Add -plugin-package-db for cross-compilation plugin support

compiler/GHC/Driver/DynFlags.hs

@@ -363,6 +363,11 @@ data DynFlags = DynFlags {
   pluginPackageFlags    :: [PackageFlag],
         -- ^ The @-plugin-package-id@ flags from command line.
         -- In *reverse* order that they're specified on the command line.
+  pluginPackageDBFlags  :: [PackageDBFlag],
+        -- ^ The @-plugin-package-db@ flags from command line.
+        -- When non-empty, a separate 'UnitState' is built from these
+        -- databases for plugin loading (host packages for cross-compilation).
+        -- In *reverse* order that they're specified on the command line.
   trustFlags            :: [TrustFlag],
         -- ^ The @-trust@ and @-distrust@ flags.
         -- In *reverse* order that they're specified on the command line.
@@ -687,6 +692,7 @@ defaultDynFlags mySettings =
         packageDBFlags          = [],
         packageFlags            = [],
         pluginPackageFlags      = [],
+        pluginPackageDBFlags    = [],
         ignorePackageFlags      = [],
         trustFlags              = [],
         packageEnv              = Nothing,
@@ -941,6 +947,7 @@ packageFlagsChanged idflags1 idflags0 =
   packageFlags idflags1 /= packageFlags idflags0 ||
   ignorePackageFlags idflags1 /= ignorePackageFlags idflags0 ||
   pluginPackageFlags idflags1 /= pluginPackageFlags idflags0 ||
+  pluginPackageDBFlags idflags1 /= pluginPackageDBFlags idflags0 ||
   trustFlags idflags1 /= trustFlags idflags0 ||
   packageDBFlags idflags1 /= packageDBFlags idflags0 ||
   packageGFlags idflags1 /= packageGFlags idflags0

compiler/GHC/Driver/Env.hs

@@ -15,6 +15,7 @@ module GHC.Driver.Env
    , hsc_all_home_unit_ids
    , hscUpdateLoggerFlags
    , hscUpdateHUG
+   , hscSetCurrentUnitState
    , hscInsertHPT
    , hscSetActiveHomeUnit
    , hscSetActiveUnitId
@@ -144,6 +145,15 @@ hscInsertHPT hmi hsc_env = UnitEnv.insertHpt hmi (hsc_unit_env hsc_env)
 hscUpdateHUG :: (HomeUnitGraph -> HomeUnitGraph) -> HscEnv -> HscEnv
 hscUpdateHUG f hsc_env = hsc_env { hsc_unit_env = updateHug f (hsc_unit_env hsc_env) }
 
+-- | Replace the 'UnitState' in the current 'HomeUnitEnv'.
+-- Used by plugin loading to swap in a host-side 'UnitState' built from
+-- @-plugin-package-db@ databases during cross-compilation.
+hscSetCurrentUnitState :: UnitState -> HscEnv -> HscEnv
+hscSetCurrentUnitState us hsc_env =
+  let uid = ue_currentUnit (hsc_unit_env hsc_env)
+      upd hue = hue { HUG.homeUnitEnv_units = us }
+  in hscUpdateHUG (HUG.unitEnv_adjust upd uid) hsc_env
+
 setModuleGraph :: ModuleGraph -> HscEnv -> HscEnv
 setModuleGraph mod_graph hsc_env = hsc_env { hsc_unit_env = (hsc_unit_env hsc_env) { ue_module_graph = mod_graph } }
 

compiler/GHC/Driver/Session.hs

@@ -2154,6 +2154,8 @@ package_flags_deps = [
       (NoArg (setGeneralFlag Opt_HideAllPackages))
   , make_ord_flag defFlag "hide-all-plugin-packages"
       (NoArg (setGeneralFlag Opt_HideAllPluginPackages))
+  , make_ord_flag defFlag "plugin-package-db"     (HasArg addPluginPkgDb)
+  , make_ord_flag defFlag "clear-plugin-package-dbs" (NoArg clearPluginPkgDbs)
   , make_ord_flag defFlag "package-env"           (HasArg setPackageEnv)
   , make_ord_flag defFlag "ignore-package"        (HasArg ignorePackage)
   , make_dep_flag defFlag "syslib" (HasArg exposePackage) "Use -package instead"
@@ -3175,6 +3177,16 @@ exposePluginPackage p =
 exposePluginPackageId p =
   upd (\s -> s{ pluginPackageFlags =
     parsePackageFlag "-plugin-package-id" parseUnitArg p : pluginPackageFlags s })
+
+addPluginPkgDb :: String -> DynP ()
+addPluginPkgDb path =
+  upd (\s -> s{ pluginPackageDBFlags =
+    PackageDB (PkgDbPath path) : pluginPackageDBFlags s })
+
+clearPluginPkgDbs :: DynP ()
+clearPluginPkgDbs =
+  upd (\s -> s{ pluginPackageDBFlags = ClearPackageDBs : pluginPackageDBFlags s })
+
 hidePackage p =
   upd (\s -> s{ packageFlags = HidePackage p : packageFlags s })
 ignorePackage p =

