deprecate the -n option (#5180)

compiler/main/DynFlags.hs

@@ -274,7 +274,6 @@ data DynFlag
    -- misc opts
    | Opt_Pp
    | Opt_ForceRecomp
-   | Opt_DryRun
    | Opt_ExcessPrecision
    | Opt_EagerBlackHoling
    | Opt_ReadUserPackageConf
@@ -1154,7 +1153,7 @@ allFlags = map ('-':) $
 --------------- The main flags themselves ------------------
 dynamic_flags :: [Flag (CmdLineP DynFlags)]
 dynamic_flags = [
-    Flag "n"        (NoArg (setDynFlag Opt_DryRun))
+    Flag "n"        (NoArg (addWarn "The -n flag is deprecated and no longer has any effect"))
   , Flag "cpp"      (NoArg (setExtensionFlag Opt_Cpp)) 
   , Flag "F"        (NoArg (setDynFlag Opt_Pp)) 
   , Flag "#include" 

compiler/main/SysTools.lhs

@@ -788,20 +788,16 @@ data BuildMessage
   | EOF
 
 traceCmd :: DynFlags -> String -> String -> IO () -> IO ()
--- a) trace the command (at two levels of verbosity)
+-- trace the command (at two levels of verbosity)
 traceCmd dflags phase_name cmd_line action
  = do   { let verb = verbosity dflags
         ; showPass dflags phase_name
         ; debugTraceMsg dflags 3 (text cmd_line)
         ; hFlush stderr
 
-
-           -- Test for -n flag
-        ; unless (dopt Opt_DryRun dflags) $ do {
            -- And run it!
         ; action `catchIO` handle_exn verb
-        ; action `catchIO` handle_exn verb
+        }
   where
     handle_exn _verb exn = do { debugTraceMsg dflags 2 (char '\n')
                               ; debugTraceMsg dflags 2 (ptext (sLit "Failed:") <+> text cmd_line <+> text (show exn))

docs/users_guide/flags.xml

@@ -35,13 +35,7 @@
 	      <entry>mode</entry>
 	      <entry>-</entry>
 	    </row>
-	    <row>
-	      <entry><option>-n</option></entry>
-	      <entry>do a dry run</entry>
-	      <entry>dynamic</entry>
-	      <entry>-</entry>
-	    </row>
-	    <row>
+            <row>
 	      <entry><option>-v</option></entry>
 	      <entry>verbose mode (equivalent to <option>-v3</option>)</entry>
 	      <entry>dynamic</entry>

docs/users_guide/using.xml

@@ -781,18 +781,6 @@ ghc -c Foo.hs</screen>
     <para>See also the <option>--help</option>, <option>--version</option>, <option>--numeric-version</option>,
     and <option>--print-libdir</option> modes in <xref linkend="modes"/>.</para>
     <variablelist>
-      <varlistentry>
-	<term>
-          <option>-n</option>
-          <indexterm><primary><option>-n</option></primary></indexterm>
-        </term>
-	<listitem>
-	  <para>Does a dry-run, i.e. GHC goes through all the motions
-          of compiling as normal, but does not actually run any
-          external commands.</para>
-	</listitem>
-      </varlistentry>
-
       <varlistentry>
 	<term>
           <option>-v</option>