I've noticed some suspicious code in base:GHC.IO.Encoding. setLocaleEncoding, setFileSystemEncoding, setForeignEncoding :: TextEncoding -> IO () (getLocaleEncoding, setLocaleEncoding) = mkGlobal initLocaleEncoding (getFileSystemEncoding, setFileSystemEncoding) = mkGlobal initFileSystemEncoding (getForeignEncoding, setForeignEncoding) = mkGlobal initForeignEncoding mkGlobal :: a -> (IO a, a -> IO ()) mkGlobal x = unsafePerformIO $ do x_ref <- newIORef x return (readIORef x_ref, writeIORef x_ref) I've not elided any pragmas and there's no relevant LANGUAGE or OPTIONS pragmas. With HEAD from a couple weeks ago, mkGlobal is not getting inlined. But with some of my experimental alterations of sizeExpr, it is getting inlined. So this code should probably have some of the guards for faking global variables, right?