compiler/GHC/Runtime/Loader.hs

@@ -1,3 +1,5 @@
+{-# LANGUAGE CPP #-}
+
 -- | Dynamically lookup up values from modules and loading them.
 --
 -- NOTE: This module is only compiled when flag(interpreter) is enabled
@@ -57,6 +59,7 @@ import GHC.Types.Name.Reader
 import GHC.Types.Unique.DFM
 
 import GHC.Unit.Finder         ( findPluginModule, FindResult(..) )
+import GHC.Unit.State          ( initPluginUnitConfig, mkUnitState )
 import GHC.Driver.Config.Diagnostic ( initIfaceMessageOpts )
 import GHC.Unit.Module   ( Module, ModuleName, thisGhcUnit, GenModule(moduleUnit), IsBootInterface(NotBoot) )
 import GHC.Unit.Module.ModIface
@@ -69,7 +72,6 @@ import GHC.Utils.Error
 import GHC.Utils.Outputable
 import GHC.Utils.Exception
 
-import Control.Monad     ( unless )
 import Data.Maybe        ( mapMaybe )
 import Unsafe.Coerce     ( unsafeCoerce )
 import GHC.Linker.Types
@@ -156,8 +158,12 @@ initializePlugins hsc_env
 
 loadPlugins :: HscEnv -> IO ([LoadedPlugin], [Linkable], PkgsLoaded)
 loadPlugins hsc_env
-  = do { unless (null to_load) $
-           checkExternalInterpreter hsc_env
+  = do { -- When -fexternal-interpreter is active, create a local Interp with
+         -- InternalInterp so plugins are loaded into GHC's own process.
+         -- TH/bytecode execution still goes through the external interpreter.
+       ; hsc_env' <- if null to_load then return hsc_env
+                     else withPluginInterp hsc_env
+       ; let loadPlugin = loadPlugin' (mkVarOccFS (fsLit "plugin")) pluginTyConName hsc_env'
        ; plugins_with_deps <- mapM loadPlugin to_load
        ; let (plugins, ifaces, links, pkgs) = unzip4 plugins_with_deps
        ; return (zipWith attachOptions to_load (zip plugins ifaces), concat links, foldl' plusUDFM emptyUDFM pkgs)
@@ -171,23 +177,72 @@ loadPlugins hsc_env
       where
         options = [ option | (opt_mod_nm, option) <- pluginModNameOpts dflags
                             , opt_mod_nm == mod_nm ]
-    loadPlugin = loadPlugin' (mkVarOccFS (fsLit "plugin")) pluginTyConName hsc_env
 
 
 loadFrontendPlugin :: HscEnv -> ModuleName -> IO (FrontendPlugin, [Linkable], PkgsLoaded)
 loadFrontendPlugin hsc_env mod_name = do
-    checkExternalInterpreter hsc_env
+    hsc_env' <- withPluginInterp hsc_env
     (plugin, _iface, links, pkgs)
       <- loadPlugin' (mkVarOccFS (fsLit "frontendPlugin")) frontendPluginTyConName
-           hsc_env mod_name
+           hsc_env' mod_name
     return (plugin, links, pkgs)
 
--- #14335
-checkExternalInterpreter :: HscEnv -> IO ()
-checkExternalInterpreter hsc_env = case interpInstance <$> hsc_interp hsc_env of
-  Just (ExternalInterp {})
-    -> throwIO (InstallationError "Plugins require -fno-external-interpreter")
-  _ -> pure ()
+-- | When the external interpreter is active, provide an 'HscEnv' with a
+-- local 'Interp' that uses 'InternalInterp' for loading plugins into GHC's
+-- own process. Plugins are compiler extensions that need to run in the
+-- compiler's address space — they cannot run in iserv because plugin values
+-- (records of Haskell functions) must be directly callable from GHC.
+--
+-- This decouples plugin loading from TH\/bytecode execution: plugins use the
+-- in-process RTS linker, while TH and GHCi continue to use the external
+-- interpreter (iserv) as configured.
+--
+-- For cross-compilers without an internal interpreter, this is not possible
+-- and we fall back to an error suggesting @-fplugin-library@ instead.
+-- See #14335.
+withPluginInterp :: HscEnv -> IO HscEnv
+withPluginInterp hsc_env = case interpInstance <$> hsc_interp hsc_env of
+  Just (ExternalInterp {}) ->
+#if defined(HAVE_INTERNAL_INTERPRETER)
+    do pluginInterp <- mkPluginInterp
+       let hsc_env1 = hsc_env { hsc_interp = Just pluginInterp }
+       -- When plugin-specific package DBs are configured (via
+       -- -plugin-package-db), build a separate UnitState from host
+       -- package DBs so that plugins resolve host-side libraries.
+       -- This is essential for cross-compilation where plugin packages
+       -- are built for the host, not the target.
+       case pluginPackageDBFlags (hsc_dflags hsc_env) of
+         [] -> return hsc_env1  -- no plugin DBs: use existing UnitState
+         _  -> do
+           let logger = hsc_logger hsc_env
+               dflags = hsc_dflags hsc_env
+               cfg    = initPluginUnitConfig dflags
+           (pluginUS, _dbs) <- mkUnitState logger dflags cfg
+           return $ hscSetCurrentUnitState pluginUS hsc_env1
+#else
+    throwIO (InstallationError $ unlines
+      [ "Plugins require the internal interpreter, which is not available"
+      , "in this cross-compiler build."
+      , "Use -fplugin-library to load plugins from shared libraries instead."
+      ])
+#endif
+  _ -> return hsc_env
+
+#if defined(HAVE_INTERNAL_INTERPRETER)
+-- | Create a local 'Interp' with 'InternalInterp' for loading plugins
+-- into GHC's own process. The local interpreter has its own 'Loader'
+-- state, independent of the external interpreter's, so plugin packages
+-- are loaded into GHC's address space via the in-process RTS linker.
+mkPluginInterp :: IO Interp
+mkPluginInterp = do
+  loader <- uninitializedLoader
+  symbolCache <- mkInterpSymbolCache
+  return Interp
+    { interpInstance    = InternalInterp
+    , interpLoader     = loader
+    , interpSymbolCache = symbolCache
+    }
+#endif
 
