Add “Add default-language” suggestion

As a warning (“Hackage would reject this package”), to prepare users
to GHC language editions.

Cabal-tests/tests/ParserTests/regressions/issue-6288-d.cabal

@@ -4,7 +4,8 @@ version:       6288
 build-type:    Simple
 synopsis:      default-language optionality
 category:      Test
-description:   Field is introduced in 1.10, defaulted in 3.4
+description:   Field is introduced in 1.10, defaulted in 3.4,
+               suggested in cabal 3.12.
 license:       BSD3
 license-file:  LICENSE
 maintainer:    Cabal Contributors

Cabal-tests/tests/ParserTests/regressions/issue-6288-f.check

@@ -0,0 +1 @@
+[add-language] Without `default-language`, cabal will default to Haskell98, which is probably not what you want. Please add `default-language` to all targets.

Cabal-tests/tests/ParserTests/regressions/issue-7776-a.cabal

@@ -24,3 +24,4 @@ library
             GHC.Hs.Type
         hs-source-dirs:
             compat-8.10
+    default-language: Haskell2010

Cabal-tests/tests/ParserTests/regressions/issue-7776-b.cabal

@@ -15,6 +15,7 @@ library
     -- the base lower bound makes the package not buildable with ghc < 6.8
     -- but cabal is not smart enough to know it :-P
     build-depends: base >= 4.5 && < 4.15
+    default-language: Haskell2010
 
 benchmark benchmarks
     main-is: Benchmarks.hs
@@ -25,3 +26,4 @@ benchmark benchmarks
         Ghc-options: -fwarn-tabs
     else
         other-modules: Data.Hashable.RandomSource
+    default-language: Haskell2010

Cabal-tests/tests/ParserTests/regressions/issue-7776-c.cabal

@@ -17,3 +17,4 @@ library
         compat
     other-modules:
         GHC.Hs.Type
+    default-language: Haskell2010

Cabal/src/Distribution/PackageDescription/Check/Target.hs

@@ -483,12 +483,7 @@ checkBuildInfoFeatures bi sv = do
     (isJust $ defaultLanguage bi)
     (PackageBuildWarning CVDefaultLanguage)
   -- CheckSpecVer sv.
-  checkP
-    ( sv >= CabalSpecV1_10
-        && sv < CabalSpecV3_4
-        && isNothing (defaultLanguage bi)
-    )
-    (PackageBuildWarning CVDefaultLanguageComponent)
+  checkDefaultLanguage
   -- Check use of 'extra-framework-dirs' field.
   checkSpecVer
     CabalSpecV1_24
@@ -534,6 +529,17 @@ checkBuildInfoFeatures bi sv = do
         (not . null $ cvs)
         (PackageDistInexcusable CVSources)
 
+    checkDefaultLanguage :: Monad m => CheckM m ()
+    checkDefaultLanguage = do
+      -- < 1.10 has no `default-language` field.
+      when
+        (sv >= CabalSpecV1_10 && isNothing (defaultLanguage bi))
+        -- < 3.4 mandatory, after just a suggestion.
+        ( if sv < CabalSpecV3_4
+            then tellP (PackageBuildWarning CVDefaultLanguageComponent)
+            else tellP (PackageDistInexcusable CVDefaultLanguageComponentSoft)
+        )
+
 -- Tests for extensions usage which can break Cabal < 1.4.
 checkBuildInfoExtensions :: Monad m => BuildInfo -> CheckM m ()
 checkBuildInfoExtensions bi = do

