Make sanity checking optional

src/Ormolu.hs

@@ -49,10 +49,11 @@ ormolu cfg path str = do
   let txt = printModule anns0 parsedSrc0
   -- Parse the result of pretty-printing again and make sure that AST is the
   -- same as AST of original snippet module span positions.
-  (anns1, parsedSrc1) <-
-    parseModule' cfg OrmoluOutputParsingFailed "<rendered>" (T.unpack txt)
-  when (diff (anns0, parsedSrc0) (anns1, parsedSrc1)) $
-    liftIO $ throwIO (OrmoluASTDiffers str txt)
+  when (cfgSanityCheck cfg) $ do
+    (anns1, parsedSrc1) <-
+      parseModule' cfg OrmoluOutputParsingFailed "<rendered>" (T.unpack txt)
+    when (diff (anns0, parsedSrc0) (anns1, parsedSrc1)) $
+      liftIO $ throwIO (OrmoluASTDiffers str txt)
   return txt
 
 -- | Load a file and format it. The file stays intact and the rendered

src/Ormolu/Config.hs

@@ -12,9 +12,13 @@ import qualified SrcLoc as GHC
 
 -- | Ormolu configuration.
 
-newtype Config = Config
+data Config = Config
   { cfgDynOptions :: [DynOption]
     -- ^ Dynamic options to pass to GHC parser
+  , cfgSanityCheck :: Bool
+    -- ^ Whether to parse output of formatter and compare the obtained AST
+    -- with original AST. Doing this makes the program much slower, but
+    -- it'll catch and report all possible issues.
   } deriving (Eq, Show)
 
 -- | Default 'Config'.
@@ -22,6 +26,7 @@ newtype Config = Config
 defaultConfig :: Config
 defaultConfig = Config
   { cfgDynOptions = []
+  , cfgSanityCheck = True
   }
 
 -- | A wrapper for dynamic options.