1 files changed, 79 insertions, 2 deletions

diff --git a/lib/libcxx/docs/UsingLibcxx.rst b/lib/libcxx/docs/UsingLibcxx.rst
index e10a27c598a..899656cca1d 100644
--- a/lib/libcxx/docs/UsingLibcxx.rst
+++ b/lib/libcxx/docs/UsingLibcxx.rst
@@ -203,8 +203,10 @@ thread safety annotations.
   This macro disables the additional diagnostics generated by libc++ using the
   `diagnose_if` attribute. These additional diagnostics include checks for:
 
-    * Giving `set`, `map`, `multiset`, `multimap` a comparator which is not
-      const callable.
+    * Giving `set`, `map`, `multiset`, `multimap` and their `unordered_`
+      counterparts a comparator which is not const callable.
+    * Giving an unordered associative container a hasher that is not const
+      callable.
 
 **_LIBCPP_NO_VCRUNTIME**:
   Microsoft's C and C++ headers are fairly entangled, and some of their C++
@@ -226,6 +228,26 @@ thread safety annotations.
   replacement scenarios from working, e.g. replacing `operator new` and
   expecting a non-replaced `operator new[]` to call the replaced `operator new`.
 
+**_LIBCPP_ENABLE_NODISCARD**:
+  Allow the library to add ``[[nodiscard]]`` attributes to entities not specified
+  as ``[[nodiscard]]`` by the current language dialect. This includes
+  backporting applications of ``[[nodiscard]]`` from newer dialects and
+  additional extended applications at the discretion of the library. All
+  additional applications of ``[[nodiscard]]`` are disabled by default.
+  See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>` for
+  more information.
+
+**_LIBCPP_DISABLE_NODISCARD_EXT**:
+  This macro prevents the library from applying ``[[nodiscard]]`` to entities
+  purely as an extension. See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>`
+  for more information.
+
+**_LIBCPP_ENABLE_DEPRECATION_WARNINGS**:
+  This macro enables warnings when using deprecated components. For example,
+  when compiling in C++11 mode, using `std::auto_ptr` with the macro defined
+  will trigger a warning saying that `std::auto_ptr` is deprecated. By default,
+  this macro is not defined.
+
 C++17 Specific Configuration Macros
 -----------------------------------
 **_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES**:
@@ -238,3 +260,58 @@ C++17 Specific Configuration Macros
 
 **_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR**:
   This macro is used to re-enable `std::auto_ptr` in C++17.
+
+C++2a Specific Configuration Macros:
+------------------------------------
+**_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17**:
+  This macro can be used to disable diagnostics emitted from functions marked
+  ``[[nodiscard]]`` in dialects after C++17.  See :ref:`Extended Applications of [[nodiscard]] <nodiscard extension>`
+  for more information.
+
+
+Libc++ Extensions
+=================
+
+This section documents various extensions provided by libc++, how they're
+provided, and any information regarding how to use them.
+
+.. _nodiscard extension:
+
+Extended applications of ``[[nodiscard]]``
+------------------------------------------
+
+The ``[[nodiscard]]`` attribute is intended to help users find bugs where
+function return values are ignored when they shouldn't be. After C++17 the
+C++ standard has started to declared such library functions as ``[[nodiscard]]``.
+However, this application is limited and applies only to dialects after C++17.
+Users who want help diagnosing misuses of STL functions may desire a more
+liberal application of ``[[nodiscard]]``.
+
+For this reason libc++ provides an extension that does just that! The
+extension must be enabled by defining ``_LIBCPP_ENABLE_NODISCARD``. The extended
+applications of ``[[nodiscard]]`` takes two forms:
+
+1. Backporting ``[[nodiscard]]`` to entities declared as such by the
+   standard in newer dialects, but not in the present one.
+
+2. Extended applications of ``[[nodiscard]]``, at the libraries discretion,
+   applied to entities never declared as such by the standard.
+
+Users may also opt-out of additional applications ``[[nodiscard]]`` using
+additional macros.
+
+Applications of the first form, which backport ``[[nodiscard]]`` from a newer
+dialect may be disabled using macros specific to the dialect it was added. For
+example ``_LIBCPP_DISABLE_NODISCARD_AFTER_CXX17``.
+
+Applications of the second form, which are pure extensions, may be disabled
+by defining ``_LIBCPP_DISABLE_NODISCARD_EXT``.
+
+
+Entities declared with ``_LIBCPP_NODISCARD_EXT``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This section lists all extended applications of ``[[nodiscard]]`` to entities
+which no dialect declares as such (See the second form described above).
+
+* ``get_temporary_buffer``