Add 'isolateTests' cabal flag to make plugin install cabal-fmt

For CI, we want to run the tests with a specific cabal-fmt version,
installed automatically by cabal.
However, locally, we might want to test with a locally installed
cabal-fmt version.
This flag allows developers to either let cabal install the
build-tool-depends or install a fitting version locally.

Author: fendor

Diff for: .github/workflows/test.yml

@@ -253,7 +253,7 @@ jobs:
       ## version needs to be limited since the tests depend on cabal-fmt which only builds using specific ghc versions
       - if: matrix.test && matrix.ghc == '8.10.7'
         name: Test hls-cabal-fmt-plugin test suite
-        run: cabal test hls-cabal-fmt-plugin --test-options="$TEST_OPTS" || cabal test hls-cabal-fmt-plugin --test-options="$TEST_OPTS" || LSP_TEST_LOG_COLOR=0 LSP_TEST_LOG_MESSAGES=true LSP_TEST_LOG_STDERR=true cabal test hls-cabal-fmt-plugin --test-options="$TEST_OPTS"
+        run: cabal test hls-cabal-fmt-plugin --flag=isolateTests --test-options="$TEST_OPTS" || cabal test hls-cabal-fmt-plugin --flag=isolateTests --test-options="$TEST_OPTS" || LSP_TEST_LOG_COLOR=0 LSP_TEST_LOG_MESSAGES=true LSP_TEST_LOG_STDERR=true cabal test hls-cabal-fmt-plugin --flag=isolateTests --test-options="$TEST_OPTS"
 
   test_post_job:
     if: always()

Diff for: plugins/hls-cabal-fmt-plugin/hls-cabal-fmt-plugin.cabal

@@ -14,6 +14,12 @@ category:           Development
 build-type:         Simple
 extra-source-files: LICENSE
 
+flag isolateTests
+  description: Should tests search for 'cabal-fmt' on the $PATH or shall we install it via build-tool-depends?
+  -- By default, search on the PATH
+  default:     False
+  manual:      True
+
 common warnings
   ghc-options: -Wall
 
@@ -44,8 +50,10 @@ test-suite tests
   ghc-options:        -threaded -rtsopts -with-rtsopts=-N
   build-depends:
     , base
+    , directory
     , filepath
     , hls-cabal-fmt-plugin
     , hls-test-utils        ^>=1.4
 
-  build-tool-depends: cabal-fmt:cabal-fmt ^>=0.1.6
+  if flag(isolateTests)
+    build-tool-depends: cabal-fmt:cabal-fmt ^>=0.1.6

Diff for: plugins/hls-cabal-fmt-plugin/test/Main.hs

@@ -1,33 +1,57 @@
 {-# LANGUAGE CPP               #-}
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE CPP #-}
 module Main
   ( main
   ) where
 
 import qualified Ide.Plugin.CabalFmt as CabalFmt
+import           System.Directory (findExecutable)
 import           System.FilePath
 import           Test.Hls
 
+data CabalFmtFound = Found | NotFound
+
+isTestIsolated :: Bool
+#if isolateTests
+isTestIsolated = True
+#else
+isTestIsolated = False
+#endif
+
+isCabalFmtFound :: IO CabalFmtFound
+isCabalFmtFound = case isTestIsolated of
+  True -> pure Found
+  False-> do
+    cabalFmt <- findExecutable "cabal-fmt"
+    pure $ maybe NotFound (const Found) cabalFmt
+
 main :: IO ()
-main = defaultTestRunner tests
+main = do
+  foundCabalFmt <- isCabalFmtFound
+  defaultTestRunner (tests foundCabalFmt)
 
 cabalFmtPlugin :: PluginDescriptor IdeState
 cabalFmtPlugin = CabalFmt.descriptor mempty "cabal-fmt"
 
-tests :: TestTree
-tests = testGroup "cabal-fmt"
-  [ cabalFmtGolden "formats a simple document" "simple_testdata" "formatted_document" $ \doc -> do
+tests :: CabalFmtFound -> TestTree
+tests found = testGroup "cabal-fmt"
+  [ cabalFmtGolden found "formats a simple document" "simple_testdata" "formatted_document" $ \doc -> do
       formatDoc doc (FormattingOptions 2 True Nothing Nothing Nothing)
 
-  , cabalFmtGolden "formats a document with expand:src comment" "commented_testdata" "formatted_document" $ \doc -> do
+  , cabalFmtGolden found "formats a document with expand:src comment" "commented_testdata" "formatted_document" $ \doc -> do
       formatDoc doc (FormattingOptions 2 True Nothing Nothing Nothing)
 
-  , cabalFmtGolden "formats a document with lib information" "lib_testdata" "formatted_document" $ \doc -> do
+  , cabalFmtGolden found "formats a document with lib information" "lib_testdata" "formatted_document" $ \doc -> do
       formatDoc doc (FormattingOptions 10 True Nothing Nothing Nothing)
   ]
 
-cabalFmtGolden :: TestName -> FilePath -> FilePath -> (TextDocumentIdentifier -> Session ()) -> TestTree
-cabalFmtGolden title path desc = goldenWithCabalDocFormatter cabalFmtPlugin "cabal-fmt" conf title testDataDir path desc "cabal"
+cabalFmtGolden :: CabalFmtFound -> TestName -> FilePath -> FilePath -> (TextDocumentIdentifier -> Session ()) -> TestTree
+cabalFmtGolden NotFound title _ _ _ =
+  testCase title $
+    assertFailure $  "Couldn't find cabal-fmt on PATH or this is not an isolated run. "
+                  <> "Use cabal flag 'isolateTests' to make it isolated or install cabal-fmt locally."
+cabalFmtGolden Found title path desc act = goldenWithCabalDocFormatter cabalFmtPlugin "cabal-fmt" conf title testDataDir path desc "cabal" act
   where
     conf = def