changelog: entry for OCaml 5.1.1~rc1 (#1817)

data/changelog/ocaml/2023-12-1-ocaml-5.1.1.rc1.md

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+---
+title: OCaml 5.1.1 - First Release Candidate
+description: First Release Candidate of OCaml 5.1.1
+date: "2023-12-01"
+tags: [ocaml]
+changelog: |
+  ## Changes in OCaml 5.1.1
+
+  ### Standard Library Dependency Fix:
+
+  * (*breaking change*) [#12562](https://github.com/ocaml/ocaml/issues/12562), [#12734](https://github.com/ocaml/ocaml/issues/12734), [#12783](https://github.com/ocaml/ocaml/issues/12783): Remove the `Marshal.Compression` flag to the
+    `Marshal.to_*` functions introduced in 5.1 by [#12006](https://github.com/ocaml/ocaml/issues/12006), as it cannot
+    be implemented without risking to link -lzstd with all
+    ocamlopt-generated executables.  The compilers are still able to use
+    ZSTD compression for compilation artefacts.
+    (Xavier Leroy and David Allsopp, report by Kate Deplaix, review by
+     Nicolás Ojeda Bär, Kate Deplaix, and Damien Doligez).
+  
+  ### Type System Bug Fix:
+  
+  - [#12623](https://github.com/ocaml/ocaml/issues/12623), fix the computation of variance composition
+    (Florian Angeletti, report by Vesa Karvonen, review by Gabriel Scherer)
+  
+  ### GC Performance Regression Fixes
+  
+  - [#12590](https://github.com/ocaml/ocaml/issues/12590), [#12595](https://github.com/ocaml/ocaml/issues/12595): Move `caml_collect_gc_stats_sample` in
+    `caml_empty_minor_heap_promote` before barrier arrival.
+    (B. Szilvasy, review by Gabriel Scherer)
+  
+  - [#12318](https://github.com/ocaml/ocaml/issues/12318): GC: simplify the meaning of custom_minor_max_size: blocks with
+    out-of-heap memory above this limit are now allocated directly in
+    the major heap.
+    (Damien Doligez, report by Stephen Dolan, review by Gabriel Scherer)
+  
+  - [#12439](https://github.com/ocaml/ocaml/issues/12439): Finalize and collect dead custom blocks during minor collection
+    (Damien Doligez, review by Xavier Leroy, Gabriel Scherer and KC
+    Sivaramakrishnan)
+  
+  
+  - [#12491](https://github.com/ocaml/ocaml/issues/12491), [#12493](https://github.com/ocaml/ocaml/issues/12493), [#12500](https://github.com/ocaml/ocaml/issues/12500), [#12754](https://github.com/ocaml/ocaml/issues/12754): Do not change GC pace when creating
+    sub-arrays of bigarrays
+    (Xavier Leroy, report by Ido Yariv, analysis by Gabriel Scherer,
+  
+  ### Miscellaneous Bug Fixes
+  
+  - [#12581](https://github.com/ocaml/ocaml/issues/12581), [#12609](https://github.com/ocaml/ocaml/issues/12609): Fix error on uses of packed modules outside their pack
+    to correctly handle nested packs
+    (Vincent Laviron, report by Javier Chávarri, review by Pierre Chambart)
+  
+  - [#12757](https://github.com/ocaml/ocaml/issues/12757): Fix ocamlnat (native toplevel) by registering frametables correctly
+    (Stephen Dolan, Nick Barnes and Mark Shinwell,
+     review by Vincent Laviron and Sébastien Hinderer)
+  
+  - [#12645](https://github.com/ocaml/ocaml/issues/12645), [#12649](https://github.com/ocaml/ocaml/issues/12649) fix error messages for cyclic type definitions in presence of
+    the `-short-paths` flag.
+    (Florian Angeletti, report by Vesa Karvonen, review by Gabriel Scherer)
+  
+  
+---
+
+Two months (and half) after the release of OCaml 5.1.0, we have discovered three
+significant regressions in OCaml 5.1.0, affecting the packaging of executables,
+the typechecking of OCaml programs, and the performance of numerical codes.
+
+Since those regressions affect many users and could have lasting effects, we
+have decided to cut a patch release of OCaml 5.1 with fixes for those issues
+next week.
+
+To give time for opam packages to test this patch version, we
+have just published a first release candidate for OCaml 5.1.1 .
+The full list of changes in this release candidate is available below.
+
+As a major exception to our policy for patch releases, OCaml 5.1.1 will
+contain one breaking change in the standard library: the `Compression` flag has
+been removed from the `Marshal` module.
+This drastic measure was taken because supporting zstd compression in the
+standard library made zstd a dependency of all OCaml executables. Since the
+compiler should not impose its dependency on end-users, the support for
+compressed marshaling has been moved to a compiler internal library in 5.1.1.
+This internally library might be released as an independent library in later
+releases of OCaml but not in OCaml 5.1.1 to give us time to polish the library
+integration and packaging.
+
+
+Installation Instructions
+--------------------------
+
+The base compiler can be installed as an opam switch with the following commands on opam 2.1 and later:
+
+    opam update
+    opam switch create 5.1.1~rc1
+
+The source code for the release candidate is also directly available on:
+
+* [GitHub](https://github.com/ocaml/ocaml/archive/5.1.1-rc1.tar.gz)
+* [OCaml archives at Inria](https://caml.inria.fr/pub/distrib/ocaml-5.1/ocaml-5.1.1~rc1.tar.gz)
+
+### Fine-Tuned Compiler Configuration
+
+If you want to tweak the configuration of the compiler, you can switch to the option variant with:
+
+    opam update
+    opam switch create <switch_name> ocaml-variants.5.1.1~rc1+options <option_list>
+
+where `option_list` is a space-separated list of `ocaml-option-*` packages. For instance, for a `flambda` and `no-flat-float-array` switch:
+
+    opam switch create 5.1.1~rc1+flambda+nffa ocaml-variants.5.1.1~rc1+options ocaml-option-flambda ocaml-option-no-flat-float-array
+
+All available options can be listed with `opam search ocaml-option`.
+