(c) The University of Glasgow 2006-2008
(c) The GRASP/AQUA Project, Glasgow University, 1993-1998

{-# LANGUAGE CPP, NondecreasingIndentation #-}
{-# LANGUAGE MultiWayIf #-}

-- | Module for constructing @ModIface@ values (interface files),
-- writing them to disk and comparing two versions to see if
-- recompilation is required.
module MkIface (


        writeIfaceFile, -- Write the interface file

        checkOldIface,  -- See if recompilation is required, by
                        -- comparing version information
        RecompileRequired(..), recompileRequired,

        tyThingToIfaceDecl -- Converting things to their Iface equivalents
 ) where

          Recompilation checking

A complete description of how recompilation checking works can be
found in the wiki commentary:


Please read the above page for a top-down description of how this all
works.  Notes below cover specific issues related to the implementation.

Basic idea:

  * In the mi_usages information in an interface, we record the
    fingerprint of each free variable of the module

  * In mkIface, we compute the fingerprint of each exported thing A.f.
    For each external thing that A.f refers to, we include the fingerprint
    of the external reference when computing the fingerprint of A.f.  So
    if anything that A.f depends on changes, then A.f's fingerprint will
    Also record any dependent files added with
      * addDependentFile
      * #include
      * -optP-include

  * In checkOldIface we compare the mi_usages for the module with
    the actual fingerprint for all each thing recorded in mi_usages

#include "HsVersions.h"

import GhcPrelude

import IfaceSyn
import BinFingerprint
import LoadIface
import ToIface
import FlagChecker

import DsUsage ( mkUsageInfo, mkUsedNames, mkDependencies )
import Id
import Annotations
import CoreSyn
import Class
import TyCon
import CoAxiom
import ConLike
import DataCon
import Type
import TcType
import InstEnv
import FamInstEnv
import TcRnMonad
import GHC.Hs
import HscTypes
import Finder
import DynFlags
import VarEnv
import Var
import Name
import Avail
import RdrName
import NameEnv
import NameSet
import Module
import BinIface
import ErrUtils
import Digraph
import SrcLoc
import Outputable
import BasicTypes       hiding ( SuccessFlag(..) )
import Unique
import Util             hiding ( eqListBy )
import FastString
import Maybes
import Binary
import Fingerprint
import Exception
import UniqSet
import Packages
import ExtractDocs

import Control.Monad
import Data.Function
import Data.List (find, findIndex, mapAccumL, sortBy, sort)
import qualified Data.Map as Map
import qualified Data.Set as Set
import Data.Ord
import Data.IORef
import System.Directory
import System.FilePath
import Plugins ( PluginRecompile(..), PluginWithArgs(..), LoadedPlugin(..),
                 pluginRecompile', plugins )

--Qualified import so we can define a Semigroup instance
-- but it doesn't clash with Outputable.<>
import qualified Data.Semigroup

*                                                                      *
\subsection{Completing an interface}
*                                                                      *

mkPartialIface :: HscEnv
               -> ModDetails
               -> ModGuts
               -> PartialModIface
mkPartialIface hsc_env mod_details
  ModGuts{ mg_module       = this_mod
         , mg_hsc_src      = hsc_src
         , mg_usages       = usages
         , mg_used_th      = used_th
         , mg_deps         = deps
         , mg_rdr_env      = rdr_env
         , mg_fix_env      = fix_env
         , mg_warns        = warns
         , mg_hpc_info     = hpc_info
         , mg_safe_haskell = safe_mode
         , mg_trust_pkg    = self_trust
         , mg_doc_hdr      = doc_hdr
         , mg_decl_docs    = decl_docs
         , mg_arg_docs     = arg_docs
  = mkIface_ hsc_env this_mod hsc_src used_th deps rdr_env fix_env warns hpc_info self_trust
             safe_mode usages doc_hdr decl_docs arg_docs mod_details

-- | Fully instantiate a interface
-- Adds fingerprints and potentially code generator produced information.
mkFullIface :: HscEnv -> PartialModIface -> IO ModIface
mkFullIface hsc_env partial_iface = do
    full_iface <-
      {-# SCC "addFingerprints" #-}
      addFingerprints hsc_env partial_iface

    -- Debug printing
    dumpIfSet_dyn (hsc_dflags hsc_env) Opt_D_dump_hi "FINAL INTERFACE" (pprModIface full_iface)

    return full_iface

-- | Make an interface from the results of typechecking only.  Useful
-- for non-optimising compilation, or where we aren't generating any
-- object code at all ('HscNothing').
mkIfaceTc :: HscEnv
          -> SafeHaskellMode    -- The safe haskell mode
          -> ModDetails         -- gotten from mkBootModDetails, probably
          -> TcGblEnv           -- Usages, deprecations, etc
          -> IO ModIface
mkIfaceTc hsc_env safe_mode mod_details
  tc_result@TcGblEnv{ tcg_mod = this_mod,
                      tcg_src = hsc_src,
                      tcg_imports = imports,
                      tcg_rdr_env = rdr_env,
                      tcg_fix_env = fix_env,
                      tcg_merged = merged,
                      tcg_warns = warns,
                      tcg_hpc = other_hpc_info,
                      tcg_th_splice_used = tc_splice_used,
                      tcg_dependent_files = dependent_files
  = do
          let used_names = mkUsedNames tc_result
          let pluginModules =
                map lpModule (cachedPlugins (hsc_dflags hsc_env))
          deps <- mkDependencies
                    (thisInstalledUnitId (hsc_dflags hsc_env))
                    (map mi_module pluginModules) tc_result
          let hpc_info = emptyHpcInfo other_hpc_info
          used_th <- readIORef tc_splice_used
          dep_files <- (readIORef dependent_files)
          -- Do NOT use semantic module here; this_mod in mkUsageInfo
          -- is used solely to decide if we should record a dependency
          -- or not.  When we instantiate a signature, the semantic
          -- module is something we want to record dependencies for,
          -- but if you pass that in here, we'll decide it's the local
          -- module and does not need to be recorded as a dependency.
          -- See Note [Identity versus semantic module]
          usages <- mkUsageInfo hsc_env this_mod (imp_mods imports) used_names
                      dep_files merged pluginModules

