Warn on missing home modules

Introduce a warning, -Wmissing-home-modules, to warn about home modules,
not listed in command line.

It is usefull for cabal when user fails to list a module in
`exposed-modules` and `other-modules`.

Test Plan: make TEST=MissingMod

Reviewers: mpickering, austin, bgamari

Reviewed By: bgamari

Subscribers: simonpj, mpickering, thomie

Differential Revision: https://phabricator.haskell.org/D2977

GHC Trac Issues: #13129

8 files changed, 76 insertions, 2 deletions

diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index bcd5a25836..6dbd723f9e 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -636,6 +636,7 @@ data WarningFlag =
    | Opt_WarnSimplifiableClassConstraints -- Since 8.2
    | Opt_WarnCPPUndef                     -- Since 8.2
    | Opt_WarnUnbangedStrictPatterns       -- Since 8.2
+   | Opt_WarnMissingHomeModules           -- Since 8.2
    deriving (Eq, Show, Enum)
 
 data Language = Haskell98 | Haskell2010
@@ -3443,6 +3444,7 @@ wWarningFlagsDeps = [
   flagSpec "missing-pattern-synonym-signatures"
                                     Opt_WarnMissingPatternSynonymSignatures,
   flagSpec "simplifiable-class-constraints" Opt_WarnSimplifiableClassConstraints,
+  flagSpec "missing-home-modules"        Opt_WarnMissingHomeModules,
   flagSpec "unrecognised-warning-flags"  Opt_WarnUnrecognisedWarningFlags ]
 
 -- | These @-\<blah\>@ flags can all be reversed with @-no-\<blah\>@
diff --git a/compiler/main/GhcMake.hs b/compiler/main/GhcMake.hs
index be6510bcb2..f74d09755f 100644
--- a/compiler/main/GhcMake.hs
+++ b/compiler/main/GhcMake.hs
@@ -132,9 +132,48 @@ depanal excluded_mods allow_dup_roots = do
     mod_graphE <- liftIO $ downsweep hsc_env old_graph
                                      excluded_mods allow_dup_roots
     mod_graph <- reportImportErrors mod_graphE
+
+    warnMissingHomeModules hsc_env mod_graph
+
     setSession hsc_env { hsc_mod_graph = mod_graph }
     return mod_graph
 
+-- Note [Missing home modules]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-- Sometimes user doesn't want GHC to pick up modules, not explicitly listed
+-- in a command line. For example, cabal may want to enable this warning
+-- when building a library, so that GHC warns user about modules, not listed
+-- neither in `exposed-modules`, nor in `other-modules`.
+--
+-- Here "home module" means a module, that doesn't come from an other package.
+--
+-- For example, if GHC is invoked with modules "A" and "B" as targets,
+-- but "A" imports some other module "C", then GHC will issue a warning
+-- about module "C" not being listed in a command line.
+--
+-- The warning in enabled by `-Wmissing-home-modules`. See Trac #13129
+warnMissingHomeModules :: GhcMonad m => HscEnv -> ModuleGraph -> m ()
+warnMissingHomeModules hsc_env mod_graph =
+    when (wopt Opt_WarnMissingHomeModules dflags && not (null missing)) $
+        logWarnings (listToBag [warn])
+    where
+    dflags = hsc_dflags hsc_env
+    missing = filter (`notElem` targets) imports
+    imports = map (moduleName . ms_mod) mod_graph
+    targets = map (targetid_to_name . targetId) (hsc_targets hsc_env)
+
+    msg = text "Modules are not listed in command line: "
+        <> sep (map ppr missing)
+    warn = makeIntoWarning
+      (Reason Opt_WarnMissingHomeModules)
+      (mkPlainErrMsg dflags noSrcSpan msg)
+
+    targetid_to_name (TargetModule name) = name
+    targetid_to_name (TargetFile file _) =
+      -- We can get a file even if module name in specified in command line
+      -- because it can be converted in guessTarget. So lets convert it back.
+      mkModuleName (fst $ splitExtension file)
+
 -- | Describes which modules of the module graph need to be loaded.
 data LoadHowMuch
    = LoadAllTargets
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index 21f00c454e..de660ed933 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -66,6 +66,7 @@ The following flags are simple ways to select standard "packages" of warnings:
         * :ghc-flag:`-Wmissing-local-signatures`
         * :ghc-flag:`-Wmissing-exported-signatures`
         * :ghc-flag:`-Wmissing-import-lists`
+        * :ghc-flag:`-Wmissing-home-modules`
         * :ghc-flag:`-Widentities`
 
 .. ghc-flag:: -Wcompat
@@ -1035,8 +1036,18 @@ of ``-W(no-)*``.
 
