More support for optional home-unit

This is a preliminary refactoring for #14335 (supporting plugins in
cross-compilers). In many places the home-unit must be optional because
there won't be one available in the plugin environment (we won't be
compiling anything in this environment). Hence we replace "HomeUnit"
with "Maybe HomeUnit" in a few places and we avoid the use of
"hsc_home_unit" (which is partial) in some few others.

2 files changed, 30 insertions, 13 deletions

diff --git a/compiler/GHC/Unit/Finder.hs b/compiler/GHC/Unit/Finder.hs
index cc16cd0dad..d4de80947b 100644
--- a/compiler/GHC/Unit/Finder.hs
+++ b/compiler/GHC/Unit/Finder.hs
@@ -134,18 +134,24 @@ findImportedModule
   :: FinderCache
   -> FinderOpts
   -> UnitState
-  -> HomeUnit
+  -> Maybe HomeUnit
   -> ModuleName
   -> PkgQual
   -> IO FindResult
-findImportedModule fc fopts units home_unit mod_name mb_pkg =
+findImportedModule fc fopts units mhome_unit mod_name mb_pkg =
   case mb_pkg of
     NoPkgQual  -> unqual_import
     ThisPkg _  -> home_import
     OtherPkg _ -> pkg_import
   where
-    home_import   = findHomeModule fc fopts home_unit mod_name
+    home_import
+      | Just home_unit <- mhome_unit
+      = findHomeModule fc fopts home_unit mod_name
+      | otherwise
+      = pure $ NoPackage (panic "findImportedModule: no home-unit")
+
     pkg_import    = findExposedPackageModule fc fopts units mod_name mb_pkg
+
     unqual_import = home_import
                     `orIfNotFound`
                     findExposedPackageModule fc fopts units mod_name NoPkgQual
@@ -154,11 +160,13 @@ findImportedModule fc fopts units home_unit mod_name mb_pkg =
 -- plugin.  This consults the same set of exposed packages as
 -- 'findImportedModule', unless @-hide-all-plugin-packages@ or
 -- @-plugin-package@ are specified.
-findPluginModule :: FinderCache -> FinderOpts -> UnitState -> HomeUnit -> ModuleName -> IO FindResult
-findPluginModule fc fopts units home_unit mod_name =
+findPluginModule :: FinderCache -> FinderOpts -> UnitState -> Maybe HomeUnit -> ModuleName -> IO FindResult
+findPluginModule fc fopts units (Just home_unit) mod_name =
   findHomeModule fc fopts home_unit mod_name
   `orIfNotFound`
   findExposedPluginPackageModule fc fopts units mod_name
+findPluginModule fc fopts units Nothing mod_name =
+  findExposedPluginPackageModule fc fopts units mod_name
 
 -- | Locate a specific 'Module'.  The purpose of this function is to
 -- create a 'ModLocation' for a given 'Module', that is to find out
@@ -166,11 +174,13 @@ findPluginModule fc fopts units home_unit mod_name =
 -- reading the interface for a module mentioned by another interface,
 -- for example (a "system import").
 
-findExactModule :: FinderCache -> FinderOpts -> UnitState -> HomeUnit -> InstalledModule -> IO InstalledFindResult
-findExactModule fc fopts unit_state home_unit mod = do
-  if isHomeInstalledModule home_unit mod
-    then findInstalledHomeModule fc fopts home_unit (moduleName mod)
-    else findPackageModule fc unit_state fopts mod
+findExactModule :: FinderCache -> FinderOpts -> UnitState -> Maybe HomeUnit -> InstalledModule -> IO InstalledFindResult
+findExactModule fc fopts unit_state mhome_unit mod = do
+  case mhome_unit of
+    Just home_unit
+      | isHomeInstalledModule home_unit mod
+      -> findInstalledHomeModule fc fopts home_unit (moduleName mod)
+    _ -> findPackageModule fc unit_state fopts mod
 
 -- -----------------------------------------------------------------------------
 -- Helpers
diff --git a/compiler/GHC/Unit/Home.hs b/compiler/GHC/Unit/Home.hs
index 02b60e64c9..c72d21e537 100644
--- a/compiler/GHC/Unit/Home.hs
+++ b/compiler/GHC/Unit/Home.hs
@@ -18,6 +18,7 @@ module GHC.Unit.Home
    , isHomeUnitInstanceOf
    , isHomeModule
    , isHomeInstalledModule
+   , notHomeUnitId
    , notHomeModule
    , notHomeModuleMaybe
    , notHomeInstalledModule
@@ -142,6 +143,11 @@ isHomeUnit hu u = u == homeUnitAsUnit hu
 isHomeUnitId :: GenHomeUnit u -> UnitId -> Bool
 isHomeUnitId hu uid = uid == homeUnitId hu
 
+-- | Test if the unit-id is not the home unit-id
+notHomeUnitId :: Maybe (GenHomeUnit u) -> UnitId -> Bool
+notHomeUnitId Nothing   _   = True
+notHomeUnitId (Just hu) uid = not (isHomeUnitId hu uid)
+
 -- | Test if the home unit is an instance of the given unit-id
 isHomeUnitInstanceOf :: HomeUnit -> UnitId -> Bool
 isHomeUnitInstanceOf hu u = homeUnitInstanceOf hu == u
@@ -204,8 +210,9 @@ homeModuleNameInstantiation hu mod_name =
 --       the instantiating module of @r:A@ in @p[A=q[]:B]@ is @r:A@.
 --       the instantiating module of @p:A@ in @p@ is @p:A@.
 --       the instantiating module of @r:A@ in @p@ is @r:A@.
-homeModuleInstantiation :: HomeUnit -> Module -> Module
-homeModuleInstantiation hu mod
-   | isHomeModule hu mod = homeModuleNameInstantiation hu (moduleName mod)
+homeModuleInstantiation :: Maybe HomeUnit -> Module -> Module
+homeModuleInstantiation mhu mod
+   | Just hu <- mhu
+   , isHomeModule hu mod = homeModuleNameInstantiation hu (moduleName mod)
    | otherwise           = mod