          let (doc_hdr', doc_map, arg_map) = extractDocs tc_result

          let partial_iface = mkIface_ hsc_env
                   this_mod hsc_src
                   used_th deps rdr_env
                   fix_env warns hpc_info
                   (imp_trust_own_pkg imports) safe_mode usages
                   doc_hdr' doc_map arg_map

          mkFullIface hsc_env partial_iface

mkIface_ :: HscEnv -> Module -> HscSource
         -> Bool -> Dependencies -> GlobalRdrEnv
         -> NameEnv FixItem -> Warnings -> HpcInfo
         -> Bool
         -> SafeHaskellMode
         -> [Usage]
         -> Maybe HsDocString
         -> DeclDocMap
         -> ArgDocMap
         -> ModDetails
         -> PartialModIface
mkIface_ hsc_env
         this_mod hsc_src used_th deps rdr_env fix_env src_warns
         hpc_info pkg_trust_req safe_mode usages
         doc_hdr decl_docs arg_docs
         ModDetails{  md_insts     = insts,
                      md_fam_insts = fam_insts,
                      md_rules     = rules,
                      md_anns      = anns,
                      md_types     = type_env,
                      md_exports   = exports,
                      md_complete_sigs = complete_sigs }
-- NB:  notice that mkIface does not look at the bindings
--      only at the TypeEnv.  The previous Tidy phase has
--      put exactly the info into the TypeEnv that we want
--      to expose in the interface

  = do
    let semantic_mod = canonicalizeHomeModule (hsc_dflags hsc_env) (moduleName this_mod)
        entities = typeEnvElts type_env
        decls  = [ tyThingToIfaceDecl entity
                 | entity <- entities,
                   let name = getName entity,
                   not (isImplicitTyThing entity),
                      -- No implicit Ids and class tycons in the interface file
                   not (isWiredInName name),
                      -- Nor wired-in things; the compiler knows about them anyhow
                   nameIsLocalOrFrom semantic_mod name  ]
                      -- Sigh: see Note [Root-main Id] in TcRnDriver
                      -- NB: ABSOLUTELY need to check against semantic_mod,
                      -- because all of the names in an hsig p[H=<H>]:H
                      -- are going to be for <H>, not the former id!
                      -- See Note [Identity versus semantic module]

        fixities    = sortBy (comparing fst)
          [(occ,fix) | FixItem occ fix <- nameEnvElts fix_env]
          -- The order of fixities returned from nameEnvElts is not
          -- deterministic, so we sort by OccName to canonicalize it.
          -- See Note [Deterministic UniqFM] in UniqDFM for more details.
        warns       = src_warns
        iface_rules = map coreRuleToIfaceRule rules
        iface_insts = map instanceToIfaceInst $ fixSafeInstances safe_mode insts
        iface_fam_insts = map famInstToIfaceFamInst fam_insts
        trust_info  = setSafeMode safe_mode
        annotations = map mkIfaceAnnotation anns
        icomplete_sigs = map mkIfaceCompleteSig complete_sigs

    ModIface {
          mi_module      = this_mod,
          -- Need to record this because it depends on the -instantiated-with flag
          -- which could change
          mi_sig_of      = if semantic_mod == this_mod
                            then Nothing
                            else Just semantic_mod,
          mi_hsc_src     = hsc_src,
          mi_deps        = deps,
          mi_usages      = usages,
          mi_exports     = mkIfaceExports exports,

          -- Sort these lexicographically, so that
          -- the result is stable across compilations
          mi_insts       = sortBy cmp_inst     iface_insts,
          mi_fam_insts   = sortBy cmp_fam_inst iface_fam_insts,
          mi_rules       = sortBy cmp_rule     iface_rules,

          mi_fixities    = fixities,
          mi_warns       = warns,
          mi_anns        = annotations,
          mi_globals     = maybeGlobalRdrEnv rdr_env,
          mi_used_th     = used_th,
          mi_decls       = decls,
          mi_hpc         = isHpcUsed hpc_info,
          mi_trust       = trust_info,
          mi_trust_pkg   = pkg_trust_req,
          mi_complete_sigs = icomplete_sigs,
          mi_doc_hdr     = doc_hdr,
          mi_decl_docs   = decl_docs,
          mi_arg_docs    = arg_docs,
          mi_final_exts        = () }
     cmp_rule     = comparing ifRuleName
     -- Compare these lexicographically by OccName, *not* by unique,
     -- because the latter is not stable across compilations:
     cmp_inst     = comparing (nameOccName . ifDFun)
     cmp_fam_inst = comparing (nameOccName . ifFamInstTcName)

     dflags = hsc_dflags hsc_env

     -- We only fill in mi_globals if the module was compiled to byte
     -- code.  Otherwise, the compiler may not have retained all the
     -- top-level bindings and they won't be in the TypeEnv (see
     -- Desugar.addExportFlagsAndRules).  The mi_globals field is used
     -- by GHCi to decide whether the module has its full top-level
     -- scope available. (#5534)
     maybeGlobalRdrEnv :: GlobalRdrEnv -> Maybe GlobalRdrEnv
     maybeGlobalRdrEnv rdr_env
         | targetRetainsAllBindings (hscTarget dflags) = Just rdr_env
         | otherwise                                   = Nothing

     ifFamInstTcName = ifFamInstFam

writeIfaceFile :: DynFlags -> FilePath -> ModIface -> IO ()
writeIfaceFile dflags hi_file_path new_iface
    = do createDirectoryIfMissing True (takeDirectory hi_file_path)
         writeBinIface dflags hi_file_path new_iface

-- -----------------------------------------------------------------------------
-- Look up parents and versions of Names

-- This is like a global version of the mi_hash_fn field in each ModIface.
-- Given a Name, it finds the ModIface, and then uses mi_hash_fn to get
-- the parent and version info.

