[hamishmack]

Summary

findCabalFiles UsePackageYamlFirst re-rendered a package.yaml through hpack whenever it existed, even when the package directory also had a committed .cabal file. That discarded the committed file and failed outright with ".cabal was generated with a newer version of hpack, please upgrade" (#626) or ".cabal was modified manually, please use --force" (#767).

It now uses the committed .cabal when one exists and falls back to hpack only when there is no .cabal. --ignore-package-yaml (IgnorePackageYaml) still forces cabal-only discovery.

This applies to:

• stack-to-nix (lib/Stack2nix/Project.hs).
• plan-to-nix (cabal projects). It had its own copy of findCabalFiles that still rendered package.yaml. On the Nix side, call-cabal-project-to-nix.nix only skips the shell hpack run when a .cabal exists, so cabal-install planned against the committed .cabal while plan-to-nix generated .plan.nix from hpack output and marked it cabal-generator = "hpack". comp-builder.nix then re-ran hpack at build time and could hit the same .cabal was modified manually, please use --force to overwrite. #767 error. Plan2Nix.Project is removed and plan-to-nix uses Stack2nix.Project.findCabalFiles; with an on-disk .cabal no cabal-generator is emitted, so the build-time hpack run is skipped.
• cabal-name (used to name stack git dependencies), which already shared the stack function.
• The same change in the nix-tools-sh copy, whose files were identical.

Behaviour changes to be aware of

• A stale committed .cabal wins. If package.yaml was edited without regenerating the .cabal, haskell.nix uses the committed .cabal. Stack itself would regenerate it (unless the .cabal was hand-modified or made by a newer hpack). This is deliberate: it matches what cabal-install sees, avoids running hpack, and the fix is to commit the regenerated .cabal. No staleness warning: hpack's -- hash: header hashes the .cabal body (to detect hand edits), not package.yaml, so detecting staleness would mean rendering every package.yaml through hpack, which is the cost and failure mode this change removes.
• Several .cabal files plus a package.yaml: all the .cabal files are returned (as with no package.yaml); cabal-name takes the first.

Tests

Self-contained test suite stack-project-tests (temp dir only, no external tools or network) asserting that findCabalFiles:

• prefers the committed .cabal when both files exist (the Do not call hpack if .cabal exists #626/.cabal was modified manually, please use --force to overwrite. #767 regression),
• still renders via hpack when only package.yaml exists,
• uses the .cabal when only a .cabal exists,
• ignores package.yaml under IgnorePackageYaml.

plan-to-nix now calls the same function, so these cover it too.

Verification

• Compiled and ran stack-project-tests with nixpkgs GHC 9.10 + Cabal 3.16: all pass.
• Typechecked plan-to-nix (plan2nix/Main.hs) for nix-tools, and for nix-tools-sh against nix-tools' cabal2nix library (the -sh one needs the stable-haskell Cabal fork). No new warnings.
• Full nix-tools build and test run are left to CI (check-nix-tools).

Notes

• Release-gated: haskell.nix uses nix-tools as a prebuilt static release binary, so this reaches haskell.nix builds only after the next nix-tools release (cf. error in flake. #2506).
• A --use-package-yaml opt-out (stack's regenerate-always behaviour) was considered and skipped; easy to add later if wanted.

Closes #626.
Closes #767.

[Copilot]

🟢 Approval recommended

Static review found no blocking issues in the focused discovery change or test integration; compilation and test execution remain unverified.

No open issues 🎉

What changed in this PR

Updates Stack package discovery to prefer an existing .cabal file over regenerating it with hpack, aligning with the Cabal path and addressing #626 and #767.

Changes:

• Uses hpack only when no .cabal file exists, preserving IgnorePackageYaml behavior.
• Adds and registers four self-contained tests covering file preference and fallback behavior.
• Delivery to haskell.nix builds remains gated on a nix-tools release.

File	Description
nix-tools/​nix-tools/​tests/​StackProjectTests.hs	Adds regression tests for discovery and hpack fallback.
nix-tools/​nix-tools/​nix-tools.cabal	Registers the stack-project-tests suite.
nix-tools/​nix-tools/​lib/​Stack2nix/​Project.hs	Prefers on-disk .cabal files before invoking hpack.

_{Review effort: Balanced}