HscMain.hs 73 KB
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{-# LANGUAGE BangPatterns, CPP, MagicHash, NondecreasingIndentation #-}

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-------------------------------------------------------------------------------
--
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-- | Main API for compiling plain Haskell source code.
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--
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-- This module implements compilation of a Haskell source. It is
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-- /not/ concerned with preprocessing of source files; this is handled
-- in "DriverPipeline".
--
-- There are various entry points depending on what mode we're in:
-- "batch" mode (@--make@), "one-shot" mode (@-c@, @-S@ etc.), and
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-- "interactive" mode (GHCi). There are also entry points for
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-- individual passes: parsing, typechecking/renaming, desugaring, and
-- simplification.
--
-- All the functions here take an 'HscEnv' as a parameter, but none of
-- them return a new one: 'HscEnv' is treated as an immutable value
-- from here on in (although it has mutable components, for the
-- caches).
--
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-- We use the Hsc monad to deal with warning messages consistently:
-- specifically, while executing within an Hsc monad, warnings are
-- collected. When a Hsc monad returns to an IO monad, the
-- warnings are printed, or compilation aborts if the @-Werror@
-- flag is enabled.
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--
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-- (c) The GRASP/AQUA Project, Glasgow University, 1993-2000
--
-------------------------------------------------------------------------------

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module HscMain
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    (
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    -- * Making an HscEnv
      newHscEnv

    -- * Compiling complete source files
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    , Messager, batchMsg
    , HscStatus (..)
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    , hscIncrementalCompile
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    , hscCompileCmmFile
    , hscCompileCore

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    , hscIncrementalFrontend
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    , genModDetails
    , hscSimpleIface
    , hscWriteIface
    , hscNormalIface
    , hscGenHardCode
    , hscInteractive

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    -- * Running passes separately
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    , hscParse
    , hscTypecheckRename
    , hscDesugar
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    , makeSimpleIface
    , makeSimpleDetails
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    , hscSimplify -- ToDo, shouldn't really export this

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    -- * Safe Haskell
    , hscCheckSafe
    , hscGetSafe

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    -- * Support for interactive evaluation
    , hscParseIdentifier
    , hscTcRcLookupName
    , hscTcRnGetInfo
#ifdef GHCI
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    , hscIsGHCiMonad
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    , hscGetModuleInterface
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    , hscRnImportDecls
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    , hscTcRnLookupRdrName
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    , hscStmt, hscStmtWithLocation, hscParsedStmt
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    , hscDecls, hscDeclsWithLocation
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    , hscTcExpr, hscImport, hscKcType
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    , hscParseExpr
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    , hscCompileCoreExpr
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    -- * Low-level exports for hooks
    , hscCompileCoreExpr'
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#endif
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      -- We want to make sure that we export enough to be able to redefine
      -- hscFileFrontEnd in client code
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    , hscParse', hscSimplify', hscDesugar', tcRnModule'
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    , getHscEnv
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    , hscSimpleIface', hscNormalIface'
    , oneShotMsg
    , hscFileFrontEnd, genericHscFrontend, dumpIfaceStats
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    ) where
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#ifdef GHCI
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import Id
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import BasicTypes       ( HValue )
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import ByteCodeGen      ( byteCodeGen, coreExprToBCOs )
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import Linker
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import CoreTidy         ( tidyExpr )
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import Type             ( Type )
import {- Kind parts of -} Type         ( Kind )
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import CoreLint         ( lintInteractiveExpr )
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import VarEnv           ( emptyTidyEnv )
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import THNames          ( templateHaskellNames )
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import ConLike
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import GHC.Exts
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#endif

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import Panic
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import Module
import Packages
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import RdrName
import HsSyn
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import CoreSyn
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import StringBuffer
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import Parser
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import Lexer
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import SrcLoc
import TcRnDriver
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import TcIface          ( typecheckIface )
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import TcRnMonad
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import IfaceEnv         ( initNameCache )
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import LoadIface        ( ifaceStats, initExternalPackageState
                        , findAndReadIface )
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import PrelInfo
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import MkIface
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import Desugar
import SimplCore
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import TidyPgm
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import CorePrep
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import CoreToStg        ( coreToStg )
import qualified StgCmm ( codeGen )
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import StgSyn
import CostCentre
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import ProfInit
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import TyCon
import Name
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import SimplStg         ( stg2stg )
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import Cmm
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import CmmParse         ( parseCmmFile )
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import CmmBuildInfoTables
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import CmmPipeline
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import CmmInfo
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import CodeOutput
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import NameEnv          ( emptyNameEnv )
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import InstEnv
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import FamInstEnv
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import Fingerprint      ( Fingerprint )
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import Hooks
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import Maybes
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import DynFlags
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import ErrUtils
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import Outputable
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import HscStats         ( ppSourceStats )
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import HscTypes
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import FastString
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import UniqSupply
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import Bag
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import Exception
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import qualified Stream
import Stream (Stream)