        :: HscEnv                       -- needed to look up versions
        -> ExternalPackageState         -- ditto
        -> (Name -> IO Fingerprint)
mkHashFun hsc_env eps name
  | isHoleModule orig_mod
  = lookup (mkModule (thisPackage dflags) (moduleName orig_mod))
  | otherwise
  = lookup orig_mod
      dflags = hsc_dflags hsc_env
      hpt = hsc_HPT hsc_env
      pit = eps_PIT eps
      occ = nameOccName name
      orig_mod = nameModule name
      lookup mod = do
        MASSERT2( isExternalName name, ppr name )
        iface <- case lookupIfaceByModule hpt pit mod of
                  Just iface -> return iface
                  Nothing -> do
                      -- This can occur when we're writing out ifaces for
                      -- requirements; we didn't do any /real/ typechecking
                      -- so there's no guarantee everything is loaded.
                      -- Kind of a heinous hack.
                      iface <- initIfaceLoad hsc_env . withException
                            $ loadInterface (text "lookupVers2") mod ImportBySystem
                      return iface
        return $ snd (mi_hash_fn (mi_final_exts iface) occ `orElse`
                  pprPanic "lookupVers1" (ppr mod <+> ppr occ))

-- ---------------------------------------------------------------------------
-- Compute fingerprints for the interface

Note [Fingerprinting IfaceDecls]

The general idea here is that we first examine the 'IfaceDecl's and determine
the recursive groups of them. We then walk these groups in dependency order,
serializing each contained 'IfaceDecl' to a "Binary" buffer which we then
hash using MD5 to produce a fingerprint for the group.

However, the serialization that we use is a bit funny: we override the @putName@
operation with our own which serializes the hash of a 'Name' instead of the
'Name' itself. This ensures that the fingerprint of a decl changes if anything
in its transitive closure changes. This trick is why we must be careful about
traversing in dependency order: we need to ensure that we have hashes for
everything referenced by the decl which we are fingerprinting.

Moreover, we need to be careful to distinguish between serialization of binding
Names (e.g. the ifName field of a IfaceDecl) and non-binding (e.g. the ifInstCls
field of a IfaceClsInst): only in the non-binding case should we include the
fingerprint; in the binding case we shouldn't since it is merely the name of the
thing that we are currently fingerprinting.

-- | Add fingerprints for top-level declarations to a 'ModIface'.
-- See Note [Fingerprinting IfaceDecls]
        :: HscEnv
        -> PartialModIface
        -> IO ModIface
addFingerprints hsc_env iface0
 = do
   eps <- hscEPS hsc_env
       decls = mi_decls iface0
       warn_fn = mkIfaceWarnCache (mi_warns iface0)
       fix_fn = mkIfaceFixCache (mi_fixities iface0)

        -- The ABI of a declaration represents everything that is made
        -- visible about the declaration that a client can depend on.
        -- see IfaceDeclABI below.
       declABI :: IfaceDecl -> IfaceDeclABI
       -- TODO: I'm not sure if this should be semantic_mod or this_mod.
       -- See also Note [Identity versus semantic module]
       declABI decl = (this_mod, decl, extras)
        where extras = declExtras fix_fn ann_fn non_orph_rules non_orph_insts
                                  non_orph_fis top_lvl_name_env decl

       -- This is used for looking up the Name of a default method
       -- from its OccName. See Note [default method Name]
       top_lvl_name_env =
         mkOccEnv [ (nameOccName nm, nm)
                  | IfaceId { ifName = nm } <- decls ]

       -- Dependency edges between declarations in the current module.
       -- This is computed by finding the free external names of each
       -- declaration, including IfaceDeclExtras (things that a
       -- declaration implicitly depends on).
       edges :: [ Node Unique IfaceDeclABI ]
       edges = [ DigraphNode abi (getUnique (getOccName decl)) out
               | decl <- decls
               , let abi = declABI decl
               , let out = localOccs $ freeNamesDeclABI abi

       name_module n = ASSERT2( isExternalName n, ppr n ) nameModule n
       localOccs =
         map (getUnique . getParent . getOccName)
                        -- NB: names always use semantic module, so
                        -- filtering must be on the semantic module!
                        -- See Note [Identity versus semantic module]
                        . filter ((== semantic_mod) . name_module)
                        . nonDetEltsUniqSet
                   -- It's OK to use nonDetEltsUFM as localOccs is only
                   -- used to construct the edges and
                   -- stronglyConnCompFromEdgedVertices is deterministic
                   -- even with non-deterministic order of edges as
                   -- explained in Note [Deterministic SCC] in Digraph.
          where getParent :: OccName -> OccName
                getParent occ = lookupOccEnv parent_map occ `orElse` occ

        -- maps OccNames to their parents in the current module.
        -- e.g. a reference to a constructor must be turned into a reference
        -- to the TyCon for the purposes of calculating dependencies.
       parent_map :: OccEnv OccName
       parent_map = foldl' extend emptyOccEnv decls
          where extend env d =
                  extendOccEnvList env [ (b,n) | b <- ifaceDeclImplicitBndrs d ]
                  where n = getOccName d

        -- Strongly-connected groups of declarations, in dependency order
       groups :: [SCC IfaceDeclABI]
       groups = stronglyConnCompFromEdgedVerticesUniq edges

       global_hash_fn = mkHashFun hsc_env eps

        -- How to output Names when generating the data to fingerprint.
        -- Here we want to output the fingerprint for each top-level
        -- Name, whether it comes from the current module or another
        -- module.  In this way, the fingerprint for a declaration will
        -- change if the fingerprint for anything it refers to (transitively)
        -- changes.
       mk_put_name :: OccEnv (OccName,Fingerprint)
                   -> BinHandle -> Name -> IO  ()
       mk_put_name local_env bh name
          | isWiredInName name  =  putNameLiterally bh name
           -- wired-in names don't have fingerprints
          | otherwise
          = ASSERT2( isExternalName name, ppr name )
            let hash | nameModule name /= semantic_mod =  global_hash_fn name
                     -- Get it from the REAL interface!!
                     -- This will trigger when we compile an hsig file
                     -- and we know a backing impl for it.
                     -- See Note [Identity versus semantic module]
                     | semantic_mod /= this_mod
                     , not (isHoleModule semantic_mod) = global_hash_fn name
                     | otherwise = return (snd (lookupOccEnv local_env (getOccName name)
                           `orElse` pprPanic "urk! lookup local fingerprint"
                                       (ppr name $$ ppr local_env)))
                -- This panic indicates that we got the dependency
                -- analysis wrong, because we needed a fingerprint for
                -- an entity that wasn't in the environment.  To debug
                -- it, turn the panic into a trace, uncomment the
                -- pprTraces below, run the compile again, and inspect
                -- the output and the generated .hi file with
                -- --show-iface.
            in hash >>= put_ bh

