UsingLibcxx.rst b/docs/UsingLibcxx.rst
index 514ed14..8234994 100644
--- a/docs/UsingLibcxx.rst
+++ b/docs/UsingLibcxx.rst
@@ -123,3 +123,29 @@
Known 3rd Party Implementations Include:
* `Koutheir's libc++ pretty-printers <https://github.com/koutheir/libcxx-pretty-printers>`_.
+
+
+Libc++ Configuration Macros
+===========================
+
+Libc++ provides a number of configuration macros which can be used to enable
+or disable extended libc++ behavior, including enabling "debug mode" or
+thread safety annotations.
+
+**_LIBCPP_DEBUG**:
+ This macro is used to enable assertions and other debugging checks within
+ libc++. All debugging checks are disabled by default.
+
+ **Values**: ``0``, ``1``
+
+ Defining ``_LIBCPP_DEBUG`` to ``0`` or greater enables most of libc++'s
+ assertions. Defining ``_LIBCPP_DEBUG`` to ``1`` enables "iterator debugging"
+ which provides additional assertions about the validity of iterators used by
+ the program.
+
+ Note that this option has no effect on libc++'s ABI
+
+**_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS**:
+ This macro is used to enable -Wthread-safety annotations on libc++'s
+ ``std::mutex`` and ``std::lock_guard``. By default these annotations are
+ disabled and must be manually enabled by the user.