Detect deprecation warnings during recompilation and warn the user

Deprecation warnings were suppressed during recompilation, which made it
difficult to improve things in the codebase and API as users did not
(know they) have a transition period to adapt their configuration.

See additional discussions about deprecations:

 - https://github.com/xmonad/xmonad-contrib/pull/404#issuecomment-731607447
 - https://github.com/xmonad/xmonad-contrib/pull/410#issuecomment-732841235

Fixes: https://github.com/xmonad/xmonad/issues/304
Related: https://github.com/xmonad/xmonad-contrib/pull/404
Related: https://github.com/xmonad/xmonad-contrib/pull/410

src/XMonad/Core.hs

@@ -45,6 +45,7 @@ import Data.Semigroup
 import Data.Traversable (for)
 import Data.Time.Clock (UTCTime)
 import Data.Default.Class
+import Data.List (isInfixOf)
 import System.FilePath
 import System.IO
 import System.Info
@@ -683,6 +684,22 @@ compile dirs method =
         ExitSuccess -> cmd2
         e -> pure e
 
+-- | Check GHC output for deprecation warnings and notify the user if there
+-- were any. Report success otherwise.
+checkCompileWarnings :: Directories -> IO ()
+checkCompileWarnings dirs = do
+    ghcErr <- readFile (errFileName dirs)
+    if "-Wdeprecations" `isInfixOf` ghcErr
+      then do
+        let msg = unlines $
+                ["Deprecations detected while compiling xmonad config: " <> srcFileName dirs]
+                ++ lines ghcErr
+                ++ ["","Please correct them or silence using {-# OPTIONS_GHC -Wno-deprecations #-}."]
+        trace msg
+        xmessage msg
+      else
+        trace "XMonad recompilation process exited with success!"
+
 -- | Notify the user that compilation failed and what was wrong.
 compileFailed :: Directories -> ExitCode -> IO ()
 compileFailed dirs status = do
@@ -726,7 +743,7 @@ recompile dirs force = io $ do
       then do
         status <- compile dirs method
         if status == ExitSuccess
-            then trace "XMonad recompilation process exited with success!"
+            then checkCompileWarnings dirs
             else compileFailed dirs status
         pure $ status == ExitSuccess
       else