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diff --git a/hadrian/src/Rules/ToolArgs.hs b/hadrian/src/Rules/ToolArgs.hs
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+module Rules.ToolArgs(toolArgsTarget) where
+
+import qualified Rules.Generate
+import Development.Shake
+import Target
+import Context
+import Stage
+import Expression
+
+import Packages
+import Settings
+import Hadrian.Oracles.Cabal
+import Hadrian.Haskell.Cabal.Type
+import System.Directory (canonicalizePath)
+
+-- | @tool:@ is used by tooling in order to get the arguments necessary
+-- to set up a GHC API session which can compile modules from GHC. When
+-- run, the target prints out the arguments that would be passed to @ghc@
+-- during normal compilation to @stdout@ for the file passed as an
+-- argument.
+--
+-- This target is called by the `ghci.sh` script in order to load all of GHC's
+-- modules into GHCi. It is invoked with argument `tool:ghc/Main.hs` in
+-- that script so that we can load the whole library and executable
+-- components into GHCi.
+--
+-- In the future where we have multi-component ghci this code can be
+-- modified to supply the right arguments for that. At the moment it is
+-- also used for GHC's support for multi-component ghcide (see the
+-- `hadrian/hie-bios` script).
+
+
+-- | A phony target of form `tool:path/to/file.hs` which returns the
+-- options needed to compile the specific file.
+toolArgsTarget :: Rules ()
+toolArgsTarget = do
+  phonys (\s -> if "tool:" `isPrefixOf` s then Just (toolRuleBody (drop 5 s)) else Nothing)
+
+toolRuleBody :: FilePath -> Action ()
+toolRuleBody fp = do
+  mm <- dirMap
+  cfp <- liftIO $ canonicalizePath fp
+  case find (flip isPrefixOf cfp . fst) mm  of
+    Just (_, (p, extra)) -> mkToolTarget extra p
+    Nothing -> fail $ "No prefixes matched " ++ show fp ++ " IN\n " ++ show mm
+
+mkToolTarget :: [String] -> Package -> Action ()
+mkToolTarget es p = do
+    -- This builds automatically generated dependencies. Not sure how to do
+    -- this generically yet.
+    allDeps
+    let fake_target = target (Context Stage0 p (if windowsHost then vanilla else dynamic))
+                        (Ghc ToolArgs Stage0) [] ["ignored"]
+    arg_list <- interpret fake_target getArgs
+    liftIO $ putStrLn (intercalate "\n" (arg_list ++ es))
+allDeps :: Action ()
+allDeps = do
+   do
+    -- We can't build DLLs on Windows (yet). Actually we should only
+    -- include the dynamic way when we have a dynamic host GHC, but just
+    -- checking for Windows seems simpler for now.
+    let fake_target = target (Context Stage0 compiler (if windowsHost then vanilla else dynamic))
+                             (Ghc ToolArgs Stage0) [] ["ignored"]
+
+    -- need the autogenerated files so that they are precompiled
+    includesDependencies Stage0 >>= need
+    interpret fake_target Rules.Generate.compilerDependencies >>= need
+
+    root <- buildRoot
+    let dir = buildDir (vanillaContext Stage0 compiler)
+    need [ root -/- dir -/- "Config.hs" ]
+    need [ root -/- dir -/- "GHC" -/- "Parser.hs" ]
+    need [ root -/- dir -/- "GHC" -/- "Parser" -/- "Lexer.hs" ]
+    need [ root -/- dir -/- "GHC" -/- "Cmm" -/- "Parser.hs" ]
+    need [ root -/- dir -/- "GHC" -/- "Cmm" -/- "Lexer.hs"  ]
+
+-- This list is quite a lot like stage0packages but doesn't include
+-- critically the `exe:ghc` component as that depends on the GHC library
+-- which takes a while to compile.
+toolTargets :: [Package]
+toolTargets = [ array
+             , bytestring
+             , templateHaskell
+             , containers
+             , deepseq
+             , directory
+             , exceptions
+             , filepath
+             , compiler
+             , ghcCompact
+             , ghcPrim
+             --, haskeline
+             , hp2ps
+             , hsc2hs
+             , pretty
+             , process
+             , rts
+             , stm
+             , time
+             , unlit
+             , xhtml ]
+
+-- | Create a mapping from files to which component it belongs to.
+dirMap :: Action [(FilePath, (Package, [String]))]
+dirMap = do
+  auto <- concatMapM go toolTargets
+  -- Mush the ghc executable into the compiler component so the whole of ghc is not built when
+  -- configuring
+  ghc_exe <- mkGhc
+  return (auto ++ [ghc_exe])
+
+  where
+    -- Make a separate target for the exe:ghc target because otherwise
+    -- configuring would build the whole GHC library which we probably
+    -- don't want to do.
+    mkGhc = do
+      let c = (Context Stage0 compiler (if windowsHost then vanilla else dynamic))
+      cd <- readContextData c
+      fp <- liftIO $ canonicalizePath "ghc/"
+      return (fp, (compiler, "-ighc" : modules cd ++ otherModules cd ++ ["ghc/Main.hs"]))
+    go p = do
+      let c = (Context Stage0 p (if windowsHost then vanilla else dynamic))
+      -- readContextData has the effect of configuring the package so all
+      -- dependent packages will also be built.
+      cd <- readContextData c
+      ids <- liftIO $ mapM canonicalizePath [pkgPath p </> i | i <- srcDirs cd]
+      return $ map (,(p, modules cd ++ otherModules cd)) ids
+