What happens when you use NOINLINE on a function defined in a where clause? Hint: optimization levels matter!

NoInlineWhere.hs

import System.IO.Unsafe

main :: IO ()
main = do
  thing 1
  thing 2
  thing 3

  thing' 1
  thing' 2
  thing' 3

thing :: Int -> IO ()
thing n = print $ n + cachedExpr
  where
  {-# NOINLINE cachedExpr #-}
  cachedExpr :: Int
  cachedExpr = unsafePerformIO $ do
    putStrLn "computing cachedExpr..."
    pure 5


thing' :: Int -> IO ()
thing' n = print $ n + cachedExpr'

{-# NOINLINE cachedExpr' #-}
cachedExpr' :: Int
cachedExpr' = unsafePerformIO $ do
  putStrLn "computing cachedExpr'..."
  pure 5

{-

% ghc -O0 NoInlineWhere.hs -o noinlinewhere && ./noinlinewhere
[1 of 1] Compiling Main             ( NoInlineWhere.hs, NoInlineWhere.o )
Linking noinlinewhere ...
computing cachedExpr...
6
computing cachedExpr...
7
computing cachedExpr...
8
computing cachedExpr'...
6
7
8

% rm noinlinewhere NoInlineWhere.{hi,o}

% ghc -O2 NoInlineWhere.hs -o noinlinewhere && ./noinlinewhere
[1 of 1] Compiling Main             ( NoInlineWhere.hs, NoInlineWhere.o )
Linking noinlinewhere ...
computing cachedExpr...
6
7
8
computing cachedExpr'...
6
7
8

-}