 loadPlugin' :: OccName -> Name -> HscEnv -> ModuleName -> IO (a, ModIface, [Linkable], PkgsLoaded)
 loadPlugin' occ_name plugin_name hsc_env mod_name

compiler/GHC/Unit/State.hs

@@ -13,6 +13,8 @@ module GHC.Unit.State (
         UnitErr (..),
         emptyUnitState,
         initUnits,
+        initPluginUnitConfig,
+        mkUnitState,
         readUnitDatabases,
         readUnitDatabase,
         getUnitDbRefs,
@@ -79,6 +81,7 @@ import GHC.Prelude
 import GHC.Driver.DynFlags
 
 import GHC.Platform
+import GHC.Platform.Host (hostPlatformArchOS)
 import GHC.Platform.Ways
 
 import GHC.Unit.Database
@@ -415,6 +418,39 @@ initUnitConfig dflags cached_dbs home_units =
     offsetPackageDb (Just offset) (PackageDB (PkgDbPath p)) | OsPath.isRelative p = PackageDB (PkgDbPath (OsPath.unsafeEncodeUtf offset OsPath.</> p))
     offsetPackageDb _ p = p
 
+-- | Build a 'UnitConfig' for plugin packages using host-side package
+-- databases. Used in cross-compilation scenarios where plugin packages
+-- are built for the host platform, not the target.
+--
+-- When @-plugin-package-db@ flags are specified, this config is used to
+-- create a separate 'UnitState' so that plugins resolve from host package
+-- DBs while TH\/bytecode uses the target 'UnitState'.
+initPluginUnitConfig :: DynFlags -> UnitConfig
+initPluginUnitConfig dflags =
+   UnitConfig
+      { unitConfigPlatformArchOS = hostPlatformArchOS
+      , unitConfigProgramName    = programName dflags
+      , unitConfigWays           = ways dflags
+      , unitConfigAllowVirtual   = False
+
+      , unitConfigGlobalDB       = globalPackageDatabasePath dflags
+      , unitConfigGHCDir         = topDir dflags
+      , unitConfigDBName         = "package.conf.d"
+
+      , unitConfigAutoLink       = []
+      , unitConfigDistrustAll    = False
+      , unitConfigHideAll        = gopt Opt_HideAllPluginPackages dflags
+      , unitConfigHideAllPlugins = gopt Opt_HideAllPluginPackages dflags
+
+      , unitConfigDBCache        = Nothing  -- no cache, read from plugin DBs
+      , unitConfigFlagsDB        = pluginPackageDBFlags dflags
+      , unitConfigFlagsExposed   = pluginPackageFlags dflags
+      , unitConfigFlagsIgnored   = []
+      , unitConfigFlagsTrusted   = []
+      , unitConfigFlagsPlugins   = pluginPackageFlags dflags
+      , unitConfigHomeUnits      = Set.empty
+      }
+
 
 -- | Map from 'ModuleName' to a set of module providers (i.e. a 'Module' and
 -- its 'ModuleOrigin').

testsuite/tests/plugins/T14335.stderr

@@ -1 +1,3 @@
-ghc: Plugins require -fno-external-interpreter
+Simple Plugin Passes Queried
+Got options: 
+Simple Plugin Pass Run

testsuite/tests/plugins/all.T

@@ -150,7 +150,7 @@ test('T14335',
      [extra_files(['simple-plugin/', 'plugins01.hs']),
 
       pre_cmd('$MAKE -s --no-print-directory -C simple-plugin package.plugins01 TOP={top}')],
-     compile_fail,
+     compile,
      ['-package-db simple-plugin/pkg.plugins01/local.package.conf -fplugin Simple.Plugin \
        -fexternal-interpreter -package simple-plugin ' + config.plugin_way_flags])