Crash when compiling diffast-cli.0.3.5.1 on Windows

[jmid]

Spotted a dune crash under Windows MinGW (Cygwin) for PR ocaml/opam-repository#28652 releasing diffast.

Expected Behavior

A successful compilation or a compiler error.

Actual Behavior

https://github.com/ocaml/opam-repository/actions/runs/18267213877/job/52003369231?pr=28652

  The following actions will be performed:
  === install 2 packages
    - install diffast-cli 0.3.5.1
    - install diffast-etc 0.3.5.1 [required by diffast-cli]
  
  <><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
  -> retrieved diffast-cli.0.3.5.1, diffast-etc.0.3.5.1  (cached)
  -> installed diffast-etc.0.3.5.1
  [ERROR] The compilation of diffast-cli.0.3.5.1 failed at "dune build -p diffast-cli -j 3 --promote-install-files=false @install".
  
  #=== ERROR while compiling diffast-cli.0.3.5.1 ================================#
  # context     2.4.1 | win32/x86_64 | ocaml.5.3.0 | file://D:/a/opam-repository/opam-repository
  # path        D:\opamroot\default\.opam-switch\build\diffast-cli.0.3.5.1
  # command     D:\opamroot\default\bin\dune.exe build -p diffast-cli -j 3 --promote-install-files=false @install
  # exit-code   1
  # env-file    D:\opamroot\log\diffast-cli-8592-0ccee9.env
  # output-file D:\opamroot\log\diffast-cli-8592-0ccee9.out
  ### output ###
  # [...]
  # Called from Fiber__Core.O.(>>|).(fun) in file "fiber__Core.ml", line 254,
  #   characters 36-41
  # Called from Fiber__Scheduler.exec in file "fiber__Scheduler.ml", line 77,
  #   characters 8-11
  # -> required by ("<unnamed>", ())
  # -> required by ("toplevel", ())
  # 
  # I must not crash.  Uncertainty is the mind-killer. Exceptions are the
  # little-death that brings total obliteration.  I will fully express my cases. 
  # Execution will pass over me and through me.  And when it has gone past, I
  # will unwind the stack along its path.  Where the cases are handled there will
  # be nothing.  Only I will remain.

Reproduction

ocaml/opam-repository#28652

To reproduce:

1. fork the opam repository
2. create a branch and cherry-pick one of the diffast-cli.0.3.5.1 release commits, e.g., b06809989dc39dbeb85dc21158ee87ea7e82a147 or 38b9f2258058f05f151a3dc793688222d69ae160
3. push the branch to GitHub and create a opam-repo self-PR to trigger a Windows GHA run

Specifications

• Version of dune (output of dune --version): 3.20.2
• Version of ocaml (output of ocamlc --version): 5.3.0
• Operating system (distribution and version): Microsoft Windows Server 2025 10.0.26100

  Image: windows-2025
  Version: 20250929.44.1
  Included Software: https://github.com/actions/runner-images/blob/win25/20250929.44/images/windows/Windows2025-Readme.md
  Image Release: https://github.com/actions/runner-images/releases/tag/win25%2F20250929.44

https://github.com/ocaml/opam-repository/blob/master/.github/workflows/windows.yml

[Alizter]

The log is truncated and not very useful here. Here is screencap from a repro in a vm:

[Alizter]

I've created a repro case in #12754. I don't yet know what the best fix for this issue is yet however.