     This flag warns whenever you write a pattern that binds a variable whose
     type is unlifted, and yet the pattern is not a bang pattern nor a bare variable.
-    See :ref:`glasgow-unboxed` for informatino about unlifted types.
-    
+    See :ref:`glasgow-unboxed` for information about unlifted types.
+
+.. ghc-flag:: -Wmissing-home-modules
+
+    :since: 8.2
+
+    When a module provided by the package currently being compiled
+    (i.e. the "home" package) is imported, but not explicitly listed in
+    command line as a target. Useful for Cabal to ensure GHC won't
+    pick up modules, not listed neither in ``exposed-modules``, nor in
+    ``other-modules``.
+
 If you're feeling really paranoid, the :ghc-flag:`-dcore-lint` option is a good choice.
 It turns on heavyweight intra-pass sanity-checking within GHC. (It checks GHC's
 sanity, not yours.)
diff --git a/testsuite/tests/warnings/should_compile/MissingMod.hs b/testsuite/tests/warnings/should_compile/MissingMod.hs
new file mode 100644
index 0000000000..eaf7983c06
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/MissingMod.hs
@@ -0,0 +1,4 @@
+module MissingMod
+where
+
+import MissingMod1
diff --git a/testsuite/tests/warnings/should_compile/MissingMod.stderr b/testsuite/tests/warnings/should_compile/MissingMod.stderr
new file mode 100644
index 0000000000..0045092696
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/MissingMod.stderr
@@ -0,0 +1,5 @@
+
+<no location info>: warning: [-Wmissing-home-modules]
+    Modules are not listed in command line: MissingMod1
+[1 of 2] Compiling MissingMod1      ( MissingMod1.hs, MissingMod1.o )
+[2 of 2] Compiling MissingMod       ( MissingMod.hs, MissingMod.o )
diff --git a/testsuite/tests/warnings/should_compile/MissingMod1.hs b/testsuite/tests/warnings/should_compile/MissingMod1.hs
new file mode 100644
index 0000000000..2e78c2311f
--- /dev/null
+++ b/testsuite/tests/warnings/should_compile/MissingMod1.hs
@@ -0,0 +1,2 @@
+module MissingMod1
+where
diff --git a/testsuite/tests/warnings/should_compile/all.T b/testsuite/tests/warnings/should_compile/all.T
index bb347b065a..f7f0194011 100644
--- a/testsuite/tests/warnings/should_compile/all.T
+++ b/testsuite/tests/warnings/should_compile/all.T
@@ -24,3 +24,5 @@ test('DeprU',
 
 test('Werror01', normal, compile, [''])
 test('Werror02', normal, compile, [''])
+
+test('MissingMod', normal, multimod_compile, ['MissingMod', '-Wmissing-home-modules'])
diff --git a/utils/mkUserGuidePart/Options/Warnings.hs b/utils/mkUserGuidePart/Options/Warnings.hs
index f18222e130..f242fb062a 100644
--- a/utils/mkUserGuidePart/Options/Warnings.hs
+++ b/utils/mkUserGuidePart/Options/Warnings.hs
@@ -445,4 +445,13 @@ warningsOptions =
          , flagType = DynamicFlag
          , flagReverse = "-Wno-deriving-typeable"
          }
+  , flag { flagName = "-Wmissing-home-modules"
+         , flagDescription =
+           "warn when encountering a home module imported, but not listed "++
+           "on the command line. Useful for cabal to ensure GHC won't pick "++
+           "up modules, not listed neither in ``exposed-modules``, nor in "++
+           "``other-modules``."
+         , flagType = DynamicFlag
+         , flagReverse = "-Wno-missing-home-modules"
+         }
   ]