Add parse error location to unparsable environment file warning

cabal-install/src/Distribution/Client/CmdInstall.hs

@@ -144,7 +144,7 @@ import Distribution.Simple.Flag
 import Distribution.Simple.GHC
   ( GhcEnvironmentFileEntry (..)
   , GhcImplInfo (..)
-  , ParseErrorExc
+  , ParseErrorExc (..)
   , getGhcAppDir
   , getImplInfo
   , ghcPlatformAndVersionString
@@ -1294,12 +1294,13 @@ getExistingEnvEntries verbosity compilerFlavor supportsPkgEnvFiles envFile = do
     if (compilerFlavor == GHC || compilerFlavor == GHCJS)
       && supportsPkgEnvFiles
       && envFileExists
-      then catch ((True,) <$> readGhcEnvironmentFile envFile) $ \(_ :: ParseErrorExc) ->
+      then catch ((True,) <$> readGhcEnvironmentFile envFile) $ \(ParseErrorExc parseError) ->
         warn
           verbosity
           ( "The environment file "
               ++ envFile
-              ++ " is unparsable. Libraries cannot be installed."
+              ++ " is unparsable. Libraries cannot be installed.\n"
+              ++ show parseError
           )
           >> return (False, [])
       else return (False, [])

cabal-testsuite/PackageTests/EnvironmentFile/UnparsableWarning/UnparsableWarning.cabal

@@ -0,0 +1,9 @@
+cabal-version: 2.4
+name: UnparsableWarning
+version: 0.1.0.0
+
+executable UnparsableWarning
+    main-is: Main.hs
+    hs-source-dirs: app
+    build-depends: base
+    default-language: Haskell2010

cabal-testsuite/PackageTests/EnvironmentFile/UnparsableWarning/app/Main.hs

@@ -0,0 +1,4 @@
+module Main where
+
+main :: IO ()
+main = return ()

cabal-testsuite/PackageTests/EnvironmentFile/UnparsableWarning/bad.environment

@@ -0,0 +1 @@
+this is not a valid environment file

cabal-testsuite/PackageTests/EnvironmentFile/UnparsableWarning/cabal.out

@@ -0,0 +1,7 @@
+# cabal install
+Wrote tarball sdist to <ROOT>/cabal.dist/work/./dist/sdist/UnparsableWarning-0.1.0.0.tar.gz
+Warning: The environment file <ROOT>/bad.environment is unparsable. Libraries cannot be installed.
+"<ROOT>/bad.environment" (line 1, column 1):
+unexpected 't'
+expecting "--", "package-id", "global-package-db", "user-package-db", "package-db", "clear-package-db" or end of input
+Resolving dependencies...

cabal-testsuite/PackageTests/EnvironmentFile/UnparsableWarning/cabal.test.hs

@@ -0,0 +1,15 @@
+import Test.Cabal.Prelude
+import System.FilePath
+
+main = cabalTest $ recordMode DoNotRecord $ do
+    env <- getTestEnv
+    let envFile = testCurrentDir env </> "bad.environment"
+    -- Pointing cabal at an existing, malformed environment file should produce
+    -- a warning that names the location (line and column) of the parse error
+    -- and the reason, not merely state that the file is unparsable.
+    -- See https://github.com/haskell/cabal/issues/11963
+    res <- cabal' "install" ["--lib", "base", "--package-env=" ++ envFile]
+    assertOutputContains "is unparsable" res
+    assertOutputContains "(line 1, column 1):" res
+    assertOutputContains "unexpected 't'" res
+    assertOutputContains "expecting" res

changelog.d/11963.md

@@ -0,0 +1,10 @@
+---
+synopsis: Include parse error location in unparsable environment file warning
+packages: [cabal-install]
+issues: 11963
+prs: 11996
+---
+When a GHC environment file cannot be parsed, the warning now includes the
+location (line and column) and the reason for the failure, taken from the
+underlying parser, in addition to the existing message. This
+makes it possible to find and fix the offending line.