[libc++][doc] Update the release notes

[Mordante]

Summary

I had a look at the changes since the last release and updated the
release notes with interesting changes.

If there are more interesting changes, please let me know and I'll
update the patch. Branching will be done the 27th of July. Landing this
patch before the 27th seems unrealistic. I'd like to land it before July
30th. July 30th is the planned release date of 13.0.0-rc1.

[mstorsjo]

More things I'd like to add:

std::filesystem has been implemented for Windows, and the implementation should be on par with libc++'s implementation for other platforms, modulo what's possible on Windows. (Do note that it's disabled by default for MSVC configurations due to missing int128 support, see https://reviews.llvm.org/D91139#2429595 for discussion on the matter; the CI configuration runs with -D_LIBCPP_HAS_NO_INT128 to make it usable in MSVC configurations, but that's not the intended permanent ABI. For MinGW configurations, it's enabled by default and should work without gotchas. I'm not sure how to write this concicsely though...)

Windows is now (again?) a formally supported platform (by having CI for it, for x86_64).

[ldionne]

Thanks for doing that!

libcxx/docs/ReleaseNotes.rst

49–68

This shouldn't be under "New features". Either move to API Changes or put directly under "What's new in Libc++".

52

I wouldn't put this under "New Features" either.

57

We have to talk about this, but I was thinking about *not* shipping our incomplete Ranges implementation in LLVM 13. I was thinking about waiting until we have a proper implementation and we can guarantee ABI stability with more confidence.

While we have a decent part of Ranges implemented, some key parts are still missing and I'm not sure we want to ship in that state - most users are just going to get confused about what works and what doesn't.

That wouldn't impact the concepts.

@zoecarver @cjdb

[Mordante]

In D106770#2903228, @mstorsjo wrote:

More things I'd like to add:

std::filesystem has been implemented for Windows,

Thanks for the info, I'll add it.

Windows is now (again?) a formally supported platform (by having CI for it, for x86_64).

Yes, it's listed as a supported platform :-)

[Mordante]

Addresses most review comments.
Based on @mstorsjo's review comments I noticed the documentation for the
CMake option LIBCXX_ENABLE_FILESYSTEM was incorrect.

[Mordante]

Based in the decision for <ranges> the document needs to be updated. When we disable the header by default we can mention how to enable it with a macro.

libcxx/docs/ReleaseNotes.rst

52

I move it to What's new. I don't mind to remove it, but then we should be sure the refactoring didn't accidentally removed an include.

[mstorsjo]

In D106770#2904305, @Mordante wrote:

Addresses most review comments.
Based on @mstorsjo's review comments I noticed the documentation for the
CMake option LIBCXX_ENABLE_FILESYSTEM was incorrect.

Thanks! Those bits of this patch look great to me now!

[Mordante]

Remove the ranges as a feature
Document the new cmake option LIBCXX_ENABLE_INCOMPLETE_FEATURES

[Mordante]

Rebased on the LLVM 13 release branch.

[ldionne]

LGTM.

@tstellar , can we merge this to release/13.x? It's only updating the release notes with some stuff we forgot to mention previously.

[tstellar]

Yes, go ahead and merge this to the branch.

[Mordante]

It seems Phabricator doesn't see commits to the release branch. The patch has landed as 1d99cadfbc58057a9a1503172439a20aa41b004b.

libcxx/docs/BuildingLibcxx.rst

.. option:: LIBCXX_HERMETIC_STATIC_LIBRARY:BOOL
 
   Do not export any symbols from the static libc++ library.
   This is useful when the static libc++ library is being linked into shared
   libraries that may be used in with other shared libraries that use different
   C++ library. We want to avoid exporting any libc++ symbols in that case.
 
 .. option:: LIBCXX_ENABLE_FILESYSTEM:BOOL
 
-   **Default**: ``ON`` except on Windows.
+   **Default**: ``ON`` except on Windows when using MSVC.
 
    This option can be used to enable or disable the filesystem components on
-   platforms that may not support them. For example on Windows.
+   platforms that may not support them. For example on Windows when using MSVC.
 
 .. option:: LIBCXX_ENABLE_INCOMPLETE_FEATURES:BOOL
 
   **Default**: ``ON``
 
   Whether to enable support for incomplete library features. Incomplete features
   are new library features under development. These features don't guarantee
   ABI stability nor the quality of completed library features. Vendors

libcxx/docs/ReleaseNotes.rst

 Note that if you are reading this file from a Git checkout or the
 main Libc++ web page, this document applies to the *next* release, not
 the current one. To see the release notes for a specific release, please
 see the `releases page <https://llvm.org/releases/>`_.
 
 What's New in Libc++ 13.0.0?
 ============================
 
+- Support for older compilers has been removed. Several additional platforms
+  are now officially supported. :ref:`platform_and_compiler_support` contains
+  the complete overview of platforms and compilers supported by libc++.
+- The large headers ``<algorithm>``, ``<iterator>``, and ``<utility>`` have
+  been split in more granular headers. This reduces the size of included code
+  when using libc++. This may lead to missing includes after upgrading to
+  libc++13.
+
 New Features
 ------------
 
-- ...
+- ``std::filesystem`` is now feature complete for the Windows platform using
+  MinGW. MSVC isn't supported since it lacks 128-bit integer support.
+- The implementation of the C++20 concepts library has been completed.
+- Several C++20 ``constexpr`` papers have been completed:
+
+  - `P0879R0 <https://wg21.link/P0879R0>`_ ``constexpr`` for ``std::swap()``
+    and swap related functions
+  - `P1032R1 <https://wg21.link/P1032R1>`_ Misc ``constexpr`` bits
+  - `P0883 <https://wg21.link/P0883>`_ Fixing Atomic Initialization
+
+- More C++20 features have been implemented. :doc:`Status/Cxx20` has the full
+  overview of libc++'s C++20 implementation status.
+- More C++2b features have been implemented. :doc:`Status/Cxx2b` has the
+  full overview of libc++'s C++2b implementation status.
+- The CMake option ``LIBCXX_ENABLE_INCOMPLETE_FEATURES`` has been added. This
+  option allows libc++ vendors to disable headers that aren't production
+  quality yet. Currently, turning the option off disables the headers
+  ``<format>`` and ``<ranges>``.
+- The documentation conversion from html to restructured text has been
+  completed.
 
 API Changes
 -----------
 
 - There has been several changes in the tuple constructors provided by libc++.
   Those changes were made as part of an effort to regularize libc++'s tuple
   implementation, which contained several subtle bugs due to these extensions.
   If you notice a build breakage when initializing a tuple, make sure you

libcxx/docs/index.rst

* STLport and the Apache libstdcxx library are two other popular
   experience of libstdc++ developers) is that adding support for C++11 (in
   particular rvalue references and move-only types) requires changes to
   almost every class and function, essentially amounting to a rewrite.
   Faced with a rewrite, we decided to start from scratch and evaluate every
   design decision from first principles based on experience.
   Further, both projects are apparently abandoned: STLport 5.2.1 was
   released in Oct'08, and STDCXX 4.2.1 in May'08.
 
+.. _platform_and_compiler_support:
 
 Platform and Compiler Support
 =============================
 
 Libc++ aims to support common compilers that implement the C++11 Standard. In order to strike a
 good balance between stability for users and maintenance cost, testing coverage and development
 velocity, libc++ drops support for older compilers as newer ones are released.