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

1 files changed, 39 insertions, 0 deletions

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