driver: Fix recompilation for modules importing GHC.Prim

The GHC.Prim module is quite special as there is no interface file,
therefore it doesn't appear in ms_textual_imports, but the ghc-prim
package does appear in the direct package dependencies. This confused
the recompilation checking which couldn't find any modules from ghc-prim
and concluded that the package was no longer a dependency.

The fix is to keep track of whether GHC.Prim is imported separately in
the relevant places.

Fixes #20084

10 files changed, 45 insertions, 15 deletions

diff --git a/compiler/GHC/Driver/Backpack.hs b/compiler/GHC/Driver/Backpack.hs
index 2190bdd753..8581865dbf 100644
--- a/compiler/GHC/Driver/Backpack.hs
+++ b/compiler/GHC/Driver/Backpack.hs
@@ -769,6 +769,7 @@ summariseRequirement pn mod_name = do
         ms_hie_date = hie_timestamp,
         ms_srcimps = [],
         ms_textual_imps = extra_sig_imports,
+        ms_ghc_prim_import = False,
         ms_parsed_mod = Just (HsParsedModule {
                 hpm_module = L loc (HsModule {
                         hsmodAnn = noAnn,
@@ -854,8 +855,9 @@ hsModuleToModSummary pn hsc_src modname
     let (src_idecls, ord_idecls) = partition ((== IsBoot) . ideclSource . unLoc) imps
 
              -- GHC.Prim doesn't exist physically, so don't go looking for it.
-        ordinary_imps = filter ((/= moduleName gHC_PRIM) . unLoc . ideclName . unLoc)
-                               ord_idecls
+        (ordinary_imps, ghc_prim_import)
+          = partition ((/= moduleName gHC_PRIM) . unLoc . ideclName . unLoc)
+              ord_idecls
 
         implicit_prelude = xopt LangExt.ImplicitPrelude dflags
         implicit_imports = mkPrelImports modname loc
@@ -884,6 +886,7 @@ hsModuleToModSummary pn hsc_src modname
             ms_hspp_opts = dflags,
             ms_hspp_buf = Nothing,
             ms_srcimps = map convImport src_idecls,
+            ms_ghc_prim_import = not (null ghc_prim_import),
             ms_textual_imps = normal_imports
                            -- We have to do something special here:
                            -- due to merging, requirements may end up with
diff --git a/compiler/GHC/Driver/Make.hs b/compiler/GHC/Driver/Make.hs
index aef6953a30..51fb469828 100644
--- a/compiler/GHC/Driver/Make.hs
+++ b/compiler/GHC/Driver/Make.hs
@@ -2538,6 +2538,7 @@ makeNewModSummary hsc_env MakeNewModSummary{..} = do
         , ms_hspp_buf  = Just pi_hspp_buf
         , ms_parsed_mod = Nothing
         , ms_srcimps = pi_srcimps
+        , ms_ghc_prim_import = pi_ghc_prim_import
         , ms_textual_imps =
             pi_theimps ++
             extra_sig_imports ++
@@ -2556,6 +2557,7 @@ data PreprocessedImports
       { pi_local_dflags :: DynFlags
       , pi_srcimps  :: [(Maybe FastString, Located ModuleName)]
       , pi_theimps  :: [(Maybe FastString, Located ModuleName)]
+      , pi_ghc_prim_import :: Bool
       , pi_hspp_fn  :: FilePath
       , pi_hspp_buf :: StringBuffer
       , pi_mod_name_loc :: SrcSpan
@@ -2575,7 +2577,7 @@ getPreprocessedImports hsc_env src_fn mb_phase maybe_buf = do
   (pi_local_dflags, pi_hspp_fn)
       <- ExceptT $ preprocess hsc_env src_fn (fst <$> maybe_buf) mb_phase
   pi_hspp_buf <- liftIO $ hGetStringBuffer pi_hspp_fn
-  (pi_srcimps, pi_theimps, L pi_mod_name_loc pi_mod_name)
+  (pi_srcimps, pi_theimps, pi_ghc_prim_import, L pi_mod_name_loc pi_mod_name)
       <- ExceptT $ do
           let imp_prelude = xopt LangExt.ImplicitPrelude pi_local_dflags
               popts = initParserOpts pi_local_dflags
diff --git a/compiler/GHC/Driver/Pipeline/Execute.hs b/compiler/GHC/Driver/Pipeline/Execute.hs
index 57d491104e..022e8ce1a1 100644
--- a/compiler/GHC/Driver/Pipeline/Execute.hs
+++ b/compiler/GHC/Driver/Pipeline/Execute.hs
@@ -632,15 +632,15 @@ runHscPhase pipe_env hsc_env0 input_fn src_flavour = do
 
