Add useLocalGhcLib project option#2510
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Surface what `modules/configuration-nix.nix` used to do unconditionally as an opt-in `useLocalGhcLib` flag, so the `packages.ghc.src` override only fires when a project actually constrains the `ghc` package (e.g. `ghc-lib-reinstallable`). Four pieces: * `modules/project-common.nix`: add the `useLocalGhcLib` option (default `false`). * `modules/configuration-nix.nix`: drop the unconditional `packages.ghc.src` / `packages.ghc.package-description-override` overrides — they're moved into the per-project wiring below. * `modules/stack-project.nix`: under `useLocalGhcLib`, re-apply the `packages.ghc.src` post-plan override. Stack-to-nix can't use the cabal-project route, so this keeps the existing behaviour for stack users who flip the flag. * `modules/cabal-project.nix`: under `useLocalGhcLib`, inject a `source-repository-package` block into `cabalProjectLocal` pointing at the configured-src + generated GHC tree, and add an `inputMap` entry so haskell.nix doesn't try to fetch the URL. Cabal then hashes the wrapped repo's content into `pkg-src-sha256` and installs `lib:ghc` like any other reinstallable dep. Projects that need the previous always-on behaviour now set `useLocalGhcLib = true` on the project module; everyone else gets a smaller plan-nix and avoids the unconditional `configured-src` materialisation. Pulled out of #2504 (`hkm/builder-v2`).
Flips the new opt-in flag in both test variants and adds a changelog entry telling users to do the same when they constrain `lib:ghc`.
hamishmack
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May 25, 2026
Migrates the in-tree test suite to work under `builderVersion = 2`.
Adds `docs/dev/migrating-to-v2.md` documenting the patterns we hit,
keyed off the symptoms users are likely to see when flipping the
flag on their own projects.
Patterns covered (with file-level examples):
* Library / executable profiling — `.profiled` overlay replaced by
a sibling project whose `cabalProjectLocal` carries
`library-profiling: True` / `profiling: True`
(`test/exe-dlls`, `test/exe-lib-dlls`, `test/gi-gtk`,
`test/th-dlls`, `test/th-dlls-minimal`, `test/js-template-haskell`,
`test/cabal-simple-prof`).
* Coverage — module-level `doCoverage` mirrored into
`cabalProjectLocal` `coverage: True` (`test/coverage`).
* DWARF — `.dwarf` overlay dropped, replaced by
`compilerSelection = … c.dwarf …` + `debug-info: 2`
(`test/cabal-simple-debug`).
* Sublib haddock — `documentation: True` in `cabalProjectLocal`;
output path moved so `find -name <Module.html>` accepts both v1
and v2 layouts (`test/sublib-docs`).
* Per-package `ghc-options` — `HsOpenSSL` workaround moved from
`test/modules.nix` to `test/cabal.project.local` (`test/modules.nix`).
* Per-package `extra-lib-dirs` — Windows `test-clib` paths moved
into `cabalProjectLocal` (`test/th-dlls-minimal`).
* `ghcOptions` scope — moved from per-component to per-package
(`test/th-dlls`).
* `shellFor` migration — `component.shell` / `component.env` →
`project.shellFor { exposePackagesVia = "ghc-pkg"; }`
(`test/with-packages`, `test/shell-for`,
`test/shell-for-setup-deps`).
* `withHoogle = false` on static — avoids `.dyn_hi` lookup failures
in hoogle's haddock chain (`test/with-packages`).
* `origSrc` / `origSubDir` workaround — synthesised package-root
drv with `.git` at the top, source-cleaning disabled via
`outPath` + `filterPath` (`test/githash`).
* Custom-setup `base` / `directory` — added to explicit
`setup-depends` (`test/setup-deps`, `test/shell-for-setup-deps`).
* Native-musl `git` — host vs build-build picker keyed on
`haskellLib.isNativeMusl` to avoid glibc/musl libc mixing
(`test/githash`).
* Android cross — `test/cabal.project.android` injected
conditionally (`test/exe-dlls`, `test/exe-lib-dlls`,
`test/th-dlls`).
* `useLocalGhcLib = true` — earlier change (PR #2510) covered
separately; referenced in the guide for completeness.
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Surfaces what
modules/configuration-nix.nixused to do unconditionally as an opt-inuseLocalGhcLibflag. Thepackages.ghc.srcoverride now only fires when a project actually constrains theghcpackage (e.g.ghc-lib-reinstallable).Four pieces:
modules/project-common.nix— add theuseLocalGhcLiboption (defaultfalse).modules/configuration-nix.nix— drop the unconditionalpackages.ghc.src/packages.ghc.package-description-overrideoverrides; they're moved into the per-project wiring below.modules/stack-project.nix— underuseLocalGhcLib, re-apply thepackages.ghc.srcpost-plan override. Stack-to-nix can't use the cabal-project route, so this keeps the existing behaviour for stack users who flip the flag.modules/cabal-project.nix— underuseLocalGhcLib, inject asource-repository-packageblock intocabalProjectLocalpointing at the configured-src + generated GHC tree, and add aninputMapentry so haskell.nix doesn't try to fetch the URL. Cabal then hashes the wrapped repo's content intopkg-src-sha256and installslib:ghclike any other reinstallable dep.Projects that need the previous always-on behaviour now set
useLocalGhcLib = trueon the project module; everyone else gets a smaller plan-nix and avoids the unconditionalconfigured-srcmaterialisation.Pulled out of #2504 (
hkm/builder-v2).