Implement -Wunrecognised-warning-flag

This allows the user to avoid warnings for warning flags that GHC
doesn't recognise. See #11429 for details..

Test Plan: Validate with T11429[abc] tests

Reviewers: austin, hvr

Reviewed By: hvr

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1830

GHC Trac Issues: #11429

9 files changed, 50 insertions, 4 deletions

diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index 0a7b6d21a1..f6496d5fa6 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -597,6 +597,7 @@ data WarningFlag =
    | Opt_WarnNonCanonicalMonadFailInstances -- since 8.0
    | Opt_WarnNonCanonicalMonoidInstances  -- since 8.0
    | Opt_WarnMissingPatSynSigs            -- since 8.0
+   | Opt_WarnUnrecognisedWarningFlags     -- since 8.0
    deriving (Eq, Show, Enum)
 
 data Language = Haskell98 | Haskell2010
@@ -2744,7 +2745,8 @@ dynamic_flags = [
  ++ map (mkFlag turnOff "XNo"       unSetExtensionFlag) xFlags
  ++ map (mkFlag turnOn  "X"         setLanguage       ) languageFlags
  ++ map (mkFlag turnOn  "X"         setSafeHaskell    ) safeHaskellFlags
- ++ [ defFlag "XGenerics"
+ ++ [ unrecognisedWarning
+    , defFlag "XGenerics"
         (NoArg (deprecate $
                   "it does nothing; look into -XDefaultSignatures " ++
                   "and -XDeriveGeneric for generic programming support."))
@@ -2753,6 +2755,16 @@ dynamic_flags = [
                   "it does nothing; look into -XDefaultSignatures and " ++
                   "-XDeriveGeneric for generic programming support.")) ]
 
+-- | This is where we handle unrecognised warning flags. We only issue a warning
+-- if -Wunrecognised-warning-flags is set. See Trac #11429 for context.
+unrecognisedWarning :: Flag (CmdLineP DynFlags)
+unrecognisedWarning = defFlag "W" (Prefix action)
+  where
+    action :: String -> EwM (CmdLineP DynFlags) ()
+    action flag = do
+      f <- wopt Opt_WarnUnrecognisedWarningFlags <$> liftEwM getCmdLineState
+      when f $ addWarn $ "unrecognised warning flag: -W"++flag
+
 -- See Note [Supporting CLI completion]
 package_flags :: [Flag (CmdLineP DynFlags)]
 package_flags = [
@@ -2965,7 +2977,8 @@ wWarningFlags = [
   flagSpec "unused-top-binds"            Opt_WarnUnusedTopBinds,
   flagSpec "warnings-deprecations"       Opt_WarnWarningsDeprecations,
   flagSpec "wrong-do-bind"               Opt_WarnWrongDoBind,
-  flagSpec "missing-pat-syn-sigs"        Opt_WarnMissingPatSynSigs]
+  flagSpec "missing-pat-syn-sigs"        Opt_WarnMissingPatSynSigs,
+  flagSpec "unrecognised-warning-flags"  Opt_WarnUnrecognisedWarningFlags ]
 
 -- | These @-\<blah\>@ flags can all be reversed with @-no-\<blah\>@
 negatableFlags :: [FlagSpec GeneralFlag]
@@ -3464,7 +3477,7 @@ optLevelFlags -- see Note [Documenting optimisation flags]
 -- please remember to update the User's Guide. The relevant file is:
 --
 --  * utils/mkUserGuidePart/
---  * docs/users_guide/using.rst
+--  * docs/users_guide/using-warnings.rst
 
 -- | Warnings enabled unless specified otherwise
 standardWarnings :: [WarningFlag]
@@ -3487,7 +3500,8 @@ standardWarnings -- see Note [Documenting warning flags]
         Opt_WarnInlineRuleShadowing,
         Opt_WarnAlternativeLayoutRuleTransitional,
         Opt_WarnUnsupportedLlvmVersion,
-        Opt_WarnTabs
+        Opt_WarnTabs,
+        Opt_WarnUnrecognisedWarningFlags
       ]
 
 -- | Things you get with -W
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index 4f9a7416d8..10fc9dfb10 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -31,6 +31,7 @@ generally likely to indicate bugs in your program. These are:
     * :ghc-flag:`-Winline-rule-shadowing`
     * :ghc-flag:`-Wunsupported-llvm-version`
     * :ghc-flag:`-Wtabs`
+    * :ghc-flag:`-Wunrecognised-warning-flags`
 
 The following flags are simple ways to select standard "packages" of warnings:
 
@@ -106,6 +107,13 @@ command line. For backwards compatibility with GHC versions prior to 8.0,
 all these warnings can still be controlled with ``-f(no-)warn-*`` instead
 of ``-W(no-)*``.
 