        -- take a strongly-connected group of declarations and compute
        -- its fingerprint.

       fingerprint_group :: (OccEnv (OccName,Fingerprint),
                         -> SCC IfaceDeclABI
                         -> IO (OccEnv (OccName,Fingerprint),

       fingerprint_group (local_env, decls_w_hashes) (AcyclicSCC abi)
          = do let hash_fn = mk_put_name local_env
                   decl = abiDecl abi
               --pprTrace "fingerprinting" (ppr (ifName decl) ) $ do
               hash <- computeFingerprint hash_fn abi
               env' <- extend_hash_env local_env (hash,decl)
               return (env', (hash,decl) : decls_w_hashes)

       fingerprint_group (local_env, decls_w_hashes) (CyclicSCC abis)
          = do let decls = map abiDecl abis
               local_env1 <- foldM extend_hash_env local_env
                                   (zip (repeat fingerprint0) decls)
               let hash_fn = mk_put_name local_env1
               -- pprTrace "fingerprinting" (ppr (map ifName decls) ) $ do
               let stable_abis = sortBy cmp_abiNames abis
                -- put the cycle in a canonical order
               hash <- computeFingerprint hash_fn stable_abis
               let pairs = zip (repeat hash) decls
               local_env2 <- foldM extend_hash_env local_env pairs
               return (local_env2, pairs ++ decls_w_hashes)

       -- we have fingerprinted the whole declaration, but we now need
       -- to assign fingerprints to all the OccNames that it binds, to
       -- use when referencing those OccNames in later declarations.
       extend_hash_env :: OccEnv (OccName,Fingerprint)
                       -> (Fingerprint,IfaceDecl)
                       -> IO (OccEnv (OccName,Fingerprint))
       extend_hash_env env0 (hash,d) = do
          return (foldr (\(b,fp) env -> extendOccEnv env b (b,fp)) env0
                 (ifaceDeclFingerprints hash d))

   (local_env, decls_w_hashes) <-
       foldM fingerprint_group (emptyOccEnv, []) groups

   -- when calculating fingerprints, we always need to use canonical
   -- ordering for lists of things.  In particular, the mi_deps has various
   -- lists of modules and suchlike, so put these all in canonical order:
   let sorted_deps = sortDependencies (mi_deps iface0)

   -- The export hash of a module depends on the orphan hashes of the
   -- orphan modules below us in the dependency tree.  This is the way
   -- that changes in orphans get propagated all the way up the
   -- dependency tree.
   -- Note [A bad dep_orphs optimization]
   -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   -- In a previous version of this code, we filtered out orphan modules which
   -- were not from the home package, justifying it by saying that "we'd
   -- pick up the ABI hashes of the external module instead".  This is wrong.
   -- Suppose that we have:
   --       module External where
   --           instance Show (a -> b)
   --       module Home1 where
   --           import External
   --       module Home2 where
   --           import Home1
   -- The export hash of Home1 needs to reflect the orphan instances of
   -- External. It's true that Home1 will get rebuilt if the orphans
   -- of External, but we also need to make sure Home2 gets rebuilt
   -- as well.  See #12733 for more details.
   let orph_mods
        = filter (/= this_mod) -- Note [Do not update EPS with your own hi-boot]
        $ dep_orphs sorted_deps
   dep_orphan_hashes <- getOrphanHashes hsc_env orph_mods

   -- Note [Do not update EPS with your own hi-boot]
   -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   -- (See also #10182).  When your hs-boot file includes an orphan
   -- instance declaration, you may find that the dep_orphs of a module you
   -- import contains reference to yourself.  DO NOT actually load this module
   -- or add it to the orphan hashes: you're going to provide the orphan
   -- instances yourself, no need to consult hs-boot; if you do load the
   -- interface into EPS, you will see a duplicate orphan instance.

   orphan_hash <- computeFingerprint (mk_put_name local_env)
                                     (map ifDFun orph_insts, orph_rules, orph_fis)

   -- the export list hash doesn't depend on the fingerprints of
   -- the Names it mentions, only the Names themselves, hence putNameLiterally.
   export_hash <- computeFingerprint putNameLiterally
                      (mi_exports iface0,
                       dep_pkgs (mi_deps iface0),
                       -- See Note [Export hash depends on non-orphan family instances]
                       dep_finsts (mi_deps iface0),
                        -- dep_pkgs: see "Package Version Changes" on
                        -- wiki/commentary/compiler/recompilation-avoidance
                       mi_trust iface0)
                        -- Make sure change of Safe Haskell mode causes recomp.

   -- Note [Export hash depends on non-orphan family instances]
   -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   -- Suppose we have:
   --   module A where
   --       type instance F Int = Bool
   --   module B where
   --       import A
   --   module C where
   --       import B
   -- The family instance consistency check for C depends on the dep_finsts of
   -- B.  If we rename module A to A2, when the dep_finsts of B changes, we need
   -- to make sure that C gets rebuilt. Effectively, the dep_finsts are part of
   -- the exports of B, because C always considers them when checking
   -- consistency.
   -- A full discussion is in #12723.
   -- We do NOT need to hash dep_orphs, because this is implied by
   -- dep_orphan_hashes, and we do not need to hash ordinary class instances,
   -- because there is no eager consistency check as there is with type families
   -- (also we didn't store it anywhere!)

