Make enable/disable nix flag easier to read

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

@@ -357,10 +357,14 @@ globalCommand commands = CommandUI {
          globalHttpTransport (\v flags -> flags { globalHttpTransport = v })
          (reqArgFlag "HttpTransport")
 
-      ,option [] ["nix"]
-         "Nix integration: run commands through nix-shell if a 'shell.nix' file exists"
-         globalNix (\v flags -> flags { globalNix = v })
-         (boolOpt [] [])
+      ,multiOption "nix"
+        globalNix (\v flags -> flags { globalNix = v })
+        [
+          noArg (Flag True) [] ["enable-nix"] 
+          "Enable Nix integration: run commands through nix-shell if a 'shell.nix' file exists",
+          noArg (Flag False) [] ["disable-nix"] 
+          "Disable Nix integration"
+        ]
 
       ,option [] ["store-dir", "storedir"]
          "The location of the build store"

cabal-install/tests/IntegrationTests2.hs

@@ -49,7 +49,9 @@ import Distribution.Package
 import Distribution.PackageDescription
 import Distribution.InstalledPackageInfo (InstalledPackageInfo)
 import Distribution.Simple.Setup (toFlag, HaddockFlags(..), defaultHaddockFlags)
+import Distribution.Client.Setup (globalCommand)
 import Distribution.Simple.Compiler
+import Distribution.Simple.Command
 import Distribution.System
 import Distribution.Version
 import Distribution.ModuleName (ModuleName)
@@ -71,6 +73,9 @@ import Test.Tasty.Options
 import Data.Tagged (Tagged(..))
 
 import qualified Data.ByteString as BS
+import Distribution.Client.GlobalFlags (GlobalFlags, globalNix)
+import Distribution.Simple.Flag (Flag (Flag, NoFlag))
+import Data.Maybe (fromJust)
 
 #if !MIN_VERSION_directory(1,2,7)
 removePathForcibly :: FilePath -> IO ()
@@ -139,9 +144,12 @@ tests config =
     , testCase "program options scope local" (testProgramOptionsLocal config)
     , testCase "program options scope specific" (testProgramOptionsSpecific config)
     ]
+  , testGroup "Flag tests" $
+    [
+      testCase "Test Nix Flag" testNixFlags
+    ]
   ]
 
-
 testFindProjectRoot :: Assertion
 testFindProjectRoot = do
     Left (BadProjectRootExplicitFile file) <- findProjectRoot (Just testdir)
@@ -1927,3 +1935,25 @@ tryFewTimes action = go (3 :: Int) where
         hPutStrLn stderr $ "Trying " ++ show n ++ " after " ++ show e
         threadDelay 10000
         go (n - 1)
+
+testNixFlags :: Assertion
+testNixFlags = do
+  let gc = globalCommand []
+  -- changing from the v1 to v2 build command does not change whether the "--enable-nix" flag
+  -- sets the globalNix param of the GlobalFlags type to True even though the v2 command doesn't use it
+  let nixEnabledFlags = getFlags gc . commandParseArgs gc True $ ["--enable-nix", "build"]
+  let nixDisabledFlags = getFlags gc . commandParseArgs gc True $ ["--disable-nix", "build"]
+  let nixDefaultFlags = getFlags gc . commandParseArgs gc True $ ["build"]
+  True @=? isJust nixDefaultFlags
+  True @=? isJust nixEnabledFlags
+  True @=? isJust nixDisabledFlags
+  Just True @=? (fromFlag . globalNix . fromJust $ nixEnabledFlags)
+  Just False @=? (fromFlag . globalNix . fromJust $ nixDisabledFlags)
+  Nothing @=? (fromFlag . globalNix . fromJust $ nixDefaultFlags)
+  where
+    fromFlag :: Flag Bool -> Maybe Bool
+    fromFlag (Flag x) = Just x
+    fromFlag NoFlag = Nothing
+    getFlags :: CommandUI GlobalFlags -> CommandParse (GlobalFlags -> GlobalFlags, [String]) -> Maybe GlobalFlags
+    getFlags cui (CommandReadyToGo (mkflags, _)) = Just . mkflags . commandDefaultFlags $ cui
+    getFlags _ _ = Nothing

changelog.d/issue-8036

@@ -0,0 +1,4 @@
+synopsis: Make enable/disable nix flags easier to read
+packages: cabal-install
+issues: #8036
+prs: #8054