+.. ghc-flag:: -Wunrecognised-warning-flags
+
+    Enables warnings when the compiler encounters a ``-W...`` flag that is not
+    recognised.
+
+    This warning is on by default.
+
 .. ghc-flag:: -Wtyped-holes
 
     Determines whether the compiler reports typed holes warnings. Has no
diff --git a/testsuite/tests/driver/T11429a.hs b/testsuite/tests/driver/T11429a.hs
new file mode 100644
index 0000000000..c81fb82437
--- /dev/null
+++ b/testsuite/tests/driver/T11429a.hs
@@ -0,0 +1,2 @@
+main :: IO ()
+main = putStrLn "hello world"
diff --git a/testsuite/tests/driver/T11429a.stderr b/testsuite/tests/driver/T11429a.stderr
new file mode 100644
index 0000000000..c52b89e33d
--- /dev/null
+++ b/testsuite/tests/driver/T11429a.stderr
@@ -0,0 +1,2 @@
+
+on the commandline: warning: unrecognised warning flag: -Wfoobar
diff --git a/testsuite/tests/driver/T11429b.hs b/testsuite/tests/driver/T11429b.hs
new file mode 100644
index 0000000000..c81fb82437
--- /dev/null
+++ b/testsuite/tests/driver/T11429b.hs
@@ -0,0 +1,2 @@
+main :: IO ()
+main = putStrLn "hello world"
diff --git a/testsuite/tests/driver/T11429c.hs b/testsuite/tests/driver/T11429c.hs
new file mode 100644
index 0000000000..c81fb82437
--- /dev/null
+++ b/testsuite/tests/driver/T11429c.hs
@@ -0,0 +1,2 @@
+main :: IO ()
+main = putStrLn "hello world"
diff --git a/testsuite/tests/driver/T11429c.stderr b/testsuite/tests/driver/T11429c.stderr
new file mode 100644
index 0000000000..19e269b2d0
--- /dev/null
+++ b/testsuite/tests/driver/T11429c.stderr
@@ -0,0 +1,5 @@
+
+<no location info>: error: 
+Failing due to -Werror.
+
+on the commandline: warning: unrecognised warning flag: -Wfoobar
diff --git a/testsuite/tests/driver/all.T b/testsuite/tests/driver/all.T
index 69d18d90fa..5a19366e23 100644
--- a/testsuite/tests/driver/all.T
+++ b/testsuite/tests/driver/all.T
@@ -467,3 +467,7 @@ test('T10970a', normal, compile_and_run, [''])
 test('T4931', normal, compile_and_run, [''])
 test('T11182', normal, compile_and_run, [''])
 test('T11381', normal, compile_fail, [''])
+test('T11429a', normal, compile, ['-Wunrecognised-warning-flags -Wfoobar'])
+test('T11429b', normal, compile, ['-Wno-unrecognised-warning-flags -Wfoobar'])
+test('T11429c', normal, compile_fail, ['-Wunrecognised-warning-flags -Werror -Wfoobar'])
+
diff --git a/utils/mkUserGuidePart/Options/Warnings.hs b/utils/mkUserGuidePart/Options/Warnings.hs
index 3c69de7bd5..256d01f9fa 100644
--- a/utils/mkUserGuidePart/Options/Warnings.hs
+++ b/utils/mkUserGuidePart/Options/Warnings.hs
@@ -36,6 +36,13 @@ warningsOptions =
          , flagType = DynamicFlag
          , flagReverse = "-Werror"
          }
+  , flag { flagName = "-Wunrecognised-warning-flags"
+         , flagDescription =
+           "throw a warning when an unreconised ``-W...`` flag is "++
+           "encountered on the command line."
+         , flagType = DynamicFlag
+         , flagReverse = "-Wno-unrecognised-warning-flags"
+         }
   , flag { flagName = "-fdefer-type-errors"
          , flagDescription =
            "Turn type errors into warnings, :ref:`deferring the error until "++