Cabal: tell configure scripts which compiler is in use by andreabedini · Pull Request #12340 · haskell/cabal

andreabedini · 2026-09-09T03:35:29Z

Fixes #7452. First step of #2947.

runConfigureScript passes --with-compiler= to a package's configure script, but takes the
value from configHcPath, and cabal-install never sets that: it hands the compiler over as a
program path override (--ghc=/path) and leaves configHcPath empty on purpose. So the script
gets the flavour name and, following autoconf convention, goes looking for ghc on PATH.

With cabal-install 3.16.1.0 and a configure script that just echoes its arguments:

$ cabal build -w ~/.ghcup/bin/ghc-9.12.2
configure args: --with-compiler=ghc --prefix=... CC=/usr/lib64/ccache/gcc CXX=/usr/lib64/ccache/g++
GHC=<unset>

A script that needs the actual compiler runs the wrong one. GHC's own ghc-internal does
exactly that ($GHC --print-prim-module to generate GHC/Internal/Prim.hs) and fails under any
other ghc, which is how I ran into it: building GHC's stage2 libraries with the stage1 compiler
via cabal picked up the ghc on PATH.

The fix takes the compiler and its package tool from the configured ProgramDb when the flags
do not name them, and passes them as:

--with-compiler=/path and --with-hc-pkg=/path (what existing scripts look at);
HC=/path as a configure argument, the standard autoconf form suggested in Replace the --with-compiler configure script argument with HC=/path/to/hc #2947, alongside
the existing CC=/CXX=;
GHC and GHC_PKG in the environment, for GHC, for scripts that use AC_ARG_VAR([GHC])
rather than an option.

Explicit --with-compiler/--with-hc-pkg flags are left as given. With the fix:

configure args: --with-compiler=/home/andrea/.ghcup/bin/ghc-9.12.2 --with-hc-pkg=/home/andrea/.ghcup/ghc/9.12.2/bin/ghc-pkg-9.12.2 ... CC=... CXX=... HC=/home/andrea/.ghcup/bin/ghc-9.12.2
GHC=/home/andrea/.ghcup/bin/ghc-9.12.2

I have not removed --with-compiler (the second half of #2947); the six or so packages on
Hackage that implement it still work, and dropping it is a separate compatibility decision.

Testing

New cabal-testsuite/PackageTests/ConfigureCompiler: a build-type: Configure package whose
hand-written configure checks that --with-compiler/--with-hc-pkg are executable paths,
that HC= matches --with-compiler, that GHC/GHC_PKG are set, and that the compiler runs.
It fails on master ([ -x ghc ] is false) and passes with the change. The existing Configure
tests (Configure, BuildTypeConfigure, ConfigureCXX, ConfigureComponent) still pass.
Skipped on Windows (POSIX shell script).

QA notes

Create a package with build-type: Configure whose configure script prints its arguments and
$GHC, then cabal build -w /full/path/to/ghc. Before: --with-compiler=ghc, GHC unset.
After: the full path in --with-compiler, --with-hc-pkg, HC= and GHC.

Copilot

🟡 Changes recommended

The new HC= and GHC/GHC_PKG exports can disagree with an explicitly provided --with-compiler/--with-hc-pkg, causing inconsistent inputs to configure scripts.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR fixes how build-type: Configure packages learn which Haskell compiler Cabal is using, ensuring configure scripts receive the actual configured compiler path (not just the flavor name like ghc) and adding a regression test to prevent future breakage.

Changes:

Recover compiler and package-tool paths from the configured ProgramDb and pass them to configure via --with-compiler, --with-hc-pkg, HC=..., and (for GHC) GHC/GHC_PKG env vars.
Add a new testsuite package (PackageTests/ConfigureCompiler) whose configure script asserts the above are executable paths and that the compiler runs.
Document the behavior change in the changelog.

File summaries

File	Description
changelog.d/configure-script-compiler.md	Changelog entry describing the new `configure` script compiler-path propagation.
Cabal/src/Distribution/Simple/ConfigureScript.hs	Implements compiler/path recovery from `ProgramDb` and exports it to `configure` via args and env.
cabal-testsuite/PackageTests/ConfigureCompiler/configure-compiler.cabal	New minimal `build-type: Configure` test package.
cabal-testsuite/PackageTests/ConfigureCompiler/configure	New `configure` script asserting correct `--with-compiler`/`HC=`/env behavior.
cabal-testsuite/PackageTests/ConfigureCompiler/cabal.test.hs	New testsuite driver (skips on Windows, runs `v2-build`).
cabal-testsuite/PackageTests/ConfigureCompiler/cabal.project	New test project file for the testsuite package.
cabal-testsuite/PackageTests/ConfigureCompiler/cabal.out	Golden output for the new test.
cabal-testsuite/PackageTests/ConfigureCompiler/A.hs	Minimal library module for the test package.

Review details

Suppressed comments (1)

Cabal/src/Distribution/Simple/ConfigureScript.hs:222

HC= is currently derived from mHcPath (the ProgramDb value) rather than the effective --with-compiler value passed via configureArgs cfg'. This can make HC= disagree with --with-compiler when configHcPath is explicitly set. Prefer using configHcPath cfg' so all configure-script spellings stay in sync.

