ghc: Use --hash-unit-ids when supported

[mpickering]

The compilers produced by nixpkgs don't use the detailed ABI hash.

("Project Unit Id","ghc-9.10.1-inplace")

which should rather be

("Project Unit Id","ghc-9.10.1-"))

This flag is supported on ghc-9.8 and upwards.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[sternenseemann]

Why is this not the default behavior of hadrian? (Given that it's not documented it's not very discoverable…) Would there be any reason to turn this off?

[mpickering]

@sternenseemann It is going to become the default, you should also enable it when distributing. I think it has been opt-in until now just-in-case there were any unforeseen issues with the change to using hashes.

See MR: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/13444

[sternenseemann]

@mpickering Then I'd actually lean towards making this unconditional and now adding an hashUnitIds input.

[mpickering]

@sternenseemann Now that https://gitlab.haskell.org/ghc/ghc/-/merge_requests/13444 has landed, hash-unit-ids will be the default when you use the release flavour in 9.14.

I updated the patch to always enable it for supported versions. Can you take it from here?