[libc++][doc] Update the release notes for LLVM 19. (#99061)

This is a preparation for the upcoming LLVM 19 release.

libcxx/docs/ReleaseNotes/19.rst

@@ -35,6 +35,20 @@ see the `releases page <https://llvm.org/releases/>`_.
 What's New in Libc++ 19.0.0?
 ==============================
 
+The main focus of the libc++ team has been to implement new C++20, C++23,
+and C++26 features.
+
+Experimental support for the time zone database has progressed.
+
+Work on the ranges support has progressed. See
+:ref:`ranges-status` for the current status.
+
+Work on the experimental C++17 Parallel STL has progressed. See
+:ref:`pstl-status` for the current status.
+
+Work on the C++17 mathematical special functions has started. See
+:ref:`special-math-status` for the current status.
+
 Implemented Papers
 ------------------
 
@@ -59,22 +73,29 @@ Implemented Papers
 - P0019R8 - ``std::atomic_ref``
 - P2389R2 - Alias template ``dims`` for the ``extents`` of ``mdspan``
 - P1223R5 - ``ranges::find_last()``, ``ranges::find_last_if()``, and ``ranges::find_last_if_not()``
+- P2602R2 - Poison Pills are Too Toxic
+- P1981R0 - Rename ``leap`` to ``leap_second``
+- P1982R0 - Rename ``link`` to ``time_zone_link``
+
 
 Improvements and New Features
 -----------------------------
 
 - The performance of growing ``std::vector`` has been improved for trivially relocatable types.
-- A lot of types are considered trivially relocatable now, including ``vector`` and ``string``.
-- The performance of ``ranges::fill`` and ``ranges::fill_n`` has been improved for ``vector<bool>::iterator``\s,
+
+- A lot of types are considered trivially relocatable now, including ``std::vector`` and ``std::string``.
+
+- The performance of ``std::ranges::fill`` and ``std::ranges::fill_n`` has been improved for ``std::vector<bool>::iterator``\s,
   resulting in a performance increase of up to 1400x.
+
 - The ``std::mismatch`` algorithm has been optimized for integral types, which can lead up to 40x performance
   improvements.
 
 - The ``std::ranges::minmax`` algorithm has been optimized for integral types, resulting in a performance increase of
   up to 100x.
 
 - The ``std::set_intersection`` and ``std::ranges::set_intersection`` algorithms have been optimized to fast-forward over
-  contiguous ranges of non-matching values, reducing the number of comparisons from linear to 
+  contiguous ranges of non-matching values, reducing the number of comparisons from linear to
   logarithmic growth with the number of elements in best-case scenarios.
 
 - The ``_LIBCPP_ENABLE_CXX26_REMOVED_STRSTREAM`` macro has been added to make the declarations in ``<strstream>`` available.
@@ -101,15 +122,18 @@ Improvements and New Features
 
   Note: bounded iterators currently are not supported for ``vector<bool>``.
 
+- In C++23 and C++26 the number of transitive includes in several headers has been reduced, improving the compilation speed.
+
+
 Deprecations and Removals
 -------------------------
 
-- The C++20 synchronization library (``<barrier>``, ``<latch>``, ``atomic::wait``, etc.) has been deprecated
+- The C++20 synchronization library (``<barrier>``, ``<latch>``, ``std::atomic::wait``, etc.) has been deprecated
   in language modes prior to C++20. If you are using these features prior to C++20, please update to ``-std=c++20``.
   In LLVM 20, the C++20 synchronization library will be removed entirely in language modes prior to C++20.
 
 - ``_LIBCPP_DISABLE_NODISCARD_EXT`` has been removed. ``[[nodiscard]]`` applications are now unconditional.
-  This decision is based on LEWGs discussion on `P3122 <https://wg21.link/P3122>` and `P3162 <https://wg21.link/P3162>`
+  This decision is based on LEWGs discussion on `P3122 <https://wg21.link/P3122>`_ and `P3162 <https://wg21.link/P3162>`_
   to not use ``[[nodiscard]]`` in the standard.
 
