Remove extra call to newHscEnvEqWithImportPaths (#3676)

* Remove extra call to newHscEnvEqWithImportPaths

* Revert "Remove extra call to newHscEnvEqWithImportPaths"

This reverts commit 376ada0.

* Add updateHscEnvEq helper function

* Add comment to updateHscEnvEq call

* Update HscEnvEq equality comment

---------

Co-authored-by: Michael Peyton Jones <me@michaelpj.com>

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

@@ -799,7 +799,12 @@ ghcSessionDepsDefinition fullModSummary GhcSessionDepsConfig{..} env file = do
 #endif
             session' <- liftIO $ mergeEnvs hsc moduleNode inLoadOrder depSessions
 
-            Just <$> liftIO (newHscEnvEqWithImportPaths (envImportPaths env) session' [])
+            -- Here we avoid a call to to `newHscEnvEqWithImportPaths`, which creates a new
+            -- ExportsMap when it is called. We only need to create the ExportsMap once per
+            -- session, while `ghcSessionDepsDefinition` will be called for each file we need
+            -- to compile. `updateHscEnvEq` will refresh the HscEnv (session') and also
+            -- generate a new Unique.
+            Just <$> liftIO (updateHscEnvEq env session')
 
 -- | Load a iface from disk, or generate it if there isn't one or it is out of date
 -- This rule also ensures that the `.hie` and `.o` (if needed) files are written out.

ghcide/src/Development/IDE/Types/HscEnvEq.hs

@@ -4,6 +4,7 @@ module Development.IDE.Types.HscEnvEq
     hscEnvWithImportPaths,
     newHscEnvEqPreserveImportPaths,
     newHscEnvEqWithImportPaths,
+    updateHscEnvEq,
     envImportPaths,
     envPackageExports,
     envVisibleModuleNames,
@@ -32,7 +33,8 @@ import           System.Directory                (makeAbsolute)
 import           System.FilePath
 
 -- | An 'HscEnv' with equality. Two values are considered equal
---   if they are created with the same call to 'newHscEnvEq'.
+--   if they are created with the same call to 'newHscEnvEq' or
+--   'updateHscEnvEq'.
 data HscEnvEq = HscEnvEq
     { envUnique             :: !Unique
     , hscEnv                :: !HscEnv
@@ -51,6 +53,11 @@ data HscEnvEq = HscEnvEq
         -- If Nothing, 'listVisibleModuleNames' panic
     }
 
+updateHscEnvEq :: HscEnvEq -> HscEnv -> IO HscEnvEq
+updateHscEnvEq oldHscEnvEq newHscEnv = do
+  let update newUnique = oldHscEnvEq { envUnique = newUnique, hscEnv = newHscEnv }
+  update <$> Unique.newUnique
+
 -- | Wrap an 'HscEnv' into an 'HscEnvEq'.
 newHscEnvEq :: FilePath -> HscEnv -> [(UnitId, DynFlags)] -> IO HscEnvEq
 newHscEnvEq cradlePath hscEnv0 deps = do