Cabal/src/Distribution/PackageDescription/Check/Warning.hs

@@ -233,6 +233,7 @@ data CheckExplanation
   | CVTestSuite
   | CVDefaultLanguage
   | CVDefaultLanguageComponent
+  | CVDefaultLanguageComponentSoft
   | CVExtraDocFiles
   | CVMultiLib
   | CVReexported
@@ -394,6 +395,7 @@ data CheckExplanationID
   | CICVTestSuite
   | CICVDefaultLanguage
   | CICVDefaultLanguageComponent
+  | CICVDefaultLanguageComponentSoft
   | CICVExtraDocFiles
   | CICVMultiLib
   | CICVReexported
@@ -534,6 +536,7 @@ checkExplanationId (FilePathEmpty{}) = CIFilePathEmpty
 checkExplanationId (CVTestSuite{}) = CICVTestSuite
 checkExplanationId (CVDefaultLanguage{}) = CICVDefaultLanguage
 checkExplanationId (CVDefaultLanguageComponent{}) = CICVDefaultLanguageComponent
+checkExplanationId (CVDefaultLanguageComponentSoft{}) = CICVDefaultLanguageComponentSoft
 checkExplanationId (CVExtraDocFiles{}) = CICVExtraDocFiles
 checkExplanationId (CVMultiLib{}) = CICVMultiLib
 checkExplanationId (CVReexported{}) = CICVReexported
@@ -679,6 +682,7 @@ ppCheckExplanationId CIFilePathEmpty = "empty-path"
 ppCheckExplanationId CICVTestSuite = "test-cabal-ver"
 ppCheckExplanationId CICVDefaultLanguage = "default-language"
 ppCheckExplanationId CICVDefaultLanguageComponent = "no-default-language"
+ppCheckExplanationId CICVDefaultLanguageComponentSoft = "add-language"
 ppCheckExplanationId CICVExtraDocFiles = "extra-doc-files"
 ppCheckExplanationId CICVMultiLib = "multilib"
 ppCheckExplanationId CICVReexported = "reexported-modules"
@@ -1163,6 +1167,10 @@ ppExplanation CVDefaultLanguageComponent =
     ++ "Haskell98 or Haskell2010). If a component uses different languages "
     ++ "in different modules then list the other ones in the "
     ++ "'other-languages' field."
+ppExplanation CVDefaultLanguageComponentSoft =
+  "Without `default-language`, cabal will default to Haskell98, which is "
+    ++ "probably not what you want. Please add `default-language` to all "
+    ++ "targets."
 ppExplanation CVExtraDocFiles =
   "To use the 'extra-doc-files' field the package needs to specify "
     ++ "'cabal-version: 1.18' or higher."

cabal-testsuite/PackageTests/Check/ConfiguredPackage/CabalVersion/DefaultLanguageSoft/cabal.out

@@ -0,0 +1,4 @@
+# cabal check
+The following errors will cause portability problems on other environments:
+Error: [add-language] Without `default-language`, cabal will default to Haskell98, which is probably not what you want. Please add `default-language` to all targets.
+Error: Hackage would reject this package.

cabal-testsuite/PackageTests/Check/ConfiguredPackage/CabalVersion/DefaultLanguageSoft/cabal.test.hs

@@ -0,0 +1,5 @@
+import Test.Cabal.Prelude
+
+-- Please specify `default-language`.
+main = cabalTest $
+  fails $ cabal "check" []

cabal-testsuite/PackageTests/Check/ConfiguredPackage/CabalVersion/DefaultLanguageSoft/pkg.cabal

@@ -0,0 +1,11 @@
+cabal-version: 3.4
+name: pkg
+synopsis: synopsis
+description: description
+version: 0
+category: example
+maintainer: none@example.com
+license: GPL-3.0-or-later
+
+library
+  exposed-modules: Module

changelog.d/pr-9766

@@ -0,0 +1,11 @@
+synopsis: Warn on missing `default-language`
+packages: Cabalcabal-install
+prs: #9766
+issues: #9620
+
+description: {
+
+- To help the adoption of GHC language editions, `cabal check` will now
+  warn about missing `default-language`.
+
+}

doc/cabal-commands.rst

@@ -1276,6 +1276,7 @@ A list of all warnings with their constructor:
 - ``test-cabal-ver``: ``test-suite`` used with ``cabal-version`` < 1.10.
 - ``default-language``: ``default-language`` used with ``cabal-version`` < 1.10.
 - ``no-default-language``: missing ``default-language``.
+- ``add-default-language``: suggested ``default-language``.
 - ``extra-doc-files``: ``extra-doc-files`` used with ``cabal-version`` < 1.18.
 - ``multilib``: multiple ``library`` sections with ``cabal-version`` < 2.0.
 - ``reexported-modules``: ``reexported-modules`` with ``cabal-version`` < 1.22.