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import Util
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import Data.List
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import Control.Monad
import Data.IORef
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import System.FilePath as FilePath
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import System.Directory
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import qualified Data.Map as Map
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#include "HsVersions.h"
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{- **********************************************************************
%*                                                                      *
                Initialisation
%*                                                                      *
%********************************************************************* -}
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newHscEnv :: DynFlags -> IO HscEnv
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newHscEnv dflags = do
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    eps_var <- newIORef initExternalPackageState
    us      <- mkSplitUniqSupply 'r'
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    nc_var  <- newIORef (initNameCache us allKnownKeyNames)
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    fc_var  <- newIORef emptyModuleEnv
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    return HscEnv {  hsc_dflags       = dflags,
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                     hsc_targets      = [],
                     hsc_mod_graph    = [],
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                     hsc_IC           = emptyInteractiveContext dflags,
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                     hsc_HPT          = emptyHomePackageTable,
                     hsc_EPS          = eps_var,
                     hsc_NC           = nc_var,
                     hsc_FC           = fc_var,
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                     hsc_type_env_var = Nothing }
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allKnownKeyNames :: [Name]      -- Put here to avoid loops involving DsMeta,
allKnownKeyNames =              -- where templateHaskellNames are defined
    knownKeyNames
#ifdef GHCI
        ++ templateHaskellNames
#endif

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-- -----------------------------------------------------------------------------
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getWarnings :: Hsc WarningMessages
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getWarnings = Hsc $ \_ w -> return (w, w)
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clearWarnings :: Hsc ()
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clearWarnings = Hsc $ \_ _ -> return ((), emptyBag)
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logWarnings :: WarningMessages -> Hsc ()
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logWarnings w = Hsc $ \_ w0 -> return ((), w0 `unionBags` w)
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getHscEnv :: Hsc HscEnv
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getHscEnv = Hsc $ \e w -> return (e, w)
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handleWarnings :: Hsc ()
handleWarnings = do
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    dflags <- getDynFlags
    w <- getWarnings
    liftIO $ printOrThrowWarnings dflags w
    clearWarnings
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-- | log warning in the monad, and if there are errors then
-- throw a SourceError exception.
logWarningsReportErrors :: Messages -> Hsc ()
logWarningsReportErrors (warns,errs) = do
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    logWarnings warns
    when (not $ isEmptyBag errs) $ throwErrors errs
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-- | Throw some errors.
throwErrors :: ErrorMessages -> Hsc a
throwErrors = liftIO . throwIO . mkSrcErr
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-- | Deal with errors and warnings returned by a compilation step
--
-- In order to reduce dependencies to other parts of the compiler, functions
-- outside the "main" parts of GHC return warnings and errors as a parameter
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-- and signal success via by wrapping the result in a 'Maybe' type. This
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-- function logs the returned warnings and propagates errors as exceptions
-- (of type 'SourceError').
--
-- This function assumes the following invariants:
--
--  1. If the second result indicates success (is of the form 'Just x'),
--     there must be no error messages in the first result.
--
--  2. If there are no error messages, but the second result indicates failure
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--     there should be warnings in the first result. That is, if the action
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--     failed, it must have been due to the warnings (i.e., @-Werror@).
ioMsgMaybe :: IO (Messages, Maybe a) -> Hsc a
ioMsgMaybe ioA = do
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    ((warns,errs), mb_r) <- liftIO ioA
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    logWarnings warns
    case mb_r of
        Nothing -> throwErrors errs
        Just r  -> ASSERT( isEmptyBag errs ) return r
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-- | like ioMsgMaybe, except that we ignore error messages and return
-- 'Nothing' instead.
ioMsgMaybe' :: IO (Messages, Maybe a) -> Hsc (Maybe a)
ioMsgMaybe' ioA = do
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    ((warns,_errs), mb_r) <- liftIO $ ioA
    logWarnings warns
    return mb_r
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-- -----------------------------------------------------------------------------
-- | Lookup things in the compiler's environment

#ifdef GHCI
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hscTcRnLookupRdrName :: HscEnv -> Located RdrName -> IO [Name]
hscTcRnLookupRdrName hsc_env0 rdr_name 
  = runInteractiveHsc hsc_env0 $ 
    do { hsc_env <- getHscEnv
       ; ioMsgMaybe $ tcRnLookupRdrName hsc_env rdr_name }
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#endif

