Add tests to ensure the Stan plugin detects a module's language extensions

Includes test cases for both `LANGUAGE` pragmas and extensions enabled
in a project's `.cabal` file.

Author: keithfancher

plugins/hls-stan-plugin/test/Main.hs

@@ -40,6 +40,18 @@ tests =
           diags <- waitForDiagnosticsFromSource doc "stan"
           liftIO $ length diags @?= 0
           return ()
+    , testCase "respects LANGUAGE pragmas in the source file" $
+        runStanSession "" $ do
+          doc <- openDoc "extension-tests/language-pragma/LanguagePragmaTest.hs" "haskell"
+          diags <- waitForDiagnosticsFromSource doc "stan"
+          liftIO $ length diags @?= 0
+          return ()
+    , testCase "respects language extensions defined in the .cabal file" $
+        runStanSession "" $ do
+          doc <- openDoc "extension-tests/cabal-file/CabalFileTest.hs" "haskell"
+          diags <- waitForDiagnosticsFromSource doc "stan"
+          liftIO $ length diags @?= 0
+          return ()
     ]
 
 testDir :: FilePath

plugins/hls-stan-plugin/test/testdata/extension-tests/cabal-file/CabalFileTest.hs

@@ -0,0 +1,4 @@
+module CabalFileTest () where
+
+-- With `StrictData` enabled in the `.cabal` file, Stan shouldn't complain here:
+data A = A Int Int

plugins/hls-stan-plugin/test/testdata/extension-tests/cabal-file/cabal-file-test.cabal

@@ -0,0 +1,9 @@
+cabal-version: 3.0
+name: cabal-file-test
+version: 0.0.0.0
+
+library
+  exposed-modules: CabalFileTest
+  hs-source-dirs: extension-tests/cabal-file
+  -- Specifically, we're testing that Stan respects the following extension definition:
+  default-extensions: StrictData

plugins/hls-stan-plugin/test/testdata/extension-tests/language-pragma/LanguagePragmaTest.hs

@@ -0,0 +1,6 @@
+{-# LANGUAGE StrictData #-}
+
+module LanguagePragmaTest () where
+
+-- With the above `StrictData` language pragma, Stan shouldn't complain here:
+data A = A Int Int

plugins/hls-stan-plugin/test/testdata/extension-tests/language-pragma/language-pragma-test.cabal

@@ -0,0 +1,11 @@
+cabal-version: 3.0
+name: language-pragma-test
+version: 0.0.0.0
+
+-- Without at least a minimal valid `.cabal` file, Stan won't bother building its
+-- map of language extensions. This means it also won't detect LANGUAGE pragmas
+-- without this file.
+
+library
+  exposed-modules: LanguagePragmaTest
+  hs-source-dirs: extension-tests/language-pragma