          -- The standard autoconf spelling for the Haskell compiler (#2947);
          -- '--with-compiler' above is kept for the scripts that use it.
          ++ ["HC=" ++ hcPath | Just hcPath <- [mHcPath]]

Files reviewed: 8/8 changed files
Comments generated: 1
Review effort level: Lite

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andreabedini · 2026-09-22T04:47:59Z

+      hcEnv =
+        [ (var, Just path)
+        | Just GHC <- [flagToMaybe (configHcFlavor cfg)]
+        , (var, Just path) <- [("GHC", mHcPath), ("GHC_PKG", mHcPkgPath)]
+        ]


Fair point. HC= and the GHC/GHC_PKG variables now come from cfg', the same values configureArgs turns into --with-compiler/--with-hc-pkg, so an explicit flag shows up in all of them.

Mikolaj · 2026-09-17T17:50:18Z

Could you include such a configure script that you QA note requires, e.g., in a comment in this issue?

People may be depending on this bug, so please write in your changelog snippet that this requires a Release Notes point with maybe a hint of how to get back the old behaviour.

Mikolaj · 2026-09-17T17:52:46Z

+          ++ hcEnv
          ++ cabalFlagEnv
      maybeHostFlag = ["--host=" ++ show (pretty hp) | hp /= buildPlatform]
+      backwardsCompatHack = False


what's this?

Maybe it's time to retire that?

Pre-existing: it picks --with-hc= over --with-compiler= in configureArgs, and it has been hard-wired to False since 3d9a490 (the TODO next to it mentions defaultUserHooks, which is long gone). Happy to drop the Bool, but configureArgs is exported from Distribution.Simple.Setup, so I'd do it in a separate PR.

'runConfigureScript' passes '--with-compiler=' to a package's configure script, but takes the value from 'configHcPath', which cabal-install never sets: it hands the compiler over as a program path override instead. The script therefore got the bare flavour name ("--with-compiler=ghc") and, following autoconf convention, looked up "ghc" on PATH. A script that needs the actual compiler then ran the wrong one. GHC's own ghc-internal does exactly that ('ghc --print-prim-module' to generate GHC/Internal/Prim.hs) and fails under any other ghc, which broke building GHC's stage2 libraries with the stage1 compiler. Take the compiler and its package tool from the configured ProgramDb when the flags do not name them, and pass them as '--with-compiler=' and '--with-hc-pkg=', as the standard autoconf 'HC=/path' argument next to the existing 'CC='/'CXX=' (the form haskell#2947 asks for; '--with-compiler' stays for the scripts that use it), and for GHC as GHC and GHC_PKG in the environment, for scripts that use AC_ARG_VAR([GHC]) rather than an option. Explicit '--with-compiler'/'--with-hc-pkg' flags are left as given. The new ConfigureCompiler test has a configure script that checks the arguments are executable paths, that HC= agrees with --with-compiler, and that the variables are set. Fixes haskell#7452.

andreabedini · 2026-09-22T04:48:18Z

The changelog now says this needs a release notes entry and how to get the old value back: --configure-option=--with-compiler=ghc (or configure-options: in cabal.project) comes after Cabal's own arguments and autoconf keeps the last one.

A configure script for the QA notes, next to any build-type: Configure package:

#!/bin/sh
echo "configure args: $*"
echo "GHC=${GHC:-<unset>}"

With cabal build -w /full/path/to/ghc, before this PR:

configure args: --with-compiler=ghc --prefix=... CC=/usr/lib64/ccache/gcc CXX=/usr/lib64/ccache/g++
GHC=<unset>

after:

configure args: --with-compiler=/home/andrea/.ghcup/bin/ghc --with-hc-pkg=/home/andrea/.ghcup/ghc/9.12.2/bin/ghc-pkg-9.12.2 --prefix=... CC=/usr/lib64/ccache/gcc CXX=/usr/lib64/ccache/g++ HC=/home/andrea/.ghcup/bin/ghc
GHC=/home/andrea/.ghcup/bin/ghc

Copilot AI lite review requested due to automatic review settings September 9, 2026 03:35

andreabedini force-pushed the andrea/wip/prep/configure-script-compiler branch from 559d3a0 to e90a74a Compare September 9, 2026 03:35

andreabedini mentioned this pull request Sep 9, 2026

build-type: configure configure script isn't passed the GHC exe set with -w to be used #7452

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Copilot started reviewing on behalf of andreabedini September 9, 2026 03:35 View session

andreabedini requested review from Mikolaj, philderbeast and sheaf September 9, 2026 03:37

andreabedini added the attention: needs-review label Sep 9, 2026

Copilot AI reviewed Sep 9, 2026

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zlonast added the build-type: configure label Sep 9, 2026

Mikolaj reviewed Sep 17, 2026

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andreabedini force-pushed the andrea/wip/prep/configure-script-compiler branch from e90a74a to d3c12d6 Compare September 22, 2026 04:47

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Cabal: tell configure scripts which compiler is in use - #12340

Cabal: tell configure scripts which compiler is in use#12340
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andreabedini commented Sep 9, 2026

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