hscTcRcLookupName :: HscEnv -> Name -> IO (Maybe TyThing)
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hscTcRcLookupName hsc_env0 name = runInteractiveHsc hsc_env0 $ do
  hsc_env <- getHscEnv
  ioMsgMaybe' $ tcRnLookupName hsc_env name
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      -- ignore errors: the only error we're likely to get is
      -- "name not found", and the Maybe in the return type
      -- is used to indicate that.
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hscTcRnGetInfo :: HscEnv -> Name -> IO (Maybe (TyThing, Fixity, [ClsInst], [FamInst]))
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hscTcRnGetInfo hsc_env0 name
  = runInteractiveHsc hsc_env0 $
    do { hsc_env <- getHscEnv
       ; ioMsgMaybe' $ tcRnGetInfo hsc_env name }
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#ifdef GHCI
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hscIsGHCiMonad :: HscEnv -> String -> IO Name
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hscIsGHCiMonad hsc_env name
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  = runHsc hsc_env $ ioMsgMaybe $ isGHCiMonad hsc_env name
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hscGetModuleInterface :: HscEnv -> Module -> IO ModIface
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hscGetModuleInterface hsc_env0 mod = runInteractiveHsc hsc_env0 $ do
  hsc_env <- getHscEnv
  ioMsgMaybe $ getModuleInterface hsc_env mod
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-- -----------------------------------------------------------------------------
-- | Rename some import declarations
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hscRnImportDecls :: HscEnv -> [LImportDecl RdrName] -> IO GlobalRdrEnv
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hscRnImportDecls hsc_env0 import_decls = runInteractiveHsc hsc_env0 $ do
  hsc_env <- getHscEnv
  ioMsgMaybe $ tcRnImportDecls hsc_env import_decls
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#endif

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-- -----------------------------------------------------------------------------
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-- | parse a file, returning the abstract syntax
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hscParse :: HscEnv -> ModSummary -> IO HsParsedModule
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hscParse hsc_env mod_summary = runHsc hsc_env $ hscParse' mod_summary

-- internal version, that doesn't fail due to -Werror
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hscParse' :: ModSummary -> Hsc HsParsedModule
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hscParse' mod_summary = do
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    dflags <- getDynFlags
    let src_filename  = ms_hspp_file mod_summary
        maybe_src_buf = ms_hspp_buf  mod_summary
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    --------------------------  Parser  ----------------
    liftIO $ showPass dflags "Parser"
    {-# SCC "Parser" #-} do
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    -- sometimes we already have the buffer in memory, perhaps
    -- because we needed to parse the imports out of it, or get the
    -- module name.
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    buf <- case maybe_src_buf of
               Just b  -> return b
               Nothing -> liftIO $ hGetStringBuffer src_filename
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    let loc = mkRealSrcLoc (mkFastString src_filename) 1 1
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    case unP parseModule (mkPState dflags buf loc) of
        PFailed span err ->
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            liftIO $ throwOneError (mkPlainErrMsg dflags span err)
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        POk pst rdr_module -> do
            logWarningsReportErrors (getMessages pst)
            liftIO $ dumpIfSet_dyn dflags Opt_D_dump_parsed "Parser" $
                                   ppr rdr_module
            liftIO $ dumpIfSet_dyn dflags Opt_D_source_stats "Source Statistics" $
                                   ppSourceStats False rdr_module
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            -- To get the list of extra source files, we take the list
            -- that the parser gave us,
            --   - eliminate files beginning with '<'.  gcc likes to use
            --     pseudo-filenames like "<built-in>" and "<command-line>"
            --   - normalise them (elimiante differences between ./f and f)
            --   - filter out the preprocessed source file
            --   - filter out anything beginning with tmpdir
            --   - remove duplicates
            --   - filter out the .hs/.lhs source filename if we have one
            --
            let n_hspp  = FilePath.normalise src_filename
                srcs0 = nub $ filter (not . (tmpDir dflags `isPrefixOf`))
                            $ filter (not . (== n_hspp))
                            $ map FilePath.normalise
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                            $ filter (not . (isPrefixOf "<"))
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                            $ map unpackFS
                            $ srcfiles pst
                srcs1 = case ml_hs_file (ms_location mod_summary) of
                          Just f  -> filter (/= FilePath.normalise f) srcs0
                          Nothing -> srcs0

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            -- sometimes we see source files from earlier
            -- preprocessing stages that cannot be found, so just
            -- filter them out:
            srcs2 <- liftIO $ filterM doesFileExist srcs1

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            return HsParsedModule {
                      hpm_module    = rdr_module,
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                      hpm_src_files = srcs2,
                      hpm_annotations
                              = (Map.fromListWith (++) $ annotations pst,
                                 Map.fromList $ ((noSrcSpan,comment_q pst)
                                                 :(annotations_comments pst)))
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                   }
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-- XXX: should this really be a Maybe X?  Check under which circumstances this
-- can become a Nothing and decide whether this should instead throw an
-- exception/signal an error.
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type RenamedStuff =
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        (Maybe (HsGroup Name, [LImportDecl Name], Maybe [LIE Name],
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                Maybe LHsDocString))
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-- | Rename and typecheck a module, additionally returning the renamed syntax
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hscTypecheckRename :: HscEnv -> ModSummary -> HsParsedModule
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                   -> IO (TcGblEnv, RenamedStuff)
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hscTypecheckRename hsc_env mod_summary rdr_module = runHsc hsc_env $ do
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    tc_result <- tcRnModule' hsc_env mod_summary True rdr_module
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        -- This 'do' is in the Maybe monad!
    let rn_info = do decl <- tcg_rn_decls tc_result
                     let imports = tcg_rn_imports tc_result
                         exports = tcg_rn_exports tc_result
                         doc_hdr = tcg_doc_hdr tc_result
                     return (decl,imports,exports,doc_hdr)
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    return (tc_result, rn_info)
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-- wrapper around tcRnModule to handle safe haskell extras
tcRnModule' :: HscEnv -> ModSummary -> Bool -> HsParsedModule
            -> Hsc TcGblEnv
tcRnModule' hsc_env sum save_rn_syntax mod = do
    tcg_res <- {-# SCC "Typecheck-Rename" #-}
               ioMsgMaybe $
                   tcRnModule hsc_env (ms_hsc_src sum) save_rn_syntax mod

