Only bring units actually depended on into scope on 9.4+

Author: wz1000

ghcide/src/Development/IDE/Import/FindImports.hs

@@ -29,6 +29,7 @@ import           Data.Maybe
 import           System.FilePath
 #if MIN_VERSION_ghc(9,3,0)
 import           GHC.Types.PkgQual
+import           GHC.Unit.State
 #endif
 
 data Import
@@ -147,13 +148,32 @@ locateModule env comp_info exts targetFor modName mbPkgName isSource = do
             map snd import_paths
 #endif
 
-      mbFile <- locateModuleFile ((homeUnitId_ dflags, importPaths dflags) : import_paths') exts targetFor isSource $ unLoc modName
+      mbFile <- locateModuleFile ((homeUnitId_ dflags, importPaths dflags) : other_imports) exts targetFor isSource $ unLoc modName
       case mbFile of
         Nothing          -> lookupInPackageDB env
         Just (uid, file) -> toModLocation uid file
   where
     dflags = hsc_dflags env
     import_paths = mapMaybe (mkImportDirs env) comp_info
+    other_imports =
+#if MIN_VERSION_ghc(9,4,0)
+      -- On 9.4+ instead of bringing all the units into scope, only bring into scope the units
+      -- this one depends on
+      -- This way if you have multiple units with the same module names, we won't get confused
+      -- For example if unit a imports module M from unit B, when there is also a module M in unit C,
+      -- and unit a only depends on unit b, without this logic there is the potential to get confused
+      -- about which module unit a imports.
+      -- Without multi-component support it is hard to recontruct the dependency environment so
+      -- unit a will have both unit b and unit c in scope.
+      map (\uid -> (uid, importPaths (homeUnitEnv_dflags (ue_findHomeUnitEnv uid ue)))) hpt_deps
+    ue = hsc_unit_env env
+    units = homeUnitEnv_units $ ue_findHomeUnitEnv (homeUnitId_ dflags) ue
+    hpt_deps :: [UnitId]
+    hpt_deps = homeUnitDepends units
+#else
+      import_paths'
+#endif
+
     toModLocation uid file = liftIO $ do
         loc <- mkHomeModLocation dflags (unLoc modName) (fromNormalizedFilePath file)
 #if MIN_VERSION_ghc(9,0,0)