 
   -- gather the imports and module name
-  (hspp_buf,mod_name,imps,src_imps) <- do
+  (hspp_buf,mod_name,imps,src_imps, ghc_prim_imp) <- do
               buf <- hGetStringBuffer input_fn
               let imp_prelude = xopt LangExt.ImplicitPrelude dflags
                   popts = initParserOpts dflags
               eimps <- getImports popts imp_prelude buf input_fn (basename <.> suff)
               case eimps of
                   Left errs -> throwErrors (GhcPsMessage <$> errs)
-                  Right (src_imps,imps,L _ mod_name) -> return
-                        (Just buf, mod_name, imps, src_imps)
+                  Right (src_imps,imps, ghc_prim_imp, L _ mod_name) -> return
+                        (Just buf, mod_name, imps, src_imps, ghc_prim_imp)
 
   -- Take -o into account if present
   -- Very like -ohi, but we must *only* do this if we aren't linking
@@ -679,6 +679,7 @@ runHscPhase pipe_env hsc_env0 input_fn src_flavour = do
                                 ms_parsed_mod   = Nothing,
                                 ms_iface_date   = hi_date,
                                 ms_hie_date     = hie_date,
+                                ms_ghc_prim_import = ghc_prim_imp,
                                 ms_textual_imps = imps,
                                 ms_srcimps      = src_imps }
 
diff --git a/compiler/GHC/Iface/Recomp.hs b/compiler/GHC/Iface/Recomp.hs
index 59c31ad566..72341ba147 100644
--- a/compiler/GHC/Iface/Recomp.hs
+++ b/compiler/GHC/Iface/Recomp.hs
@@ -452,11 +452,11 @@ checkMergedSignatures hsc_env mod_summary iface = do
 checkDependencies :: HscEnv -> ModSummary -> ModIface -> IfG RecompileRequired
 checkDependencies hsc_env summary iface
  = do
-    res <- liftIO $ fmap sequence $ traverse (\(mb_pkg, L _ mod) ->
+    res <- liftIO $ traverse (\(mb_pkg, L _ mod) ->
               let reason = moduleNameString mod ++ " changed"
               in classify reason <$> findImportedModule fc units home_unit dflags mod (mb_pkg))
               (ms_imps summary ++ ms_srcimps summary)
-    case res of
+    case sequence (res ++ [Right (fake_ghc_prim_import)| ms_ghc_prim_import summary]) of
       Left recomp -> return recomp
       Right es -> do
         let (hs, ps) = partitionEithers es
@@ -475,9 +475,15 @@ checkDependencies hsc_env summary iface
    prev_dep_pkgs = sort (dep_direct_pkgs (mi_deps iface))
    bkpk_units    = map (("Signature",) . indefUnit . instUnitInstanceOf . moduleUnit) (requirementMerges units (moduleName (mi_module iface)))
 
-
    implicit_deps = map ("Implicit",) (implicitPackageDeps dflags)
 
+   -- GHC.Prim is very special and doesn't appear in ms_textual_imps but
+   -- ghc-prim will appear in the package dependencies still. In order to not confuse
+   -- the recompilation logic we need to not forget we imported GHC.Prim.
+   fake_ghc_prim_import = if homeUnitId home_unit == primUnitId
+                            then Left (mkModuleName "GHC.Prim")
+                            else Right ("GHC.Prim", primUnitId)
+
 
    classify _ (Found _ mod)
     | isHomeUnit home_unit (moduleUnit mod) = Right (Left (moduleName mod))
diff --git a/compiler/GHC/Parser/Header.hs b/compiler/GHC/Parser/Header.hs
index 65e09bfeff..68468d89eb 100644
--- a/compiler/GHC/Parser/Header.hs
+++ b/compiler/GHC/Parser/Header.hs
@@ -75,6 +75,7 @@ getImports :: ParserOpts   -- ^ Parser options
                (Messages PsMessage)
                ([(Maybe FastString, Located ModuleName)],
                 [(Maybe FastString, Located ModuleName)],
+                Bool, -- Is GHC.Prim imported or not
                 Located ModuleName))
               -- ^ The source imports and normal imports (with optional package
               -- names from -XPackageImports), and the module name.
@@ -100,17 +101,19 @@ getImports popts implicit_prelude buf filename source_filename = do
                 (src_idecls, ord_idecls) = partition ((== IsBoot) . ideclSource . unLoc) imps
 
