Fix performance regression

A tiny fix, that tested on a closed-source codebase of over 16k
modules took formatting inplace from ~10.5 minutes to ~19seconds.

Author: telser

CHANGELOG.md

@@ -1,3 +1,8 @@
+## Unreleased
+
+* Fixed a performance regression introduced in 0.8.0.0. [Issue
+  1176](https://github.com/tweag/ormolu/issues/1176).
+
 ## Ormolu 0.8.0.1
 
 * Correctly format edge cases where fully collapsing string gaps changes the
@@ -9,7 +14,7 @@
   1166](https://github.com/tweag/ormolu/issues/1166).
 
 * Make multiline function signatures in RequiredTypeArguments consistent with
-  types [PR 1170](https://github.com/tweag/ormolu/pull/1170)
+  types [PR 1170](https://github.com/tweag/ormolu/pull/1170).
 
 * Correctly format single-line `MultiWayIf`s. [Issue
   1171](https://github.com/tweag/ormolu/issues/1171).

src/Ormolu/Utils/IO.hs

@@ -10,7 +10,6 @@ module Ormolu.Utils.IO
 where
 
 import Control.Exception (catch, throwIO)
-import Control.Monad (void)
 import Control.Monad.IO.Class
 import Data.IORef
 import Data.Map.Lazy (Map)
@@ -67,5 +66,5 @@ withCache (Cache cacheVar) k action = do
     Just v -> pure v
     Nothing -> do
       v <- action
-      void $ atomicModifyIORef cacheVar (pure . M.insert k v)
+      atomicModifyIORef cacheVar ((,()) . M.insert k v)
       pure v