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    -- See Note [Safe Haskell Overlapping Instances Implementation]
    -- although this is used for more than just that failure case.
    (tcSafeOK, whyUnsafe) <- liftIO $ readIORef (tcg_safeInfer tcg_res)
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    dflags   <- getDynFlags
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    let allSafeOK = safeInferred dflags && tcSafeOK
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    -- end of the safe haskell line, how to respond to user?
    if not (safeHaskellOn dflags) || (safeInferOn dflags && not allSafeOK)
        -- if safe Haskell off or safe infer failed, mark unsafe
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        then markUnsafeInfer tcg_res whyUnsafe
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        -- module (could be) safe, throw warning if needed
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        else do
            tcg_res' <- hscCheckSafeImports tcg_res
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            safe <- liftIO $ fst <$> readIORef (tcg_safeInfer tcg_res')
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            when safe $ do
              case wopt Opt_WarnSafe dflags of
                True -> (logWarnings $ unitBag $ mkPlainWarnMsg dflags
                       (warnSafeOnLoc dflags) $ errSafe tcg_res')
                False | safeHaskell dflags == Sf_Trustworthy &&
                        wopt Opt_WarnTrustworthySafe dflags ->
                  (logWarnings $ unitBag $ mkPlainWarnMsg dflags
                    (trustworthyOnLoc dflags) $ errTwthySafe tcg_res')
                False -> return ()
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            return tcg_res'
  where
    pprMod t  = ppr $ moduleName $ tcg_mod t
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    errSafe t = quotes (pprMod t) <+> text "has been inferred as safe!"
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    errTwthySafe t = quotes (pprMod t)
      <+> text "is marked as Trustworthy but has been inferred as safe!"
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-- | Convert a typechecked module to Core
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hscDesugar :: HscEnv -> ModSummary -> TcGblEnv -> IO ModGuts
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hscDesugar hsc_env mod_summary tc_result =
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    runHsc hsc_env $ hscDesugar' (ms_location mod_summary) tc_result
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hscDesugar' :: ModLocation -> TcGblEnv -> Hsc ModGuts
hscDesugar' mod_location tc_result = do
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    hsc_env <- getHscEnv
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    r <- ioMsgMaybe $
      {-# SCC "deSugar" #-}
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      deSugar hsc_env mod_location tc_result
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    -- always check -Werror after desugaring, this is the last opportunity for
    -- warnings to arise before the backend.
    handleWarnings
    return r
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-- | Make a 'ModIface' from the results of typechecking. Used when
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-- not optimising, and the interface doesn't need to contain any
-- unfoldings or other cross-module optimisation info.
-- ToDo: the old interface is only needed to get the version numbers,
-- we should use fingerprint versions instead.
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makeSimpleIface :: HscEnv -> Maybe ModIface -> TcGblEnv -> ModDetails
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                -> IO (ModIface,Bool)
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makeSimpleIface hsc_env maybe_old_iface tc_result details = runHsc hsc_env $ do
    safe_mode <- hscGetSafeMode tc_result
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    liftIO $ do
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        mkIfaceTc hsc_env (fmap mi_iface_hash maybe_old_iface) safe_mode
                  details tc_result
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-- | Make a 'ModDetails' from the results of typechecking. Used when
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-- typechecking only, as opposed to full compilation.
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makeSimpleDetails :: HscEnv -> TcGblEnv -> IO ModDetails
makeSimpleDetails hsc_env tc_result = mkBootModDetailsTc hsc_env tc_result
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{- **********************************************************************
%*                                                                      *
                The main compiler pipeline
%*                                                                      *
%********************************************************************* -}

{-
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                   --------------------------------
                        The compilation proper
                   --------------------------------

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It's the task of the compilation proper to compile Haskell, hs-boot and core
files to either byte-code, hard-code (C, asm, LLVM, ect) or to nothing at all
(the module is still parsed and type-checked. This feature is mostly used by
IDE's and the likes). Compilation can happen in either 'one-shot', 'batch',
'nothing', or 'interactive' mode. 'One-shot' mode targets hard-code, 'batch'
mode targets hard-code, 'nothing' mode targets nothing and 'interactive' mode
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targets byte-code.
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The modes are kept separate because of their different types and meanings:

 * In 'one-shot' mode, we're only compiling a single file and can therefore
 discard the new ModIface and ModDetails. This is also the reason it only
 targets hard-code; compiling to byte-code or nothing doesn't make sense when
 we discard the result.

 * 'Batch' mode is like 'one-shot' except that we keep the resulting ModIface
 and ModDetails. 'Batch' mode doesn't target byte-code since that require us to
 return the newly compiled byte-code.

