Mark public libs as an experimental feature

Cabal/Cabal.cabal

@@ -215,6 +215,10 @@ extra-source-files:
   tests/ParserTests/regressions/pre-1.6-glob.check
   tests/ParserTests/regressions/pre-2.4-globstar.cabal
   tests/ParserTests/regressions/pre-2.4-globstar.check
+  tests/ParserTests/regressions/public-multilib-1.cabal
+  tests/ParserTests/regressions/public-multilib-1.check
+  tests/ParserTests/regressions/public-multilib-2.cabal
+  tests/ParserTests/regressions/public-multilib-2.check
   tests/ParserTests/regressions/shake.cabal
   tests/ParserTests/regressions/shake.expr
   tests/ParserTests/regressions/shake.format

Cabal/Distribution/Parsec/Warning.hs

@@ -38,6 +38,8 @@ data PWarnType
 
     | PWTVersionOperator       -- ^ Version operators used (without cabal-version: 1.8)
     | PWTVersionWildcard       -- ^ Version wildcard used (without cabal-version: 1.6)
+
+    | PWTExperimental          -- ^ Experimental feature
     deriving (Eq, Ord, Show, Enum, Bounded, Generic)
 
 instance Binary PWarnType

Cabal/Distribution/Types/Dependency.hs

@@ -126,6 +126,7 @@ instance Parsec Dependency where
         libs <- option mainLib $ do
           _ <- char ':'
           versionGuardMultilibs
+          parsecWarning PWTExperimental "colon specifier is experimental feature (issue #5660)"
           Set.singleton <$> parseLib <|> parseMultipleLibs
 
         spaces -- https://github.com/haskell/cabal/issues/5846

Cabal/Distribution/Types/LibraryVisibility.hs

@@ -34,6 +34,7 @@ instance Pretty LibraryVisibility where
 instance Parsec LibraryVisibility where
   parsec = do
     name <- P.munch1 isAlpha
+    parsecWarning PWTExperimental "visibility is experimental feature (issue #5660)"
     case name of
       "public"  -> return LibraryVisibilityPublic
       "private" -> return LibraryVisibilityPrivate

Cabal/tests/CheckTests.hs

@@ -40,6 +40,8 @@ checkTests = testGroup "regressions"
     , checkTest "ghc-option-j.cabal"
     , checkTest "multiple-libs-2.cabal"
     , checkTest "assoc-cpp-options.cabal"
+    , checkTest "public-multilib-1.cabal"
+    , checkTest "public-multilib-2.cabal"
     ]
 
 checkTest :: FilePath -> TestTree

Cabal/tests/ParserTests/regressions/public-multilib-1.cabal

@@ -0,0 +1,17 @@
+cabal-version: 3.0
+name:          public-multilib1
+version:       0
+synopsis:      public-multilibs are not polished enough for Hackage
+category:      Tests
+license:       MIT
+
+library
+  default-language: Haskell2010
+  build-depends:    base ^>=4.14, internal
+  exposed-modules:  Foo
+
+library internal
+  default-language: Haskell2010
+  build-depends:    base ^>=4.14
+  visibility:       public
+  exposed-modules:  Bar

Cabal/tests/ParserTests/regressions/public-multilib-1.check

@@ -0,0 +1,3 @@
+public-multilib-1.cabal:16:27: visibility is experimental feature (issue #5660)
+No 'maintainer' field.
+No 'description' field.

Cabal/tests/ParserTests/regressions/public-multilib-2.cabal

@@ -0,0 +1,14 @@
+cabal-version: 3.0
+name:          public-multilib1
+version:       0
+synopsis:      public-multilibs are not polished enough for Hackage
+category:      Tests
+license:       MIT
+
+library
+  default-language: Haskell2010
+  build-depends:
+    , base     ^>=4.14
+    , somelib:internal
+
+  exposed-modules:  Foo

Cabal/tests/ParserTests/regressions/public-multilib-2.check

@@ -0,0 +1,3 @@
+public-multilib-2.cabal:12:15: colon specifier is experimental feature (issue #5660)
+No 'maintainer' field.
+No 'description' field.