 - The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable that was used to enable the safe mode has been deprecated and setting
@@ -151,10 +175,11 @@ Deprecations and Removals
 - libc++ no longer supports ``std::allocator<const T>`` and containers of ``const``-qualified element type, such
   as ``std::vector<const T>`` and ``std::list<const T>``. This used to be supported as an undocumented extension.
   If you were using ``std::vector<const T>``, replace it with ``std::vector<T>`` instead. The
-  ``_LIBCPP_ENABLE_REMOVED_ALLOCATOR_CONST`` macro can be defined to temporarily re-enable this extension as
-  folks transition their code. This macro will be honored for one released and ignored starting in LLVM 20.
+  ``_LIBCPP_ENABLE_REMOVED_ALLOCATOR_CONST`` macro can be defined to temporarily re-enable this extension.
+  to temporarily re-enable this extension to make it easier to update user code
+  This macro will be honored for one released and ignored starting in LLVM 20.
   To assist with the clean-up process, consider running your code through Clang Tidy, with
-  `std-allocator-const <https://clang.llvm.org/extra/clang-tidy/checks/portability/std-allocator-const.html>`
+  `std-allocator-const <https://clang.llvm.org/extra/clang-tidy/checks/portability/std-allocator-const.html>`_
   enabled.
 
 - When configuring libc++ with localization or threads disabled, the library no longer emits an error when
@@ -187,6 +212,9 @@ LLVM 20
   ``_LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS`` macro that was used to re-enable this extension will be
   ignored in LLVM 20.
 
+- The ``_LIBCPP_ENABLE_REMOVED_ALLOCATOR_CONST`` macro will no longer have an effect.
+
+
 LLVM 21
 ~~~~~~~
 
@@ -197,6 +225,7 @@ LLVM 21
 
   If you are using C++03 in your project, you should consider moving to a newer version of the Standard to get the most out of libc++.
 
+
 ABI Affecting Changes
 ---------------------
 
@@ -211,7 +240,7 @@ Build System Changes
 - The ``LIBCXX_EXECUTOR`` and ``LIBCXXABI_EXECUTOR`` CMake variables have been removed. Please
   set ``LIBCXX_TEST_PARAMS`` to ``executor=<...>`` instead.
 
-- The Cmake variable ``LIBCXX_ENABLE_CLANG_TIDY`` has been removed. The build system has been changed
+- The CMake variable ``LIBCXX_ENABLE_CLANG_TIDY`` has been removed. The build system has been changed
   to automatically detect the presence of ``clang-tidy`` and the required ``Clang`` libraries.
 
 - The CMake options ``LIBCXX_INSTALL_MODULES`` now defaults to ``ON``.

libcxx/docs/ReleaseNotes/20.rst

@@ -59,16 +59,28 @@ Deprecations and Removals
   ``_LIBCPP_ENABLE_REMOVED_WEEKDAY_RELATIONAL_OPERATORS`` macro that was used to re-enable this extension will be
   ignored in LLVM 20.
 
+- TODO: The ``_LIBCPP_ENABLE_REMOVED_ALLOCATOR_CONST`` macro will no longer have an effect.
 
 Upcoming Deprecations and Removals
 ----------------------------------
 
-LLVM 21
+LLVM 20
 ~~~~~~~
 
 - TODO
 
 
+LLVM 21
+~~~~~~~
+
+- The status of the C++03 implementation will be frozen after the LLVM 21 release. This means that starting in LLVM 22, non-critical bug fixes may not be back-ported
+  to C++03, including LWG issues. C++03 is a legacy platform, where most projects are no longer actively maintained. To
+  reduce the amount of fixes required to keep such legacy projects compiling with up-to-date toolchains, libc++ will aim to freeze the status of the headers in C++03 mode to avoid unintended breaking changes.
+  See https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc for more details.
+
+  If you are using C++03 in your project, you should consider moving to a newer version of the Standard to get the most out of libc++.
+
+
 ABI Affecting Changes
 ---------------------
 

libcxx/docs/Status/SpecialMath.rst

@@ -1,4 +1,4 @@
-.. special-math-status:
+.. _special-math-status:
 
 ======================================================
 libc++ Mathematical Special Functions Status (P0226R1)