 * 'Nothing' mode has exactly the same type as 'batch' mode but they're still
 kept separate. This is because compiling to nothing is fairly special: We
 don't output any interface files, we don't run the simplifier and we don't
 generate any code.

 * 'Interactive' mode is similar to 'batch' mode except that we return the
 compiled byte-code together with the ModIface and ModDetails.

Trying to compile a hs-boot file to byte-code will result in a run-time error.
This is the only thing that isn't caught by the type-system.
-}


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type Messager = HscEnv -> (Int,Int) -> RecompileRequired -> ModSummary -> IO ()

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-- | This function runs GHC's frontend with recompilation
-- avoidance. Specifically, it checks if recompilation is needed,
-- and if it is, it parses and typechecks the input module.
-- It does not write out the results of typechecking (See
-- compileOne and hscIncrementalCompile).
hscIncrementalFrontend :: Bool -- always do basic recompilation check?
                       -> Maybe TcGblEnv
                       -> Maybe Messager
                       -> ModSummary
                       -> SourceModified
                       -> Maybe ModIface  -- Old interface, if available
                       -> (Int,Int)       -- (i,n) = module i of n (for msgs)
                       -> Hsc (Either ModIface (FrontendResult, Maybe Fingerprint))

hscIncrementalFrontend
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  always_do_basic_recompilation_check m_tc_result
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  mHscMessage mod_summary source_modified mb_old_iface mod_index
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    = do
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    hsc_env <- getHscEnv
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    let msg what = case mHscMessage of
                   Just hscMessage -> hscMessage hsc_env mod_index what mod_summary
                   Nothing -> return ()
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        skip iface = do
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            liftIO $ msg UpToDate
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            return $ Left iface
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        compile mb_old_hash reason = do
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            liftIO $ msg reason
            result <- genericHscFrontend mod_summary
            return $ Right (result, mb_old_hash)
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        stable = case source_modified of
                     SourceUnmodifiedAndStable -> True
                     _                         -> False
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    case m_tc_result of
         Just tc_result
          | not always_do_basic_recompilation_check ->
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             return $ Right (FrontendTypecheck tc_result, Nothing)
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         _ -> do
            (recomp_reqd, mb_checked_iface)
                <- {-# SCC "checkOldIface" #-}
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                   liftIO $ checkOldIface hsc_env mod_summary
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                                source_modified mb_old_iface
            -- save the interface that comes back from checkOldIface.
            -- In one-shot mode we don't have the old iface until this
            -- point, when checkOldIface reads it from the disk.
            let mb_old_hash = fmap mi_iface_hash mb_checked_iface

            case mb_checked_iface of
                Just iface | not (recompileRequired recomp_reqd) ->
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                    -- If the module used TH splices when it was last
                    -- compiled, then the recompilation check is not
                    -- accurate enough (#481) and we must ignore
                    -- it.  However, if the module is stable (none of
                    -- the modules it depends on, directly or
                    -- indirectly, changed), then we *can* skip
                    -- recompilation. This is why the SourceModified
                    -- type contains SourceUnmodifiedAndStable, and
                    -- it's pretty important: otherwise ghc --make
                    -- would always recompile TH modules, even if
                    -- nothing at all has changed. Stability is just
                    -- the same check that make is doing for us in
                    -- one-shot mode.
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                    case m_tc_result of
                    Nothing
                     | mi_used_th iface && not stable ->
                        compile mb_old_hash (RecompBecause "TH")
                    _ ->
                        skip iface
                _ ->
                    case m_tc_result of
                    Nothing -> compile mb_old_hash recomp_reqd
                    Just tc_result ->
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                        return $ Right (FrontendTypecheck tc_result, mb_old_hash)
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genericHscFrontend :: ModSummary -> Hsc FrontendResult
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genericHscFrontend mod_summary =
  getHooked hscFrontendHook genericHscFrontend' >>= ($ mod_summary)

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genericHscFrontend' :: ModSummary -> Hsc FrontendResult
genericHscFrontend' mod_summary
    | ms_hsc_src mod_summary == HsBootMerge
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    = FrontendInterface `fmap` hscMergeFrontEnd mod_summary
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    | otherwise
    = FrontendTypecheck `fmap` hscFileFrontEnd mod_summary
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--------------------------------------------------------------
-- Compilers
--------------------------------------------------------------