   -- put the declarations in a canonical order, sorted by OccName
   let sorted_decls = Map.elems $ Map.fromList $
                          [(getOccName d, e) | e@(_, d) <- decls_w_hashes]

   -- the flag hash depends on:
   --   - (some of) dflags
   -- it returns two hashes, one that shouldn't change
   -- the abi hash and one that should
   flag_hash <- fingerprintDynFlags dflags this_mod putNameLiterally

   opt_hash <- fingerprintOptFlags dflags putNameLiterally

   hpc_hash <- fingerprintHpcFlags dflags putNameLiterally

   plugin_hash <- fingerprintPlugins hsc_env

   -- the ABI hash depends on:
   --   - decls
   --   - export list
   --   - orphans
   --   - deprecations
   --   - flag abi hash
   mod_hash <- computeFingerprint putNameLiterally
                      (map fst sorted_decls,
                       export_hash,  -- includes orphan_hash
                       mi_warns iface0)

   -- The interface hash depends on:
   --   - the ABI hash, plus
   --   - the module level annotations,
   --   - usages
   --   - deps (home and external packages, dependent files)
   --   - hpc
   iface_hash <- computeFingerprint putNameLiterally
                       ann_fn (mkVarOcc "module"),  -- See mkIfaceAnnCache
                       mi_usages iface0,
                       mi_hpc iface0)

    final_iface_exts = ModIfaceBackend
      { mi_iface_hash  = iface_hash
      , mi_mod_hash    = mod_hash
      , mi_flag_hash   = flag_hash
      , mi_opt_hash    = opt_hash
      , mi_hpc_hash    = hpc_hash
      , mi_plugin_hash = plugin_hash
      , mi_orphan      = not (   all ifRuleAuto orph_rules
                                   -- See Note [Orphans and auto-generated rules]
                              && null orph_insts
                              && null orph_fis)
      , mi_finsts      = not (null (mi_fam_insts iface0))
      , mi_exp_hash    = export_hash
      , mi_orphan_hash = orphan_hash
      , mi_warn_fn     = warn_fn
      , mi_fix_fn      = fix_fn
      , mi_hash_fn     = lookupOccEnv local_env
    final_iface = iface0 { mi_decls = sorted_decls, mi_final_exts = final_iface_exts }
   return final_iface

    this_mod = mi_module iface0
    semantic_mod = mi_semantic_module iface0
    dflags = hsc_dflags hsc_env
    (non_orph_insts, orph_insts) = mkOrphMap ifInstOrph    (mi_insts iface0)
    (non_orph_rules, orph_rules) = mkOrphMap ifRuleOrph    (mi_rules iface0)
    (non_orph_fis,   orph_fis)   = mkOrphMap ifFamInstOrph (mi_fam_insts iface0)
    ann_fn = mkIfaceAnnCache (mi_anns iface0)

-- | Retrieve the orphan hashes 'mi_orphan_hash' for a list of modules
-- (in particular, the orphan modules which are transitively imported by the
-- current module).
-- Q: Why do we need the hash at all, doesn't the list of transitively
-- imported orphan modules suffice?
-- A: If one of our transitive imports adds a new orphan instance, our
-- export hash must change so that modules which import us rebuild.  If we just
-- hashed the [Module], the hash would not change even when a new instance was
-- added to a module that already had an orphan instance.
-- Q: Why don't we just hash the orphan hashes of our direct dependencies?
-- Why the full transitive closure?
-- A: Suppose we have these modules:
--      module A where
--          instance Show (a -> b) where
--      module B where
--          import A -- **
--      module C where
--          import A
--          import B
-- Whether or not we add or remove the import to A in B affects the
-- orphan hash of B.  But it shouldn't really affect the orphan hash
-- of C.  If we hashed only direct dependencies, there would be no
-- way to tell that the net effect was a wash, and we'd be forced
-- to recompile C and everything else.
getOrphanHashes :: HscEnv -> [Module] -> IO [Fingerprint]
getOrphanHashes hsc_env mods = do
  eps <- hscEPS hsc_env
    hpt        = hsc_HPT hsc_env
    pit        = eps_PIT eps
    get_orph_hash mod =
          case lookupIfaceByModule hpt pit mod of
            Just iface -> return (mi_orphan_hash (mi_final_exts iface))
            Nothing    -> do -- similar to 'mkHashFun'
                iface <- initIfaceLoad hsc_env . withException
                            $ loadInterface (text "getOrphanHashes") mod ImportBySystem
                return (mi_orphan_hash (mi_final_exts iface))

  mapM get_orph_hash mods

sortDependencies :: Dependencies -> Dependencies
sortDependencies d
 = Deps { dep_mods   = sortBy (compare `on` (moduleNameFS.fst)) (dep_mods d),
          dep_pkgs   = sortBy (compare `on` fst) (dep_pkgs d),
          dep_orphs  = sortBy stableModuleCmp (dep_orphs d),
          dep_finsts = sortBy stableModuleCmp (dep_finsts d),
          dep_plgins = sortBy (compare `on` moduleNameFS) (dep_plgins d) }

-- | Creates cached lookup for the 'mi_anns' field of ModIface
-- Hackily, we use "module" as the OccName for any module-level annotations
mkIfaceAnnCache :: [IfaceAnnotation] -> OccName -> [AnnPayload]
mkIfaceAnnCache anns
  = \n -> lookupOccEnv env n `orElse` []
    pair (IfaceAnnotation target value) =
      (case target of
          NamedTarget occn -> occn
          ModuleTarget _   -> mkVarOcc "module"
      , [value])
    -- flipping (++), so the first argument is always short
    env = mkOccEnv_C (flip (++)) (map pair anns)

*                                                                      *
          The ABI of an IfaceDecl
*                                                                      *

Note [The ABI of an IfaceDecl]
The ABI of a declaration consists of:

   (a) the full name of the identifier (inc. module and package,
       because these are used to construct the symbol name by which
       the identifier is known externally).

