nix: fix cygwin build

[corngood]

Dependency PRs:

• openssl: openssl: fix cygwin build #468144
• lowdown: lowdown: fix cygwin build #468178
• editline: editline: fix cygwin build #468180
• cmake: build-support/lib/cmake: fix CMAKE_SYSTEM_NAME in cygwin cross #468185
• bash: bash: fix cygwin build #468186
• libev: libev: fix cygwin build #468197
• libedit: libedit: fix cygwin build #468201
• cc-wrapper: cc-wrapper: set gl_cv_clean_version_stddef=yes on cygwin #468206
• gcc: gcc: remove unix define on cygwin #468215
• libblake3: libblake3: fix cygwin build #468273
• nghttp2: nghttp2: fix cygwin build #470895
• libxml2: libxml2: fix cygwin build #468277
• gnugrep: gnugrep: fix cygwin build #469010
• ngtcp2: ngtcp2: fix cygwin build #470896
• zstd: zstd: fix cygwin build #470897
• libiconv: libiconv: fix cygwin build #471411
• boost: boost: fix cygwin build #472755
• curlMinimal: curlMinimal: fix cygwin build #472401
• libarchive: libarchive: fix cygwin build #472495
• libidn2: libidn2: fix cygwin build #472851

Related:

• fix cross eval for nixpkgs-review: treewide: fix nix-env eval in cross #467887
• crossSystem support in nixpkgs-review: Add --extra-nixpkgs-args Mic92/nixpkgs-review#577
• staging: staging-next 2025-12-17 #471587
• nix: packaging: disable LTO on cygwin nix#14844

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[corngood]

@ofborg build pkgsCross.x86_64-cygwin.nix

[Ericson2314]

Very excited about this!

@alyssais how would you like the changes to be merged (preemptively create many PRs? Split out dynamically based on what needs more review?) I am hoping that these changes can be merged soon (because I am very excited by @corngood's work!) and also backported to 25.11, because I would like to use this in Nix's own CI to make sure we don't have Cygwin regressions.

Per it being draft, there is still some cleanup to be done to the stuff (my comments below are premature, @corngood is already aware of many of those things) but @corngood was wondering about what sort of approach to PR bucketting he should use (as he cleans up and gets ready to submit non-draft PRs) and so I am relaying that question to you.

[alyssais]

@alyssais how would you like the changes to be merged (preemptively create many PRs? Split out dynamically based on what needs more review?)

In my experience doing musl, the fastest way to get something like this through review and merged is:

• Open one PR per fixed package. For packages that require a dependency to be fixed first, wait until the dependency PRs are fixed before opening the PR for the dependent. This lets simple changes get finalised fast, which will make subsequent review cycles faster, because it limits what has to be rechecked repeatedly to only the controversial stuff. Smaller PRs = more distributed review, less overwhelmed reviewers, faster review cycles.
• Use a draft PR or tracking issue to let interested people find all such PRs. (I don't usually bother with this but it probably does help if you have the energy to keep it updated.)
• Request review from people generally interested in the initiative as well as the topic.
• Just send mass rebuilds to staging. The staging process has relatively fixed waiting period once changes are in it, whereas review doesn't, so getting a simple, mass-rebuild diff into staging will often end up with it in master faster than getting a PR with a more complicated rebuild-avoiding diff merged straight to master.

[corngood]

@ofborg build pkgsCross.x86_64-cygwin.nix

It doesn't look like this has worked. Any ideas for demonstrating in a PR that a package builds in cross?

[corngood]

I started tracking the dependency PRs in the top comment. Either this PR can stay for tracking only, or we can use it when it's down to changing nix only. I think it's useful to keep a rebased version of all the changes with the rebuild hacks so we can verify zero rebuilds.

[Eveeifyeve]

What is the difference between the last cygwin pr: #447520 and this?

[corngood]

What is the difference between the last cygwin pr: #447520 and this?

This one is specifically for getting a cross-build of nix working. It's a subset of the other one.

[corngood]

This PR now only contains one small change to nix packaging, which is also in NixOS/nix#14844.

It won't build in master until the next (or possibly next-next) staging merge, but it should build in staging after this and the last remaining dependency PR (libidn2) makes it there.

[Ericson2314]

Congratulations!