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-- Compile Haskell/boot in OneShot mode.
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hscIncrementalCompile :: Bool
                      -> Maybe TcGblEnv
                      -> Maybe Messager
                      -> HscEnv
                      -> ModSummary
                      -> SourceModified
                      -> Maybe ModIface
                      -> (Int,Int)
                      -- HomeModInfo does not contain linkable, since we haven't
                      -- code-genned yet
                      -> IO (HscStatus, HomeModInfo)
hscIncrementalCompile always_do_basic_recompilation_check m_tc_result
    mHscMessage hsc_env' mod_summary source_modified mb_old_iface mod_index
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  = do
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    -- One-shot mode needs a knot-tying mutable variable for interface
    -- files. See TcRnTypes.TcGblEnv.tcg_type_env_var.
    type_env_var <- newIORef emptyNameEnv
    let mod = ms_mod mod_summary
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        hsc_env = hsc_env'{ hsc_type_env_var = Just (mod, type_env_var) }
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    -- NB: enter Hsc monad here so that we don't bail out early with
    -- -Werror on typechecker warnings; we also want to run the desugarer
    -- to get those warnings too. (But we'll always exit at that point
    -- because the desugarer runs ioMsgMaybe.)
    runHsc hsc_env $ do
    let dflags = hsc_dflags hsc_env
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    e <- hscIncrementalFrontend always_do_basic_recompilation_check m_tc_result mHscMessage
            mod_summary source_modified mb_old_iface mod_index
    case e of
        Left iface -> do
            details <- liftIO $ genModDetails hsc_env iface
            return (HscUpToDate, HomeModInfo{
                hm_details = details,
                hm_iface = iface,
                hm_linkable = Nothing
            })
        Right (result, mb_old_hash) -> do
            (status, hmi, no_change) <- case result of
                FrontendTypecheck tc_result ->
                    if hscTarget dflags /= HscNothing &&
                       ms_hsc_src mod_summary == HsSrcFile
                       then finish              hsc_env mod_summary tc_result mb_old_hash
                       else finishTypecheckOnly hsc_env mod_summary tc_result mb_old_hash
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                FrontendInterface raw_iface ->
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                            finishMerge         hsc_env mod_summary raw_iface mb_old_hash
            liftIO $ hscMaybeWriteIface dflags (hm_iface hmi) no_change mod_summary
            return (status, hmi)

-- Generates and writes out the final interface for an hs-boot merge.
finishMerge :: HscEnv
            -> ModSummary
            -> ModIface
            -> Maybe Fingerprint
            -> Hsc (HscStatus, HomeModInfo, Bool)
finishMerge hsc_env summary iface0 mb_old_hash = do
    MASSERT( ms_hsc_src summary == HsBootMerge )
    (iface, changed) <- liftIO $ mkIfaceDirect hsc_env mb_old_hash iface0
    details <- liftIO $ genModDetails hsc_env iface
    let dflags = hsc_dflags hsc_env
        hsc_status =
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            case hscTarget dflags of
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                HscNothing -> HscNotGeneratingCode
                _ -> HscUpdateBootMerge
    return (hsc_status,
            HomeModInfo{ hm_details  = details,
                         hm_iface    = iface,
                         hm_linkable = Nothing },
            changed)

-- Generates and writes out the final interface for a typecheck.
finishTypecheckOnly :: HscEnv
              -> ModSummary
              -> TcGblEnv
              -> Maybe Fingerprint
              -> Hsc (HscStatus, HomeModInfo, Bool)
finishTypecheckOnly hsc_env summary tc_result mb_old_hash = do
    let dflags = hsc_dflags hsc_env
    MASSERT( hscTarget dflags == HscNothing || ms_hsc_src summary == HsBootFile )
    (iface, changed, details) <- liftIO $ hscSimpleIface hsc_env tc_result mb_old_hash
    let hsc_status =
          case (hscTarget dflags, ms_hsc_src summary) of
            (HscNothing, _) -> HscNotGeneratingCode
            (_, HsBootFile) -> HscUpdateBoot
            _ -> panic "finishTypecheckOnly"
    return (hsc_status,
            HomeModInfo{ hm_details  = details,
                         hm_iface    = iface,
                         hm_linkable = Nothing },
            changed)

-- Runs the post-typechecking frontend (desugar and simplify),
-- and then generates and writes out the final interface. We want
-- to write the interface AFTER simplification so we can get
-- as up-to-date and good unfoldings and other info as possible
-- in the interface file.  This is only ever run for HsSrcFile,
-- and NOT for HscNothing.
finish :: HscEnv
       -> ModSummary
       -> TcGblEnv
       -> Maybe Fingerprint
       -> Hsc (HscStatus, HomeModInfo, Bool)
finish hsc_env summary tc_result mb_old_hash = do
    let dflags = hsc_dflags hsc_env
    MASSERT( ms_hsc_src summary == HsSrcFile )
    MASSERT( hscTarget dflags /= HscNothing )
    guts0 <- hscDesugar' (ms_location summary) tc_result
    guts <- hscSimplify' guts0
    (iface, changed, details, cgguts) <- liftIO $ hscNormalIface hsc_env guts mb_old_hash

    return (HscRecomp cgguts summary,
            HomeModInfo{ hm_details  = details,
                         hm_iface    = iface,
                         hm_linkable = Nothing },
            changed)

hscMaybeWriteIface :: DynFlags -> ModIface -> Bool -> ModSummary -> IO ()
hscMaybeWriteIface dflags iface changed summary =
    let force_write_interface = gopt Opt_WriteInterface dflags
        write_interface = case hscTarget dflags of
                            HscNothing      -> False
                            HscInterpreted  -> False
                            _               -> True
    in when (write_interface || force_write_interface) $
            hscWriteIface dflags iface changed summary
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--------------------------------------------------------------
-- NoRecomp handlers
--------------------------------------------------------------

