Cabal: tell configure scripts which compiler is in use

Cabal/src/Distribution/Simple/ConfigureScript.hs

@@ -18,6 +18,7 @@ import Distribution.Compat.Prelude
 import Prelude ()
 
 -- local
+import Distribution.Compiler (CompilerFlavor (..))
 import Distribution.PackageDescription
 import Distribution.Pretty
 import Distribution.Simple.Configure (findDistPrefOrDefault)
@@ -86,6 +87,34 @@ runConfigureScript verbHandles cfg flags programDb hp = do
         return (Just cxxProgShort, Just cxxFlags)
       Nothing -> return (Nothing, Nothing)
 
+  -- The compiler this package is configured with. A configure script that
+  -- asks for it (autoconf's @AC_ARG_WITH([compiler])@ or @AC_ARG_VAR([GHC])@,
+  -- as GHC's own libraries do to run @ghc --print-prim-module@) must get the
+  -- compiler Cabal uses, not whatever @ghc@ happens to be on PATH. The
+  -- ConfigFlags alone do not say: cabal-install passes the compiler as a
+  -- program path override and leaves 'configHcPath' unset, in which case
+  -- 'configureArgs' would only pass the flavour name.
+  let hcPrograms = case flagToMaybe (configHcFlavor cfg) of
+        Just GHC -> Just (ghcProgram, ghcPkgProgram)
+        Just GHCJS -> Just (ghcjsProgram, ghcjsPkgProgram)
+        _ -> Nothing
+      configuredPath prog = programPath <$> lookupProgram prog programDb
+  mHcPath <- traverse getShortPathName (hcPrograms >>= configuredPath . fst)
+  mHcPkgPath <- traverse getShortPathName (hcPrograms >>= configuredPath . snd)
+  let orConfigured flag mpath = case flag of
+        Flag p -> Flag p
+        NoFlag -> maybe NoFlag Flag mpath
+      cfg' =
+        cfg
+          { configHcPath = configHcPath cfg `orConfigured` mHcPath
+          , configHcPkg = configHcPkg cfg `orConfigured` mHcPkgPath
+          }
+      hcEnv =
+        [ (var, Just path)
+        | Just GHC <- [flagToMaybe (configHcFlavor cfg)]
+        , (var, Just path) <- [("GHC", mHcPath), ("GHC_PKG", mHcPkgPath)]
+        ]
+
   let configureFile' = toUnix configureFile
   -- autoconf is fussy about filenames, and has a set of forbidden
   -- characters that can't appear in the build directory, etc:
@@ -178,13 +207,19 @@ runConfigureScript verbHandles cfg flags programDb hp = do
         ("CFLAGS", Just (mkFlagsEnv ccFlags "CFLAGS"))
           : [("CXXFLAGS", Just (mkFlagsEnv cxxFlags "CXXFLAGS")) | Just cxxFlags <- [mcxxFlags]]
           ++ [("PATH", Just pathEnv) | not (null extraPath)]
+          ++ hcEnv
           ++ cabalFlagEnv
       maybeHostFlag = ["--host=" ++ show (pretty hp) | hp /= buildPlatform]
+      backwardsCompatHack = False
+      args = configureArgs backwardsCompatHack cfg'
       args' =
         configureFile'
           : args
           ++ ["CC=" ++ ccProgShort]
           ++ ["CXX=" ++ cxxProgShort | Just cxxProgShort <- [mcxxProgShort]]
+          -- 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]]
           ++ maybeHostFlag
       shProg = simpleProgram "sh"
   progDb <- prependProgramSearchPath verbosity extraPath [] emptyProgramDb
@@ -200,9 +235,6 @@ runConfigureScript verbHandles cfg flags programDb hp = do
           { progInvokeCwd = Just build_in
           }
     Nothing -> dieWithException verbosity NotFoundMsg
-  where
-    args = configureArgs backwardsCompatHack cfg
-    backwardsCompatHack = False
 
