driver: Only check for unused package warning in after succesful downsweep

Before we would check for the unused package warning even if the module
graph was compromised due to an error in downsweep. This is easily
fixed by pushing warmUnusedPackages into depanalE, and then returning
the errors like the other downsweep errors.

Fixes #20242

compiler/GHC/Driver/Make.hs

@@ -187,9 +187,11 @@ depanalE excluded_mods allow_dup_roots = do
     (errs, mod_graph) <- depanalPartial excluded_mods allow_dup_roots
     if isEmptyMessages errs
       then do
-        warnMissingHomeModules hsc_env mod_graph
+        let unused_home_mod_err = warnMissingHomeModules hsc_env mod_graph
+            unused_pkg_err = warnUnusedPackages hsc_env mod_graph
+        logDiagnostics (GhcDriverMessage <$> (unused_home_mod_err `unionMessages` unused_pkg_err))
         setSession hsc_env { hsc_mod_graph = mod_graph }
-        pure (errs, mod_graph)
+        pure (emptyMessages, mod_graph)
       else do
         -- We don't have a complete module dependency graph,
         -- The graph may be disconnected and is unusable.
@@ -276,10 +278,11 @@ instantiationNodes unit_state = InstantiationNode <$> iuids_to_check
 -- about module "C" not being listed in a command line.
 --
 -- The warning in enabled by `-Wmissing-home-modules`. See #13129
-warnMissingHomeModules :: GhcMonad m => HscEnv -> ModuleGraph -> m ()
+warnMissingHomeModules :: HscEnv -> ModuleGraph -> DriverMessages
 warnMissingHomeModules hsc_env mod_graph =
-    when (not (null missing)) $
-        logDiagnostics (GhcDriverMessage <$> warn)
+  if null missing
+    then emptyMessages
+    else warn
   where
     dflags = hsc_dflags hsc_env
     targets = map targetId (hsc_targets hsc_env)
@@ -347,7 +350,6 @@ load :: GhcMonad m => LoadHowMuch -> m SuccessFlag
 load how_much = do
     (errs, mod_graph) <- depanalE [] False                        -- #17459
     success <- load' how_much (Just batchMsg) mod_graph
-    warnUnusedPackages mod_graph
     if isEmptyMessages errs
       then pure success
       else throwErrors (fmap GhcDriverMessage errs)
@@ -359,30 +361,29 @@ load how_much = do
 -- actually loaded packages. All the packages, specified on command line,
 -- but never loaded, are probably unused dependencies.
 
-warnUnusedPackages :: GhcMonad m => ModuleGraph -> m ()
-warnUnusedPackages mod_graph = do
-    hsc_env <- getSession
-
+warnUnusedPackages :: HscEnv -> ModuleGraph -> DriverMessages
+warnUnusedPackages hsc_env mod_graph =
     let dflags = hsc_dflags hsc_env
         state  = hsc_units  hsc_env
         diag_opts = initDiagOpts dflags
         us = hsc_units hsc_env
 
     -- Only need non-source imports here because SOURCE imports are always HPT
-    let loadedPackages = concat $
+        loadedPackages = concat $
           mapMaybe (\(fs, mn) -> lookupModulePackage us (unLoc mn) fs)
             $ concatMap ms_imps (mgModSummaries mod_graph)
 
-    let requestedArgs = mapMaybe packageArg (packageFlags dflags)
+        requestedArgs = mapMaybe packageArg (packageFlags dflags)
 
         unusedArgs
           = filter (\arg -> not $ any (matching state arg) loadedPackages)
                    requestedArgs
 
-    let warn = singleMessage $ mkPlainMsgEnvelope diag_opts noSrcSpan (DriverUnusedPackages unusedArgs)
+        warn = singleMessage $ mkPlainMsgEnvelope diag_opts noSrcSpan (DriverUnusedPackages unusedArgs)
 
-    when (not (null unusedArgs)) $
-      logDiagnostics (GhcDriverMessage <$> warn)
+    in if null unusedArgs
+        then emptyMessages
+        else warn
 
     where
         packageArg (ExposePackage _ arg _) = Just arg

testsuite/tests/cabal/t20242/BootNoHeader.cabal

+cabal-version:      2.4
+name:               BootNoHeader
+version:            0.1.0.0
+
+executable BootNoHeader
+    main-is:          Main.hs
+    other-modules:    Foo
+    build-depends:    base
+    hs-source-dirs:   .
+    default-language: Haskell2010
+
+    ghc-options:      -Werror=unused-packages

testsuite/tests/cabal/t20242/Foo.hs

+module Foo where
+
+a :: Int
+a = 5

testsuite/tests/cabal/t20242/Foo.hs-boot

+a :: Int

testsuite/tests/cabal/t20242/Main.hs

+module Main where
+
+import {-# SOURCE #-} Foo
+
+main = putStrLn "Hello, world!"

testsuite/tests/cabal/t20242/Makefile

+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk
+
+SETUP = ./Setup -v0
+
+T20242: clean
+	$(MAKE) clean
+	'$(TEST_HC)' $(TEST_HC_OPTS) -v0 --make Setup
+	$(SETUP) clean
+	$(SETUP) configure $(CABAL_MINIMAL_BUILD) --with-ghc='$(TEST_HC)' --ghc-options='$(TEST_HC_OPTS)'
+	$(SETUP) build || true
+ifneq "$(CLEANUP)" ""
+	$(MAKE) clean
+endif
+
+clean :
+	$(RM) -r */dist Setup$(exeext) *.o *.hi

testsuite/tests/cabal/t20242/Setup.hs

+import Distribution.Simple
+main = defaultMain

testsuite/tests/cabal/t20242/T20242.stderr

+
+Foo.hs-boot:1:1: error:
+    File name does not match module name:
+    Saw     : ‘Main’
+    Expected: ‘Foo’

testsuite/tests/cabal/t20242/all.T

+if config.cleanup:
+   cleanup = 'CLEANUP=1'
+else:
+   cleanup = 'CLEANUP=0'
+
+test('T20242',
+     [extra_files(['Setup.hs', 'BootNoHeader.cabal','Foo.hs', 'Foo.hs-boot', 'Main.hs'])],
+     run_command,
+     ['$MAKE -s --no-print-directory T20242 ' + cleanup])

testsuite/tests/warnings/should_compile/UnusedPackages.stderr

-[1 of 1] Compiling Main             ( UnusedPackages.hs, UnusedPackages.o )
-Linking UnusedPackages ...
 
 <no location info>: warning: [-Wunused-packages]
-     The following packages were specified via -package or -package-id flags,
-     but were not needed for compilation:
-       - ghc
-       - process
-       - bytestring
+    The following packages were specified via -package or -package-id flags,
+    but were not needed for compilation:
+      - ghc
+      - process
+      - bytestring
+[1 of 1] Compiling Main             ( UnusedPackages.hs, UnusedPackages.o )
+Linking UnusedPackages ...