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genModDetails :: HscEnv -> ModIface -> IO ModDetails
genModDetails hsc_env old_iface
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  = do
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    new_details <- {-# SCC "tcRnIface" #-}
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                   initIfaceCheck hsc_env (typecheckIface old_iface)
    dumpIfaceStats hsc_env
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    return new_details
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--------------------------------------------------------------
-- Progress displayers.
--------------------------------------------------------------

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oneShotMsg :: HscEnv -> RecompileRequired -> IO ()
oneShotMsg hsc_env recomp =
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    case recomp of
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        UpToDate ->
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            compilationProgressMsg (hsc_dflags hsc_env) $
                   "compilation IS NOT required"
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        _ ->
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            return ()

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batchMsg :: Messager
batchMsg hsc_env mod_index recomp mod_summary =
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    case recomp of
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        MustCompile -> showMsg "Compiling " ""
        UpToDate
            | verbosity (hsc_dflags hsc_env) >= 2 -> showMsg "Skipping  " ""
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            | otherwise -> return ()
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        RecompBecause reason -> showMsg "Compiling " (" [" ++ reason ++ "]")
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    where
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        dflags = hsc_dflags hsc_env
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        showMsg msg reason =
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            compilationProgressMsg dflags $
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            (showModuleIndex mod_index ++
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            msg ++ showModMsg dflags (hscTarget dflags)
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                              (recompileRequired recomp) mod_summary)
                ++ reason
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--------------------------------------------------------------
-- FrontEnds
--------------------------------------------------------------
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-- | Given an 'HsBootMerge' 'ModSummary', merges all @hs-boot@ files
-- under this module name into a composite, publically visible 'ModIface'.
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hscMergeFrontEnd :: ModSummary -> Hsc ModIface
hscMergeFrontEnd mod_summary = do
    hsc_env <- getHscEnv
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    MASSERT( ms_hsc_src mod_summary == HsBootMerge )
    let dflags = hsc_dflags hsc_env
    -- TODO: actually merge in signatures from external packages.
    -- Grovel in HPT if necessary
    -- TODO: replace with 'computeInterface'
    let hpt = hsc_HPT hsc_env
    -- TODO multiple mods
    let name = moduleName (ms_mod mod_summary)
        mod = mkModule (thisPackage dflags) name
        is_boot = True
    iface0 <- case lookupHptByModule hpt mod of
        Just hm -> return (hm_iface hm)
        Nothing -> do
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            mb_iface0 <- liftIO . initIfaceCheck hsc_env
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                    $ findAndReadIface (text "merge-requirements")
                                       mod is_boot
            case mb_iface0 of
                Succeeded (i, _) -> return i
                Failed err -> liftIO $ throwGhcExceptionIO
                                (ProgramError (showSDoc dflags err))
    let iface = iface0 {
                    mi_hsc_src = HsBootMerge,
                    -- TODO: mkDependencies doublecheck
                    mi_deps = (mi_deps iface0) {
                        dep_mods = (name, is_boot)
                                 : dep_mods (mi_deps iface0)
                      }
                    }
    return iface

-- | Given a 'ModSummary', parses and typechecks it, returning the
-- 'TcGblEnv' resulting from type-checking.
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hscFileFrontEnd :: ModSummary -> Hsc TcGblEnv
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hscFileFrontEnd mod_summary = do
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    MASSERT( ms_hsc_src mod_summary == HsBootFile || ms_hsc_src mod_summary == HsSrcFile )
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    hpm <- hscParse' mod_summary
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    hsc_env <- getHscEnv
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    tcg_env <- tcRnModule' hsc_env mod_summary False hpm
    return tcg_env
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--------------------------------------------------------------
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-- Safe Haskell
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--------------------------------------------------------------

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-- Note [Safe Haskell Trust Check]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- Safe Haskell checks that an import is trusted according to the following
-- rules for an import of module M that resides in Package P:
--
--   * If M is recorded as Safe and all its trust dependencies are OK
--     then M is considered safe.
--   * If M is recorded as Trustworthy and P is considered trusted and
--     all M's trust dependencies are OK then M is considered safe.
--
-- By trust dependencies we mean that the check is transitive. So if
-- a module M that is Safe relies on a module N that is trustworthy,
-- importing module M will first check (according to the second case)
-- that N is trusted before checking M is trusted.
--
-- This is a minimal description, so please refer to the user guide
-- for more details. The user guide is also considered the authoritative
-- source in this matter, not the comments or code.


