Deprecating --solver=modular CLI argument

cabal-install/src/Distribution/Client/Dependency/Types.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE DeriveGeneric #-}
+{-# LANGUAGE InstanceSigs #-}
 
 module Distribution.Client.Dependency.Types
   ( PreSolver (..)
@@ -9,9 +10,8 @@ module Distribution.Client.Dependency.Types
 import Distribution.Client.Compat.Prelude
 import Prelude ()
 
-import Text.PrettyPrint (text)
-
 import qualified Distribution.Compat.CharParsing as P
+import GHC.IO (unsafePerformIO)
 
 -- | All the solvers that can be selected.
 data PreSolver = AlwaysModular
@@ -27,14 +27,14 @@ instance Binary Solver
 instance Structured PreSolver
 instance Structured Solver
 
-instance Pretty PreSolver where
-  pretty AlwaysModular = text "modular"
-
 instance Parsec PreSolver where
+  parsec :: CabalParsing m => m PreSolver
   parsec = do
     name <- P.munch1 isAlpha
     case map toLower name of
-      "modular" -> return AlwaysModular
+      "modular" -> return $ unsafePerformIO $ do
+        putStrLn "[WARNING] The `solver' config option is deprecated and will be removed in a future release."
+        return AlwaysModular
       _ -> P.unexpected $ "PreSolver: " ++ name
 
 -- | Global policy for all packages to say if we prefer package versions that

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

@@ -236,6 +236,8 @@ import System.FilePath
   ( (</>)
   )
 
+import System.IO.Unsafe (unsafePerformIO)
+
 globalCommand :: [Command action] -> CommandUI GlobalFlags
 globalCommand commands =
   CommandUI
@@ -934,7 +936,13 @@ configureExOptions _showOrParseArgs src =
           )
           (map prettyShow)
       )
-  , optionSolver configSolver (\v flags -> flags{configSolver = v})
+  , optionSolver
+      configSolver
+      ( \_ flags ->
+          unsafePerformIO $ do
+            putStrLn "[WARNING] The --solver flag is deprecated and will be removed in a future release."
+            return flags
+      )
   , option
       []
       ["allow-older"]
@@ -2161,9 +2169,6 @@ defaultMaxBackjumps = 4000
 defaultSolver :: PreSolver
 defaultSolver = AlwaysModular
 
-allSolvers :: String
-allSolvers = intercalate ", " (map prettyShow ([minBound .. maxBound] :: [PreSolver]))
-
 installCommand
   :: CommandUI
       ( ConfigFlags
@@ -3424,16 +3429,16 @@ optionSolver get set =
   option
     []
     ["solver"]
-    ("Select dependency solver to use (default: " ++ prettyShow defaultSolver ++ "). Choices: " ++ allSolvers ++ ".")
+    ("[DEPRECATED] Select dependency solver to use (default: modular). Choices: modular.")
     get
     set
     ( reqArg
         "SOLVER"
         ( parsecToReadE
-            (const $ "solver must be one of: " ++ allSolvers)
+            (const $ "solver must be one of: modular")
             (toFlag `fmap` parsec)
         )
-        (flagToList . fmap prettyShow)
+        (flagToList . fmap (\_ -> "modular"))
     )
 
 optionSolverFlags

doc/cabal-commands.rst

@@ -499,7 +499,7 @@ builds with ``cabal build`` are performed with the compiler
 
 -  Any flag accepted by ``cabal configure`` beyond
    ``./Setup configure``, namely ``--cabal-lib-version``,
-   ``--constraint``, ``--preference`` and ``--solver.``
+   ``--constraint`` and ``--preference``.
 
 -  Any flag accepted by ``cabal install`` beyond ``./Setup configure``.
 

doc/cabal-project-description-file.rst

@@ -1712,6 +1712,10 @@ Most users generally won't need these.
 
     The command line variant of this field is ``--solver=modular``.
 
+    .. warning::
+
+    This CLI option has been deprecated and will be removed in a future release.
+
 .. cfg-field:: max-backjumps: nat
                --max-backjumps=N
     :synopsis: Maximum number of solver backjumps.