Soften the error down to a warning

- Use with-ghc.config trick

cabal-install-solver/src/Distribution/Solver/Types/ProjectConfigPath.hs

@@ -120,10 +120,10 @@ cyclicalImportMsg path@(ProjectConfigPath (duplicate :| _)) =
     ]
 
 -- | A message for an import that has leading or trailing spaces.
-untrimmedUriImportMsg :: ProjectConfigPath -> Doc
-untrimmedUriImportMsg path =
+untrimmedUriImportMsg :: Doc -> ProjectConfigPath -> Doc
+untrimmedUriImportMsg intro path =
     vcat
-    [ text "import has whitespace" <> semi
+    [ intro <+> text "import has leading or trailing whitespace" <> semi
     , nest 2 (docProjectConfigPath path)
     ]
 

cabal-install/src/Distribution/Client/ProjectConfig/Legacy.hs

@@ -127,6 +127,7 @@ import Distribution.Simple.Setup
 import Distribution.Simple.Utils
   ( debug
   , lowercase
+  , noticeDoc
   )
 import Distribution.Types.CondTree
   ( CondBranch (..)
@@ -273,17 +274,17 @@ parseProjectSkeleton cacheDir httpTransport verbosity projectDir source (Project
 
         debug verbosity $ "\nimport path, normalized\n=======================\n" ++ render (docProjectConfigPath normLocPath)
 
-        if
-            | isUntrimmedUriConfigPath importLocPath ->
-                pure . parseFail $ ParseUtils.FromString (render $ untrimmedUriImportMsg importLocPath) Nothing
-            | isCyclicConfigPath normLocPath ->
-                pure . parseFail $ ParseUtils.FromString (render $ cyclicalImportMsg normLocPath) Nothing
-            | otherwise -> do
-                normSource <- canonicalizeConfigPath projectDir source
-                let fs = (\z -> CondNode z [normLocPath] mempty) <$> fieldsToConfig normSource (reverse acc)
-                res <- parseProjectSkeleton cacheDir httpTransport verbosity projectDir importLocPath . ProjectConfigToParse =<< fetchImportConfig normLocPath
-                rest <- go [] xs
-                pure . fmap mconcat . sequence $ [fs, res, rest]
+        if isCyclicConfigPath normLocPath
+          then pure . parseFail $ ParseUtils.FromString (render $ cyclicalImportMsg normLocPath) Nothing
+          else do
+            when
+              (isUntrimmedUriConfigPath importLocPath)
+              (noticeDoc verbosity $ untrimmedUriImportMsg (Disp.text "Warning:") importLocPath)
+            normSource <- canonicalizeConfigPath projectDir source
+            let fs = (\z -> CondNode z [normLocPath] mempty) <$> fieldsToConfig normSource (reverse acc)
+            res <- parseProjectSkeleton cacheDir httpTransport verbosity projectDir importLocPath . ProjectConfigToParse =<< fetchImportConfig normLocPath
+            rest <- go [] xs
+            pure . fmap mconcat . sequence $ [fs, res, rest]
       (ParseUtils.Section l "if" p xs') -> do
         subpcs <- go [] xs'
         let fs = singletonProjectConfigSkeleton <$> fieldsToConfig source (reverse acc)

cabal-testsuite/PackageTests/UntrimmedImport/cabal.out

@@ -1,14 +1,30 @@
 # checking project import with trailing space
 # cabal v2-build
-Error: [Cabal-7090]
-Error parsing project file <ROOT>/trailing-space.project:
-import has whitespace;
+Warning: import has leading or trailing whitespace;
   'https://www.stackage.org/nightly-2024-12-05/cabal.config '
     imported by: trailing-space.project
+Configuration is affected by the following files:
+- https://www.stackage.org/nightly-2024-12-05/cabal.config
+    imported by: trailing-space.project
+- trailing-space.project
+- with-ghc.config
+    imported by: trailing-space.project
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following would be built:
+ - my-0.1 (lib:my) (first run)
 # checking project import with tabs and spaces
 # cabal v2-build
-Error: [Cabal-7090]
-Error parsing project file <ROOT>/tabs-and-spaces.project:
-import has whitespace;
+Warning: import has leading or trailing whitespace;
   'https://www.stackage.org/nightly-2024-12-05/cabal.config		   '
     imported by: tabs-and-spaces.project
+Configuration is affected by the following files:
+- https://www.stackage.org/nightly-2024-12-05/cabal.config
+    imported by: tabs-and-spaces.project
+- tabs-and-spaces.project
+- with-ghc.config
+    imported by: tabs-and-spaces.project
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following would be built:
+ - my-0.1 (lib:my) (first run)

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

@@ -4,9 +4,11 @@ main = cabalTest . recordMode RecordMarked $ do
   let log = recordHeader . pure
 
   log "checking project import with trailing space"
-  trailing <- fails $ cabal' "v2-build" [ "--project-file=trailing-space.project" ]
-  assertOutputContains "import has whitespace" trailing
+  trailing <- cabal' "v2-build" [ "--dry-run", "--project-file=trailing-space.project" ]
+  assertOutputContains "import has leading or trailing whitespace" trailing
   assertOutputContains "'https://www.stackage.org/nightly-2024-12-05/cabal.config '" trailing
 
   log "checking project import with tabs and spaces"
-  fails $ cabal "v2-build" [ "--project-file=tabs-and-spaces.project" ]
+  cabal "v2-build" [ "--dry-run", "--project-file=tabs-and-spaces.project" ]
+
+  return ()

cabal-testsuite/PackageTests/UntrimmedImport/my.cabal

@@ -0,0 +1,9 @@
+name:           my
+version:        0.1
+license:        BSD3
+cabal-version:  >= 1.2
+build-type:     Simple
+
+library
+    exposed-modules:    Foo
+    build-depends:      base

cabal-testsuite/PackageTests/UntrimmedImport/tabs-and-spaces.project

@@ -1,2 +1,3 @@
-optional-packages: .
+packages: .
 import:		https://www.stackage.org/nightly-2024-12-05/cabal.config		   
+import: with-ghc.config

cabal-testsuite/PackageTests/UntrimmedImport/trailing-space.project

@@ -1,2 +1,3 @@
-optional-packages: .
+packages: .
 import: https://www.stackage.org/nightly-2024-12-05/cabal.config 
+import: with-ghc.config

cabal-testsuite/PackageTests/UntrimmedImport/with-ghc.config

@@ -0,0 +1,7 @@
+-- WARNING: Override the `with-compiler: ghc-x.y.z` of the stackage import, of
+-- https://www.stackage.org/nightly-yyyy-mm-dd/cabal.config. Otherwise tests
+-- will fail with:
+--   -Error: [Cabal-5490]
+--   -Cannot find the program 'ghc'. User-specified path 'ghc-x.y.z' does not
+--    refer to an executable and the program is not on the system path.
+with-compiler: ghc