   (b) the declaration itself, as exposed to clients.  That is, the
       definition of an Id is included in the fingerprint only if
       it is made available as an unfolding in the interface.

   (c) the fixity of the identifier (if it exists)
   (d) for Ids: rules
   (e) for classes: instances, fixity & rules for methods
   (f) for datatypes: instances, fixity & rules for constrs

Items (c)-(f) are not stored in the IfaceDecl, but instead appear
elsewhere in the interface file.  But they are *fingerprinted* with
the declaration itself. This is done by grouping (c)-(f) in IfaceDeclExtras,
and fingerprinting that as part of the declaration.

type IfaceDeclABI = (Module, IfaceDecl, IfaceDeclExtras)

data IfaceDeclExtras
  = IfaceIdExtras IfaceIdExtras

  | IfaceDataExtras
       (Maybe Fixity)           -- Fixity of the tycon itself (if it exists)
       [IfaceInstABI]           -- Local class and family instances of this tycon
                                -- See Note [Orphans] in InstEnv
       [AnnPayload]             -- Annotations of the type itself
       [IfaceIdExtras]          -- For each constructor: fixity, RULES and annotations

  | IfaceClassExtras
       (Maybe Fixity)           -- Fixity of the class itself (if it exists)
       [IfaceInstABI]           -- Local instances of this class *or*
                                --   of its associated data types
                                -- See Note [Orphans] in InstEnv
       [AnnPayload]             -- Annotations of the type itself
       [IfaceIdExtras]          -- For each class method: fixity, RULES and annotations
       [IfExtName]              -- Default methods. If a module
                                -- mentions a class, then it can
                                -- instantiate the class and thereby
                                -- use the default methods, so we must
                                -- include these in the fingerprint of
                                -- a class.

  | IfaceSynonymExtras (Maybe Fixity) [AnnPayload]

  | IfaceFamilyExtras   (Maybe Fixity) [IfaceInstABI] [AnnPayload]

  | IfaceOtherDeclExtras

data IfaceIdExtras
  = IdExtras
       (Maybe Fixity)           -- Fixity of the Id (if it exists)
       [IfaceRule]              -- Rules for the Id
       [AnnPayload]             -- Annotations for the Id

-- When hashing a class or family instance, we hash only the
-- DFunId or CoAxiom, because that depends on all the
-- information about the instance.
type IfaceInstABI = IfExtName   -- Name of DFunId or CoAxiom that is evidence for the instance

abiDecl :: IfaceDeclABI -> IfaceDecl
abiDecl (_, decl, _) = decl

cmp_abiNames :: IfaceDeclABI -> IfaceDeclABI -> Ordering
cmp_abiNames abi1 abi2 = getOccName (abiDecl abi1) `compare`
                         getOccName (abiDecl abi2)

freeNamesDeclABI :: IfaceDeclABI -> NameSet
freeNamesDeclABI (_mod, decl, extras) =
  freeNamesIfDecl decl `unionNameSet` freeNamesDeclExtras extras

freeNamesDeclExtras :: IfaceDeclExtras -> NameSet
freeNamesDeclExtras (IfaceIdExtras id_extras)
  = freeNamesIdExtras id_extras
freeNamesDeclExtras (IfaceDataExtras  _ insts _ subs)
  = unionNameSets (mkNameSet insts : map freeNamesIdExtras subs)
freeNamesDeclExtras (IfaceClassExtras _ insts _ subs defms)
  = unionNameSets $
      mkNameSet insts : mkNameSet defms : map freeNamesIdExtras subs
freeNamesDeclExtras (IfaceSynonymExtras _ _)
  = emptyNameSet
freeNamesDeclExtras (IfaceFamilyExtras _ insts _)
  = mkNameSet insts
freeNamesDeclExtras IfaceOtherDeclExtras
  = emptyNameSet

freeNamesIdExtras :: IfaceIdExtras -> NameSet
freeNamesIdExtras (IdExtras _ rules _) = unionNameSets (map freeNamesIfRule rules)

instance Outputable IfaceDeclExtras where
  ppr IfaceOtherDeclExtras       = Outputable.empty
  ppr (IfaceIdExtras  extras)    = ppr_id_extras extras
  ppr (IfaceSynonymExtras fix anns) = vcat [ppr fix, ppr anns]
  ppr (IfaceFamilyExtras fix finsts anns) = vcat [ppr fix, ppr finsts, ppr anns]
  ppr (IfaceDataExtras fix insts anns stuff) = vcat [ppr fix, ppr_insts insts, ppr anns,
                                                ppr_id_extras_s stuff]
  ppr (IfaceClassExtras fix insts anns stuff defms) =
    vcat [ppr fix, ppr_insts insts, ppr anns,
          ppr_id_extras_s stuff, ppr defms]

ppr_insts :: [IfaceInstABI] -> SDoc
ppr_insts _ = text "<insts>"

ppr_id_extras_s :: [IfaceIdExtras] -> SDoc
ppr_id_extras_s stuff = vcat (map ppr_id_extras stuff)

ppr_id_extras :: IfaceIdExtras -> SDoc
ppr_id_extras (IdExtras fix rules anns) = ppr fix $$ vcat (map ppr rules) $$ vcat (map ppr anns)