                -- GHC.Prim doesn't exist physically, so don't go looking for it.
-                ordinary_imps = filter ((/= moduleName gHC_PRIM) . unLoc
-                                        . ideclName . unLoc)
-                                       ord_idecls
+                (ordinary_imps, ghc_prim_import)
+                  = partition ((/= moduleName gHC_PRIM) . unLoc
+                                  . ideclName . unLoc)
+                                 ord_idecls
 
                 implicit_imports = mkPrelImports (unLoc mod) main_loc
                                                  implicit_prelude imps
                 convImport (L _ i) = (fmap sl_fs (ideclPkgQual i), ideclName i)
               in
-              return (map convImport src_idecls,
-                      map convImport (implicit_imports ++ ordinary_imps),
-                      mod)
+              return (map convImport src_idecls
+                     , map convImport (implicit_imports ++ ordinary_imps)
+                     , not (null ghc_prim_import)
+                     , mod)
 
 mkPrelImports :: ModuleName
               -> SrcSpan    -- Attribute the "import Prelude" to this location
diff --git a/compiler/GHC/Unit/Module/ModSummary.hs b/compiler/GHC/Unit/Module/ModSummary.hs
index 339afd4564..d36636e340 100644
--- a/compiler/GHC/Unit/Module/ModSummary.hs
+++ b/compiler/GHC/Unit/Module/ModSummary.hs
@@ -90,6 +90,8 @@ data ModSummary
           -- ^ Source imports of the module
         ms_textual_imps :: [(Maybe FastString, Located ModuleName)],
           -- ^ Non-source imports of the module from the module *text*
+        ms_ghc_prim_import :: Bool,
+          -- ^ Whether the special module GHC.Prim was imported explicitliy
         ms_parsed_mod   :: Maybe HsParsedModule,
           -- ^ The parsed, nonrenamed source, if we have it.  This is also
           -- used to support "inline module syntax" in Backpack files.
@@ -111,6 +113,7 @@ ms_installed_mod = fst . getModuleInstantiation . ms_mod
 ms_mod_name :: ModSummary -> ModuleName
 ms_mod_name = moduleName . ms_mod
 
+-- | Textual imports, plus plugin imports but not SOURCE imports.
 ms_imps :: ModSummary -> [(Maybe FastString, Located ModuleName)]
 ms_imps ms =
   ms_textual_imps ms ++
diff --git a/testsuite/tests/driver/Makefile b/testsuite/tests/driver/Makefile
index 9f7716a4fe..f58fdbee4f 100644
--- a/testsuite/tests/driver/Makefile
+++ b/testsuite/tests/driver/Makefile
@@ -726,3 +726,10 @@ T14923:
 	"$(TEST_HC)" $(TEST_HC_OPTS) -c Foo.hs;
 	# And this should not
 	"$(TEST_HC)" $(TEST_HC_OPTS) -c Foo.hs;
+
+T20084:
+	# Compile
+	"$(TEST_HC)" $(TEST_HC_OPTS) T20084.hs
+	# Don't compile
+	"$(TEST_HC)" $(TEST_HC_OPTS) T20084.hs
+
diff --git a/testsuite/tests/driver/T20084.hs b/testsuite/tests/driver/T20084.hs
new file mode 100644
index 0000000000..7025a11064
--- /dev/null
+++ b/testsuite/tests/driver/T20084.hs
@@ -0,0 +1,3 @@
+module T20084 where
+
+import GHC.Prim
diff --git a/testsuite/tests/driver/T20084.stdout b/testsuite/tests/driver/T20084.stdout
new file mode 100644
index 0000000000..04cef7e49d
--- /dev/null
+++ b/testsuite/tests/driver/T20084.stdout
@@ -0,0 +1 @@
+[1 of 1] Compiling T20084           ( T20084.hs, T20084.o )
diff --git a/testsuite/tests/driver/all.T b/testsuite/tests/driver/all.T
index 2afd735fe0..6f45e1ec21 100644
--- a/testsuite/tests/driver/all.T
+++ b/testsuite/tests/driver/all.T
@@ -292,3 +292,4 @@ test('T18369', normal, compile, ['-O'])
 test('FullGHCVersion', normal, compile_and_run, ['-package ghc-boot'])
 test('OneShotTH', normal, makefile_test, [])
 test('T17481', normal, makefile_test, [])
+test('T20084', normal, makefile_test, [])