Cabal uses main as the unit id of all executable packages. This confused multi component sessions.

Solution: include the hash of the options in the unit id when the unit id is called "main".

Fixes #3513

Author: wz1000

ghcide/session-loader/Development/IDE/Session.hs

@@ -484,7 +484,24 @@ loadSessionWithOptions recorder SessionLoadingOptions{..} dir = do
         packageSetup (hieYaml, cfp, opts, libDir) = do
           -- Parse DynFlags for the newly discovered component
           hscEnv <- emptyHscEnv ideNc libDir
-          (df, targets) <- evalGhcEnv hscEnv $ setOptions opts (hsc_dflags hscEnv)
+          (df', targets) <- evalGhcEnv hscEnv $ setOptions opts (hsc_dflags hscEnv)
+          let df =
+#if MIN_VERSION_ghc(9,3,0)
+                case unitIdString (homeUnitId_ df') of
+                     -- cabal uses main for the unit id of all executable packages
+                     -- This makes multi-component sessions confused about what
+                     -- options to use for that component.
+                     -- Solution: hash the options and use that as part of the unit id
+                     -- This works because there won't be any dependencies on the
+                     -- executable unit.
+                     "main" ->
+                       let hash = B.unpack $ B16.encode $ H.finalize $ H.updates H.init (map B.pack $ componentOptions opts)
+                           hashed_uid = Compat.toUnitId (Compat.stringToUnit ("main-"++hash))
+                       in setHomeUnitId_ hashed_uid df'
+#else
+                df'
+#endif
+
           let deps = componentDependencies opts ++ maybeToList hieYaml
           dep_info <- getDependencyInfo deps
           -- Now lookup to see whether we are combining with an existing HscEnv
@@ -499,6 +516,7 @@ loadSessionWithOptions recorder SessionLoadingOptions{..} dir = do
                   -- We will modify the unitId and DynFlags used for
                   -- compilation but these are the true source of
                   -- information.
+                  
                   new_deps = RawComponentInfo (homeUnitId_ df) df targets cfp opts dep_info
                                 : maybe [] snd oldDeps
                   -- Get all the unit-ids for things in this component

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

@@ -136,17 +136,6 @@ locateModule env comp_info exts targetFor modName mbPkgName isSource = do
 #else
     Nothing -> do
 #endif
-      -- first try to find the module as a file. If we can't find it try to find it in the package
-      -- database.
-      -- Here the importPaths for the current modules are added to the front of the import paths from the other components.
-      -- This is particularly important for Paths_* modules which get generated for every component but unless you use it in
-      -- each component will end up being found in the wrong place and cause a multi-cradle match failure.
-      let import_paths' =
-#if MIN_VERSION_ghc(9,3,0)
-            import_paths
-#else
-            map snd import_paths
-#endif
 
       mbFile <- locateModuleFile ((homeUnitId_ dflags, importPaths dflags) : other_imports) exts targetFor isSource $ unLoc modName
       case mbFile of
@@ -174,6 +163,18 @@ locateModule env comp_info exts targetFor modName mbPkgName isSource = do
       import_paths'
 #endif
 
+      -- first try to find the module as a file. If we can't find it try to find it in the package
+      -- database.
+      -- Here the importPaths for the current modules are added to the front of the import paths from the other components.
+      -- This is particularly important for Paths_* modules which get generated for every component but unless you use it in
+      -- each component will end up being found in the wrong place and cause a multi-cradle match failure.
+    import_paths' =
+#if MIN_VERSION_ghc(9,3,0)
+            import_paths
+#else
+            map snd import_paths
+#endif
+
     toModLocation uid file = liftIO $ do
         loc <- mkHomeModLocation dflags (unLoc modName) (fromNormalizedFilePath file)
 #if MIN_VERSION_ghc(9,0,0)