-- This instance is used only to compute fingerprints
instance Binary IfaceDeclExtras where
  get _bh = panic "no get for IfaceDeclExtras"
  put_ bh (IfaceIdExtras extras) = do
   putByte bh 1; put_ bh extras
  put_ bh (IfaceDataExtras fix insts anns cons) = do
   putByte bh 2; put_ bh fix; put_ bh insts; put_ bh anns; put_ bh cons
  put_ bh (IfaceClassExtras fix insts anns methods defms) = do
   putByte bh 3
   put_ bh fix
   put_ bh insts
   put_ bh anns
   put_ bh methods
   put_ bh defms
  put_ bh (IfaceSynonymExtras fix anns) = do
   putByte bh 4; put_ bh fix; put_ bh anns
  put_ bh (IfaceFamilyExtras fix finsts anns) = do
   putByte bh 5; put_ bh fix; put_ bh finsts; put_ bh anns
  put_ bh IfaceOtherDeclExtras = putByte bh 6

instance Binary IfaceIdExtras where
  get _bh = panic "no get for IfaceIdExtras"
  put_ bh (IdExtras fix rules anns)= do { put_ bh fix; put_ bh rules; put_ bh anns }

declExtras :: (OccName -> Maybe Fixity)
           -> (OccName -> [AnnPayload])
           -> OccEnv [IfaceRule]
           -> OccEnv [IfaceClsInst]
           -> OccEnv [IfaceFamInst]
           -> OccEnv IfExtName          -- lookup default method names
           -> IfaceDecl
           -> IfaceDeclExtras

declExtras fix_fn ann_fn rule_env inst_env fi_env dm_env decl
  = case decl of
      IfaceId{} -> IfaceIdExtras (id_extras n)
      IfaceData{ifCons=cons} ->
                     IfaceDataExtras (fix_fn n)
                        (map ifFamInstAxiom (lookupOccEnvL fi_env n) ++
                         map ifDFun         (lookupOccEnvL inst_env n))
                        (ann_fn n)
                        (map (id_extras . occName . ifConName) (visibleIfConDecls cons))
      IfaceClass{ifBody = IfConcreteClass { ifSigs=sigs, ifATs=ats }} ->
                     IfaceClassExtras (fix_fn n) insts (ann_fn n) meths defms
            insts = (map ifDFun $ (concatMap at_extras ats)
                                    ++ lookupOccEnvL inst_env n)
                           -- Include instances of the associated types
                           -- as well as instances of the class (#5147)
            meths = [id_extras (getOccName op) | IfaceClassOp op _ _ <- sigs]
            -- Names of all the default methods (see Note [default method Name])
            defms = [ dmName
                    | IfaceClassOp bndr _ (Just _) <- sigs
                    , let dmOcc = mkDefaultMethodOcc (nameOccName bndr)
                    , Just dmName <- [lookupOccEnv dm_env dmOcc] ]
      IfaceSynonym{} -> IfaceSynonymExtras (fix_fn n)
                                           (ann_fn n)
      IfaceFamily{} -> IfaceFamilyExtras (fix_fn n)
                        (map ifFamInstAxiom (lookupOccEnvL fi_env n))
                        (ann_fn n)
      _other -> IfaceOtherDeclExtras
        n = getOccName decl
        id_extras occ = IdExtras (fix_fn occ) (lookupOccEnvL rule_env occ) (ann_fn occ)
        at_extras (IfaceAT decl _) = lookupOccEnvL inst_env (getOccName decl)

{- Note [default method Name] (see also #15970)

The Names for the default methods aren't available in the IfaceSyn.

* We originally start with a DefMethInfo from the class, contain a
  Name for the default method

* We turn that into IfaceSyn as a DefMethSpec which lacks a Name
  entirely. Why? Because the Name can be derived from the method name
  (in TcIface), so doesn't need to be serialised into the interface

But now we have to get the Name back, because the class declaration's
fingerprint needs to depend on it (this was the bug in #15970).  This
is done in a slightly convoluted way:

* Then, in addFingerprints we build a map that maps OccNames to Names

* We pass that map to declExtras which laboriously looks up in the map
  (using the derived occurrence name) to recover the Name we have just
  thrown away.

lookupOccEnvL :: OccEnv [v] -> OccName -> [v]
lookupOccEnvL env k = lookupOccEnv env k `orElse` []

-- for testing: use the md5sum command to generate fingerprints and
-- compare the results against our built-in version.
  fp' <- oldMD5 dflags bh
  if fp /= fp' then pprPanic "computeFingerprint" (ppr fp <+> ppr fp')
               else return fp

oldMD5 dflags bh = do
  tmp <- newTempName dflags CurrentModule "bin"
  writeBinMem bh tmp
  tmp2 <- newTempName dflags CurrentModule "md5"
  let cmd = "md5sum " ++ tmp ++ " >" ++ tmp2
  r <- system cmd
  case r of
    ExitFailure _ -> throwGhcExceptionIO (PhaseFailed cmd r)
    ExitSuccess -> do
        hash_str <- readFile tmp2
        return $! readHexFingerprint hash_str

-- mkOrphMap partitions instance decls or rules into
--      (a) an OccEnv for ones that are not orphans,
--          mapping the local OccName to a list of its decls
--      (b) a list of orphan decls
mkOrphMap :: (decl -> IsOrphan) -- Extract orphan status from decl
          -> [decl]             -- Sorted into canonical order
          -> (OccEnv [decl],    -- Non-orphan decls associated with their key;
                                --      each sublist in canonical order
              [decl])           -- Orphan decls; in canonical order
mkOrphMap get_key decls
  = foldl' go (emptyOccEnv, []) decls
    go (non_orphs, orphs) d
        | NotOrphan occ <- get_key d
        = (extendOccEnv_Acc (:) singleton non_orphs occ d, orphs)
        | otherwise = (non_orphs, d:orphs)

*                                                                      *
       COMPLETE Pragmas
*                                                                      *

mkIfaceCompleteSig :: CompleteMatch -> IfaceCompleteMatch
mkIfaceCompleteSig (CompleteMatch cls tc) = IfaceCompleteMatch cls tc

*                                                                      *
       Keeping track of what we've slurped, and fingerprints
*                                                                      *

mkIfaceAnnotation :: Annotation -> IfaceAnnotation
mkIfaceAnnotation (Annotation { ann_target = target, ann_value = payload })
  = IfaceAnnotation {
        ifAnnotatedTarget = fmap nameOccName target,
        ifAnnotatedValue = payload

mkIfaceExports :: [AvailInfo] -> [IfaceExport]  -- Sort to make canonical
mkIfaceExports exports
  = sortBy stableAvailCmp (map sort_subs exports)
    sort_subs :: AvailInfo -> AvailInfo
    sort_subs (Avail n) = Avail n
    sort_subs (AvailTC n [] fs) = AvailTC n [] (sort_flds fs)
    sort_subs (AvailTC n (m:ms) fs)
       | n==m      = AvailTC n (m:sortBy stableNameCmp ms) (sort_flds fs)
       | otherwise = AvailTC n (sortBy stableNameCmp (m:ms)) (sort_flds fs)
       -- Maintain the AvailTC Invariant

    sort_flds = sortBy (stableNameCmp `on` flSelector)