 -- | Convert Windows path to Unix ones
 toUnix :: String -> String

cabal-testsuite/PackageTests/ConfigureCompiler/A.hs

@@ -0,0 +1,4 @@
+module A where
+
+a :: Int
+a = 1

cabal-testsuite/PackageTests/ConfigureCompiler/cabal.out

@@ -0,0 +1,8 @@
+# cabal v2-build
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following will be built:
+ - configure-compiler-1.0 (lib) (first run)
+Configuring library for configure-compiler-1.0...
+Preprocessing library for configure-compiler-1.0...
+Building library for configure-compiler-1.0...

cabal-testsuite/PackageTests/ConfigureCompiler/cabal.project

@@ -0,0 +1 @@
+packages: .

cabal-testsuite/PackageTests/ConfigureCompiler/cabal.test.hs

@@ -0,0 +1,11 @@
+import Test.Cabal.Prelude
+
+-- A 'build-type: Configure' script must be told which compiler Cabal is
+-- configuring the package with (GHC's own libraries run it to generate
+-- sources). cabal-install passes the compiler as a program path override,
+-- so the runner has to recover it from the program db: the script checks
+-- that --with-compiler / --with-hc-pkg are executable paths and that GHC
+-- and GHC_PKG are exported.
+main = do
+  skipIfWindows "relies on a POSIX shell script"
+  cabalTest $ cabal "v2-build" []

cabal-testsuite/PackageTests/ConfigureCompiler/configure

@@ -0,0 +1,21 @@
+#!/bin/sh
+# A hand-written configure script: check that Cabal tells us which compiler
+# it is configuring the package with, by path rather than by name.
+hc=; hcpkg=; hcvar=
+for arg in "$@"; do
+  case $arg in
+    --with-compiler=*) hc=${arg#--with-compiler=} ;;
+    --with-hc-pkg=*) hcpkg=${arg#--with-hc-pkg=} ;;
+    HC=*) hcvar=${arg#HC=} ;;
+  esac
+done
+fail() { echo "configure: $1" >&2; exit 1; }
+[ -n "$hc" ] || fail "no --with-compiler argument"
+[ -x "$hc" ] || fail "--with-compiler is not an executable path: $hc"
+[ -n "$hcpkg" ] || fail "no --with-hc-pkg argument"
+[ -x "$hcpkg" ] || fail "--with-hc-pkg is not an executable path: $hcpkg"
+[ "$hcvar" = "$hc" ] || fail "HC= argument ($hcvar) differs from --with-compiler ($hc)"
+[ -n "$GHC" ] || fail "GHC is not set in the environment"
+[ "$GHC" = "$hc" ] || fail "GHC ($GHC) differs from --with-compiler ($hc)"
+[ -n "$GHC_PKG" ] || fail "GHC_PKG is not set in the environment"
+"$hc" --numeric-version > /dev/null || fail "cannot run $hc"

cabal-testsuite/PackageTests/ConfigureCompiler/configure-compiler.cabal

@@ -0,0 +1,11 @@
+cabal-version: 2.4
+name:          configure-compiler
+version:       1.0
+synopsis:      A Configure package whose script needs the compiler
+build-type:    Configure
+license:       BSD-3-Clause
+
+library
+  exposed-modules:  A
+  build-depends:    base
+  default-language: Haskell2010

changelog.d/configure-script-compiler.md

@@ -0,0 +1,14 @@
+---
+synopsis: Tell `configure` scripts which compiler Cabal is using
+packages: [Cabal]
+issues: [7452, 2947]
+prs: 12340
+---
+
+A `build-type: Configure` package's `configure` script now receives the path
+of the compiler Cabal is configuring the package with: as the standard
+`HC=/path/to/ghc` argument, in `--with-compiler` and `--with-hc-pkg`, and, for
+GHC, in the `GHC` and `GHC_PKG` environment variables. Previously, when the
+compiler was given as a program path (as cabal-install always does), the
+script only got the flavour name `ghc` and had to find the compiler on `PATH`,
+which may be a different one.