Add cabal v2-build-based component builder ("v2 builder")
Adds an alternative component builder that runs `cabal v2-build`
inside a slice derivation and emits a cabal-store layout rather
than going through Setup.hs, alongside the existing Setup-based
v1 builder. Selected per-project via the new `builderVersion`
option (default `1` for now; flip to `2` to opt in on a
per-project basis).
What the v2 builder provides:
* Per-component cabal-store slices that compose into a single
`~/.cabal/store/ghc-<version>/` layout via `composeStore`. Each
slice's UnitId matches what a downstream `cabal v2-build` would
compute against the real compiler, so a v2 store can be reused
by interactive cabal invocations without recomputation.
* Public sublibrary support (`library <n>` stanzas) end-to-end —
not currently supported by v1's Setup-based path.
* A v2 dev shell (`shell-for-v2.nix`) wired through `project.shellFor`
whenever `builderVersion = 2`. Two exposure modes:
`cabal-store` (default) pre-populates `~/.cabal/store` so plain
`cabal v2-build` reuses the slice's units; `ghc-pkg` mode wraps
`ghc` / `ghc-pkg` with `GHC_ENVIRONMENT` / `GHC_PACKAGE_PATH` so
`ghc -e` and similar see the composed store directly.
* A shared ghc shim (`ghc-shim.nix`) used by both the slice's
`cabal v2-build` and the v2 shell so the user-facing compiler
matches the slice compiler. Adds cabal near-compiler aliases,
patches the ghcjs `ar command` setting (drops missing `.o`s for
backpack-on-ghcjs), and adds native-musl `unlit` /
`ghc-iserv[-dyn|-prof]` aliases.
Plumbing changes:
* `builder/default.nix` + `builder/hspkg-builder.nix` dispatch on
`builderVersion`. v2-specific helpers (`buildCabalStoreSlice`,
`composeStore`, `makeGhcShim`, the patched `v2CabalInstall`)
live in `builder/default.nix`.
* `modules/project-common.nix` adds `builderVersion`.
`modules/component-driver.nix` propagates it (plus
`crossTemplateHaskellSupport` and `cabalProjectLocal`) into the
pkg-set config. `modules/stack-project.nix` forces v1 for
stack projects (stack-to-nix doesn't produce the plan-json
shapes v2 consumes).
* `modules/package-options.nix` adds round-trip options
(`configureOptions`, `extraLibDirs`, `extraIncludeDirs`,
`programOptions`) that v2 emits back into each slice's
cabal.project.
* `modules/install-plan/configure-args.nix` extracts those
round-trip options from plan.json's configure-args, and
filters `-hide-all-packages` for v2 (cabal injects it on every
Setup configure call; round-tripping it would fork UnitId
hashes between plan-nix and slice cabal).
* `modules/install-plan/non-reinstallable.nix` skips the
ghcjs/wasm boot-package hiding on v2 (v2 follows cabal's plan
literally rather than overlaying haskell.nix's exclusions).
* `modules/shell.nix` adds the `exposePackagesVia` option.
* `lib/check.nix` adds a v2-slice branch that runs the installed
exe under the platform's testWrapper, mirroring the v1 check
semantics for slice derivations.
* `overlays/haskell.nix` propagates `builderVersion` /
`cabalProjectLocal` through `addProjectAndPackageAttrs`,
dispatches `shellFor` on `builderVersion`, and rebuilds
iserv-proxy as both a non-profiled and profiled variant — v2
reads the prof variant from `package iserv-proxy profiling:
True` in cabal.project rather than `enableProfiling = true` on
the .override (which v2 doesn't honour).
* `overlays/linux-cross.nix` + `overlays/mingw_w64.nix` add a
`wrapGhc` derivation that bakes `-fexternal-interpreter -pgmi
<iserv-wrapper>` into ghc itself — v2 can't put those into
per-package `ghc-options:` (UnitId divergence), so it wraps the
compiler instead. The `compilerSelection` chain picks the
wrapped ghc on cross targets.
Internal projects pin v1:
* `compiler/ghc/default.nix` (hadrian) and
`overlays/bootstrap.nix` (hscolour) set `builderVersion = 1`
explicitly so v2 evolution doesn't churn GHC builds while v2 is
still settling. `overlays/tools.nix` defaults
`haskell-nix.tool` invocations to v1 for the same reason.
Tests are NOT migrated in this commit — the existing tests run
under v1 (the default) and exercise master's behaviour as before.
A follow-up will migrate or duplicate them under v2 once we know
which expectations need adjusting.
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