Fix #16805 by formatting warning message

compiler/main/GhcMake.hs

@@ -302,12 +302,10 @@ warnUnusedPackages = do
     let warn = makeIntoWarning
           (Reason Opt_WarnUnusedPackages)
           (mkPlainErrMsg dflags noSrcSpan msg)
-        msg = hang
-          ( text "The following packages were specified "
-            <> text "via -package or -package-id flags, "
-            <> text "but were not needed for compilation: ")
-          4
-          (sep (map pprUnusedArg unusedArgs))
+        msg = vcat [ text "The following packages were specified" <+>
+                     text "via -package or -package-id flags,"
+                   , text "but were not needed for compilation:"
+                   , nest 2 (vcat (map (withDash . pprUnusedArg) unusedArgs)) ]
 
     when (wopt Opt_WarnUnusedPackages dflags && not (null unusedArgs)) $
       logWarnings (listToBag [warn])
@@ -319,6 +317,8 @@ warnUnusedPackages = do
         pprUnusedArg (PackageArg str) = text str
         pprUnusedArg (UnitIdArg uid) = ppr uid
 
+        withDash = (<+>) (text "-")
+
         matchingStr :: String -> PackageConfig -> Bool
         matchingStr str p
                 =  str == sourcePackageIdString p

testsuite/tests/warnings/should_compile/UnusedPackages.stderr

@@ -2,5 +2,8 @@
 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: 
-        bytestring
+     The following packages were specified via -package or -package-id flags,
+     but were not needed for compilation:
+       - ghc
+       - process
+       - bytestring

testsuite/tests/warnings/should_compile/all.T

@@ -28,4 +28,5 @@ test('StarBinder', normal, compile, [''])
 
 test('Overflow', expect_broken_for(16543, ['hpc']), compile, [''])
 
-test('UnusedPackages', normal, multimod_compile, ['UnusedPackages.hs', '-package=bytestring -package=base -Wunused-packages'])
+test('UnusedPackages', normal, multimod_compile,
+    ['UnusedPackages.hs', '-package=bytestring -package=base -package=process -package=ghc -Wunused-packages'])