avoid double rebuilds for FOIs (haskell#2266)

With VSCode I get a WatchedFile notification immediately after of the TextDocumentDidChange notification, triggering a second rebuild unnecessarily

Author: pepeiborra

ghcide/src/Development/IDE/Core/OfInterest.hs

@@ -8,6 +8,7 @@
 --   open in the editor. The rule is 'IsFileOfInterest'
 module Development.IDE.Core.OfInterest(
     ofInterestRules,
+    getFilesOfInterest,
     getFilesOfInterestUntracked,
     addFileOfInterest,
     deleteFileOfInterest,
@@ -57,6 +58,11 @@ ofInterestRules = do
 ------------------------------------------------------------
 -- Exposed API
 
+getFilesOfInterest :: IdeState -> IO( HashMap NormalizedFilePath FileOfInterestStatus)
+getFilesOfInterest state = do
+    OfInterestVar var <- getIdeGlobalState state
+    readVar var
+
 -- | Set the files-of-interest - not usually necessary or advisable.
 --   The LSP client will keep this information up to date.
 setFilesOfInterest :: IdeState -> HashMap NormalizedFilePath FileOfInterestStatus -> IO ()

ghcide/src/Development/IDE/LSP/Notifications.hs

@@ -26,6 +26,7 @@ import qualified Data.HashSet                          as S
 import qualified Data.Text                             as Text
 
 import           Control.Monad.IO.Class
+import qualified Data.HashMap.Strict                   as HM
 import           Development.IDE.Core.FileExists       (modifyFileExists,
                                                         watchedGlobs)
 import           Development.IDE.Core.FileStore        (registerFileWatches,
@@ -84,8 +85,15 @@ descriptor plId = (defaultPluginDescriptor plId) { pluginNotificationHandlers =
         -- what we do with them
         let msg = show fileEvents
         logDebug (ideLogger ide) $ "Watched file events: " <> Text.pack msg
-        modifyFileExists ide fileEvents
-        resetFileStore ide fileEvents
+        -- filter out files of interest, since we already know all about those
+        filesOfInterest <- getFilesOfInterest ide
+        let fileEvents' =
+                [ f | f@(FileEvent uri _) <- fileEvents
+                , Just fp <- [uriToFilePath uri]
+                , not $ HM.member (toNormalizedFilePath fp) filesOfInterest
+                ]
+        modifyFileExists ide fileEvents'
+        resetFileStore ide fileEvents'
         setSomethingModified ide [] msg
 
   , mkPluginNotificationHandler LSP.SWorkspaceDidChangeWorkspaceFolders $