[ANNOUNCE] GHC 9.4.1-rc1 is now available

[Ben Gamari]

The GHC developers are happy to announce the availability of the first
(and likely last) release candidate of GHC 9.4.1. Binary distributions, source
distributions, and documentation are available at [downloads.haskell.org].

This major release will include:

 - A new profiling mode, `-fprof-late`, which adds automatic cost-center
   annotations to all top-level functions *after* Core optimisation has
   run. This provides informative profiles while interfering
   significantly less with GHC's aggressive optimisations, making it
   easier to understand the performance of programs which depend upon
   simplification..

 - A variety of plugin improvements including the introduction of a new
   plugin type, *defaulting plugins*, and the ability for typechecking
   plugins to rewrite type-families.

 - An improved constructed product result analysis, allowing unboxing of
   nested structures, and a new boxity analysis, leading to less reboxing.

 - Introduction of a tag-check elision optimisation, bringing
   significant performance improvements in strict programs.

 - Generalisation of a variety of primitive types to be levity
   polymorphic. Consequently, the `ArrayArray#` type can at long last be
   retired, replaced by standard `Array#`.

 - Introduction of the `\cases` syntax from [GHC proposal 0302]

 - A complete overhaul of GHC's Windows support. This includes a
   migration to a fully Clang-based C toolchain, a deep refactoring of
   the linker, and many fixes in WinIO.

 - Support for multiple home packages, significantly improving support
   in IDEs and other tools for multi-package projects.

 - A refactoring of GHC's error message infrastructure, allowing GHC to
   provide diagnostic information to downstream consumers as structured
   data, greatly easing IDE support.

 - Significant compile-time improvements to runtime and memory consumption.

 - On overhaul of our packaging infrastructure, allowing full
   traceability of release artifacts and more reliable binary
   distributions.

 - Reintroduction of deep subsumption (which was previously dropped with the
   *simplified subsumption* change) as a language extension.

 - ... and much more. See the [release notes] for a full accounting.

Note that, as 9.4.1 is the first release for which the released artifacts will
all be generated by our Hadrian build system, it is possible that there will be
packaging issues. If you enounter trouble while using a binary distribution,
please open a [ticket]. Likewise, if you are a downstream packager, do consider
migrating to [Hadrian] to run your build; the Hadrian build system can be built
using `cabal-install`, `stack`, or the in-tree [bootstrap script]. We will be
publishing a blog post describing the migration process to Hadrian in the
coming weeks.

We would like to thank Microsoft Azure, GitHub, IOG, the Zw3rk
stake pool, Tweag I/O, Serokell, Equinix, SimSpace, the Haskell
Foundation, and other anonymous contributors whose on-going financial
and in-kind support has facilitated GHC maintenance and release
management over the years. Finally, this release would not have been
possible without the hundreds of open-source contributors whose work
comprise this release.

As always, do give this release a try and open a [ticket] if you see
anything amiss.

Happy testing,

- Ben

[downloads.haskell.org]: https://downloads.haskell.org/ghc/9.4.1-rc1/
[GHC proposal 0302]: https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0302-cases.rst 
[ticket]: https://gitlab.haskell.org/ghc/ghc/-/issues/new
[bootstrap script]: https://gitlab.haskell.org/ghc/ghc/-/blob/e2520df3fffa0cf22fb19c5fb872832d11c07d35/hadrian/bootstrap/README.md
[Hadrian]: https://gitlab.haskell.org/ghc/ghc/-/blob/e2520df3fffa0cf22fb19c5fb872832d11c07d35/hadrian
[release notes]: https://downloads.haskell.org/~ghc/9.4.1-rc1/docs/users_guide/9.4.1-notes.html

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