Dear OCaml users,

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 executab= les,
the typechecking of OCaml programs, and the performance of numerical codes.=

Since those regressions affect many users and could have lasting effects, w= e
have decided to cut a patch release of OCaml 5.1 with fixes for those issue= s

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 late= r
releases of OCaml but not in OCaml 5.1.1 to give us time to polish the libr= ary
integration and packaging.

Happy hacking,
Florian Angeletti for the OCaml team.

Installation Instructions
--------------------------

The base compiler can be installed as an opam switch with the following com= mands 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/ocam= l-5.1.1~rc1.tar.gz

### Fine-Tuned Compiler Configuration

If you want to tweak the configuration of the compiler, you can switch to t= he 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-fl= oat-array

All available options can be listed with `opam search ocaml-option`.

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://g= ithub.com/ocaml/ocaml/issues/12783): Remove the `Marshal.Compression` flag = to the
`Marshal.to_*` functions introduced in 5.1 by [#12006](https://githu= b.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 t= o use
ZSTD compression for compilation artefacts.
(Xavier Leroy and David Allsopp, report by Kate Deplaix, review by    Nicol=E1s Ojeda B=E4r, Kate Deplaix, and Damien Doligez).

### Type System Bug Fix:

- [#12623](https://github.com/ocaml/ocaml/issues/12623), fix the computatio= n of variance composition
(Florian Angeletti, report by Vesa Karvonen, review by Gabriel Scher= er)

### GC Performance Regression Fixes

- [#12590](https://github.com/ocaml/ocaml/issues/12590), [#12595](https://g= ithub.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 m= eaning 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 colle= ct 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://g= ithub.com/ocaml/ocaml/issues/12493), [#12500](https://github.com/ocaml/ocam= l/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://g= ithub.com/ocaml/ocaml/issues/12609): Fix error on uses of packed modules ou= tside their pack
to correctly handle nested packs
(Vincent Laviron, report by Javier Ch=E1varri, review by Pierre Cham= bart)

- [#12757](https://github.com/ocaml/ocaml/issues/12757): Fix ocamlnat (nati= ve toplevel) by registering frametables correctly
(Stephen Dolan, Nick Barnes and Mark Shinwell,
   review by Vincent Laviron and S=E9bastien Hinderer)

- [#12645](https://github.com/ocaml/ocaml/issues/12645), [#12649](http= s://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 Scher= er)