Adding an explicit pgmc flag for interaction with GHC

[zlonast]

Issue #11712
Main idea #9801 (comment)

IMHO, cc-options (and ld-options and cpp-options and ...) should be always passed when invoking GHC, similarly as ghc-options should be always used when invoking ghc - regardless of what is the intention of a particular GHC-call. GHC might use or not use the options, Cabal cannot know and should not guess.

Template Α: This PR modifies behaviour or interface

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• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
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• The documentation has been updated, if necessary.
• Manual QA notes have been included.
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[zlonast]

I found out why it started working and why it didn't work before.

Since 9.4 it works like this:
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/6949

  , make_ord_flag defFlag "pgmc"
      $ hasArg $ \f -> alterToolSettings $ \s -> s { toolSettings_pgm_c   = f }

From 8.8 to 9.2 it worked like this:
https://gitlab.haskell.org/ghc/ghc/-/issues/17919

  , make_ord_flag defFlag "pgmc"
      $ hasArg $ \f -> alterToolSettings $ \s -> s
         { toolSettings_pgm_c   = f
         , -- Don't pass -no-pie with -pgmc
           -- (see #15319)
           toolSettings_ccSupportsNoPie = False
         }

This didn't work before 8.8:
https://gitlab.haskell.org/ghc/ghc/-/issues/17919

[sheaf]

I will do my best to take a look at this PR soon.

[Copilot]

Pull request overview

This PR updates Cabal’s GHC invocation to pass an explicit -pgmc (C compiler program) for GHC ≥ 9.4, with accompanying tests and output normalization to keep the testsuite stable.

Changes:

• Add version-gated -pgmc injection to componentGhcOptions (GHC ≥ 9.4).
• Update show-build-info golden tests (split expectations across GHC versions) and normalize -pgmc paths in JSON output.
• Add a new FFI regression test case that verifies -pgmc wrapper usage.

Reviewed changes

Copilot reviewed 16 out of 17 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
changelog.d/11713.md	Changelog entry describing the -pgmc-related behavior change.
cabal-testsuite/src/Test/Cabal/OutputNormalizer.hs	Normalizes -pgmc argument paths in show-build-info JSON output.
cabal-testsuite/PackageTests/ShowBuildInfo/Custom/custom.test.hs	Formatting-only adjustments to the test script.
cabal-testsuite/PackageTests/ShowBuildInfo/Complex/single.test.hs	Restricts the existing golden test to GHC ≥ 9.4 (where -pgmc now appears).
cabal-testsuite/PackageTests/ShowBuildInfo/Complex/single.out	Updates golden output to include normalized -pgmc for GHC ≥ 9.4.
cabal-testsuite/PackageTests/ShowBuildInfo/Complex/single-2.test.hs	Adds a new golden test variant for GHC 9.2.x (caret-bounded) without -pgmc.
cabal-testsuite/PackageTests/ShowBuildInfo/Complex/single-2.out	New golden output matching the GHC 9.2.x behavior.
cabal-testsuite/PackageTests/FFI/ForeignOptsPgmc/scripts/cc-wrapper.sh	Adds a shell wrapper used to validate -pgmc selection in tests.
cabal-testsuite/PackageTests/FFI/ForeignOptsPgmc/foreign-opts-pgmc.cabal	New test package using ghc-options: -pgmc scripts/cc-wrapper.sh.
cabal-testsuite/PackageTests/FFI/ForeignOptsPgmc/cbits/pgmclib.h	New C header for the FFI test.
cabal-testsuite/PackageTests/FFI/ForeignOptsPgmc/cbits/pgmclib.c	New C source asserting the wrapper-injected define is present.
cabal-testsuite/PackageTests/FFI/ForeignOptsPgmc/cabal.test.hs	New testsuite driver for the FFI -pgmc scenario.
cabal-testsuite/PackageTests/FFI/ForeignOptsPgmc/cabal.project	New cabal.project for the test package.
cabal-testsuite/PackageTests/FFI/ForeignOptsPgmc/cabal.out	Empty golden output placeholder for the new test.
cabal-testsuite/PackageTests/FFI/ForeignOptsPgmc/README.md	Documents the intent and mechanics of the new FFI test.
cabal-testsuite/PackageTests/FFI/ForeignOptsPgmc/Main.hs	New Haskell FFI entrypoint that checks the wrapper-injected value at runtime.
Cabal/src/Distribution/Simple/GHC/Internal.hs	Adds ghcOptionsSince helper and sets ghcOptCcProgram (i.e. -pgmc) for GHC ≥ 9.4.

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[Copilot]

wrapper is defined but never used. This produces an unnecessary -Wunused-local-binds warning under the testsuite’s -Wall settings; either remove the binding or use it (e.g., to assert the wrapper exists / is executable, or to pass it explicitly via configuration).

Suggested change

	cwd <- fmap testCurrentDir getTestEnv
	let wrapper = cwd </> "scripts/cc-wrapper.sh"

[ulysses4ever]

This suggestion looks legit, @zlonast ?

[zlonast]

Yes, I missed that.