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-- Note [Safe Haskell Inference]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- Safe Haskell does Safe inference on modules that don't have any specific
-- safe haskell mode flag. The basic aproach to this is:
--   * When deciding if we need to do a Safe language check, treat
--     an unmarked module as having -XSafe mode specified.
--   * For checks, don't throw errors but return them to the caller.
--   * Caller checks if there are errors:
--     * For modules explicitly marked -XSafe, we throw the errors.
--     * For unmarked modules (inference mode), we drop the errors
--       and mark the module as being Unsafe.
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--
-- It used to be that we only did safe inference on modules that had no Safe
-- Haskell flags, but now we perform safe inference on all modules as we want
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-- to allow users to set the `-fwarn-safe`, `-fwarn-unsafe` and
-- `-fwarn-trustworthy-safe` flags on Trustworthy and Unsafe modules so that a
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-- user can ensure their assumptions are correct and see reasons for why a
-- module is safe or unsafe.
--
-- This is tricky as we must be careful when we should throw an error compared
-- to just warnings. For checking safe imports we manage it as two steps. First
-- we check any imports that are required to be safe, then we check all other
-- imports to see if we can infer them to be safe.
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-- | Check that the safe imports of the module being compiled are valid.
-- If not we either issue a compilation error if the module is explicitly
-- using Safe Haskell, or mark the module as unsafe if we're in safe
-- inference mode.
hscCheckSafeImports :: TcGblEnv -> Hsc TcGblEnv
hscCheckSafeImports tcg_env = do
    dflags   <- getDynFlags
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    tcg_env' <- checkSafeImports dflags tcg_env
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    checkRULES dflags tcg_env'
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  where
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    checkRULES dflags tcg_env' = do
      case safeLanguageOn dflags of
          True -> do
              -- XSafe: we nuke user written RULES
              logWarnings $ warns dflags (tcg_rules tcg_env')
              return tcg_env' { tcg_rules = [] }
          False
                -- SafeInferred: user defined RULES, so not safe
              | safeInferOn dflags && not (null $ tcg_rules tcg_env')
              -> markUnsafeInfer tcg_env' $ warns dflags (tcg_rules tcg_env')

                -- Trustworthy OR SafeInferred: with no RULES
              | otherwise
              -> return tcg_env'

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    warns dflags rules = listToBag $ map (warnRules dflags) rules
    warnRules dflags (L loc (HsRule n _ _ _ _ _ _)) =
        mkPlainWarnMsg dflags loc $
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            text "Rule \"" <> ftext (snd $ unLoc n) <> text "\" ignored" $+$
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            text "User defined rules are disabled under Safe Haskell"
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-- | Validate that safe imported modules are actually safe.  For modules in the
-- HomePackage (the package the module we are compiling in resides) this just
-- involves checking its trust type is 'Safe' or 'Trustworthy'. For modules
-- that reside in another package we also must check that the external pacakge
-- is trusted. See the Note [Safe Haskell Trust Check] above for more
-- information.
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--
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-- The code for this is quite tricky as the whole algorithm is done in a few
-- distinct phases in different parts of the code base. See
-- RnNames.rnImportDecl for where package trust dependencies for a module are
-- collected and unioned.  Specifically see the Note [RnNames . Tracking Trust
-- Transitively] and the Note [RnNames . Trust Own Package].
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checkSafeImports :: DynFlags -> TcGblEnv -> Hsc TcGblEnv
checkSafeImports dflags tcg_env
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    = do
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        imps <- mapM condense imports'
        let (safeImps, regImps) = partition (\(_,_,s) -> s) imps

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        -- We want to use the warning state specifically for detecting if safe
        -- inference has failed, so store and clear any existing warnings.
        oldErrs <- getWarnings
        clearWarnings

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        -- Check safe imports are correct
        safePkgs <- mapM checkSafe safeImps
        safeErrs <- getWarnings
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        clearWarnings

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        -- Check non-safe imports are correct if inferring safety
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        -- See the Note [Safe Haskell Inference]
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        (infErrs, infPkgs) <- case (safeInferOn dflags) of
          False -> return (emptyBag, [])
          True -> do infPkgs <- mapM checkSafe regImps
                     infErrs <- getWarnings
                     clearWarnings
                     return (infErrs, infPkgs)

        -- restore old errors
        logWarnings oldErrs

        case (isEmptyBag safeErrs) of
          -- Failed safe check
          False -> liftIO . throwIO . mkSrcErr $ safeErrs

          -- Passed safe check
          True -> do
            let infPassed = isEmptyBag infErrs
            tcg_env' <- case (not infPassed) of
              True  -> markUnsafeInfer tcg_env infErrs
              False -> return tcg_env
            when (packageTrustOn dflags) $ checkPkgTrust dflags pkgReqs
            let newTrust = pkgTrustReqs safePkgs infPkgs infPassed
            return tcg_env' { tcg_imports = impInfo `plusImportAvails` newTrust }
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  where
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    impInfo  = tcg_imports tcg_env     -- ImportAvails
    imports  = imp_mods impInfo        -- ImportedMods
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    imports' = moduleEnvToList imports -- (Module, [ImportedModsVal])
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    pkgReqs  = imp_trust_pkgs impInfo  -- [UnitId]
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    condense :: (Module, [ImportedModsVal]) -> Hsc (Module, SrcSpan, IsSafeImport)
    condense (_, [])   = panic "HscMain.condense: Pattern match failure!"