Note [Original module]
Consider this:
        module X where { data family T }
        module Y( T(..) ) where { import X; data instance T Int = MkT Int }
The exported Avail from Y will look like
        X.T{X.T, Y.MkT}
That is, in Y,
  - only MkT is brought into scope by the data instance;
  - but the parent (used for grouping and naming in T(..) exports) is X.T
  - and in this case we export X.T too

In the result of MkIfaceExports, the names are grouped by defining module,
so we may need to split up a single Avail into multiple ones.

Note [Internal used_names]
Most of the used_names are External Names, but we can have Internal
Names too: see Note [Binders in Template Haskell] in Convert, and
#5362 for an example.  Such Names are always
  - Such Names are always for locally-defined things, for which we
    don't gather usage info, so we can just ignore them in ent_map
  - They are always System Names, hence the assert, just as a double check.

*                                                                      *
        Load the old interface file for this module (unless
        we have it already), and check whether it is up to date
*                                                                      *

data RecompileRequired
  = UpToDate
       -- ^ everything is up to date, recompilation is not required
  | MustCompile
       -- ^ The .hs file has been touched, or the .o/.hi file does not exist
  | RecompBecause String
       -- ^ The .o/.hi files are up to date, but something else has changed
       -- to force recompilation; the String says what (one-line summary)
   deriving Eq

instance Semigroup RecompileRequired where
  UpToDate <> r = r
  mc <> _       = mc

instance Monoid RecompileRequired where
  mempty = UpToDate

recompileRequired :: RecompileRequired -> Bool
recompileRequired UpToDate = False
recompileRequired _ = True

-- | Top level function to check if the version of an old interface file
-- is equivalent to the current source file the user asked us to compile.
-- If the same, we can avoid recompilation. We return a tuple where the
-- first element is a bool saying if we should recompile the object file
-- and the second is maybe the interface file, where Nothing means to
-- rebuild the interface file and not use the existing one.
  :: HscEnv
  -> ModSummary
  -> SourceModified
  -> Maybe ModIface         -- Old interface from compilation manager, if any
  -> IO (RecompileRequired, Maybe ModIface)

checkOldIface hsc_env mod_summary source_modified maybe_iface
  = do  let dflags = hsc_dflags hsc_env
        showPass dflags $
            "Checking old interface for " ++
              (showPpr dflags $ ms_mod mod_summary) ++
              " (use -ddump-hi-diffs for more details)"
        initIfaceCheck (text "checkOldIface") hsc_env $
            check_old_iface hsc_env mod_summary source_modified maybe_iface

  :: HscEnv
  -> ModSummary
  -> SourceModified
  -> Maybe ModIface
  -> IfG (RecompileRequired, Maybe ModIface)

check_old_iface hsc_env mod_summary src_modified maybe_iface
  = let dflags = hsc_dflags hsc_env
        getIface =
            case maybe_iface of
                Just _  -> do
                    traceIf (text "We already have the old interface for" <+>
                      ppr (ms_mod mod_summary))
                    return maybe_iface
                Nothing -> loadIface

        loadIface = do
             let iface_path = msHiFilePath mod_summary
             read_result <- readIface (ms_mod mod_summary) iface_path
             case read_result of
                 Failed err -> do
                     traceIf (text "FYI: cannot read old interface file:" $$ nest 4 err)
                     traceHiDiffs (text "Old interface file was invalid:" $$ nest 4 err)
                     return Nothing
                 Succeeded iface -> do
                     traceIf (text "Read the interface file" <+> text iface_path)
                     return $ Just iface

            | gopt Opt_ForceRecomp (hsc_dflags hsc_env) = True
            | SourceModified <- src_modified = True
            | otherwise = False
    in do
        when src_changed $
            traceHiDiffs (nest 4 $ text "Source file changed or recompilation check turned off")

        case src_changed of
            -- If the source has changed and we're in interactive mode,
            -- avoid reading an interface; just return the one we might
            -- have been supplied with.
            True | not (isObjectTarget $ hscTarget dflags) ->
                return (MustCompile, maybe_iface)

            -- Try and read the old interface for the current module
            -- from the .hi file left from the last time we compiled it
            True -> do
                maybe_iface' <- getIface
                return (MustCompile, maybe_iface')

            False -> do
                maybe_iface' <- getIface
                case maybe_iface' of
                    -- We can't retrieve the iface
                    Nothing    -> return (MustCompile, Nothing)

                    -- We have got the old iface; check its versions
                    -- even in the SourceUnmodifiedAndStable case we
                    -- should check versions because some packages
                    -- might have changed or gone away.
                    Just iface -> checkVersions hsc_env mod_summary iface

-- | Check if a module is still the same 'version'.
-- This function is called in the recompilation checker after we have
-- determined that the module M being checked hasn't had any changes
-- to its source file since we last compiled M. So at this point in general
-- two things may have changed that mean we should recompile M:
--   * The interface export by a dependency of M has changed.
--   * The compiler flags specified this time for M have changed
--     in a manner that is significant for recompilation.
-- We return not just if we should recompile the object file but also
-- if we should rebuild the interface file.
checkVersions :: HscEnv
              -> ModSummary
              -> ModIface       -- Old interface
              -> IfG (RecompileRequired, Maybe ModIface)
checkVersions hsc_env mod_summary iface
  = do { traceHiDiffs (text "Considering whether compilation is required for" <+>
                        ppr (mi_module iface) <> colon)

       -- readIface will have verified that the InstalledUnitId matches,
       -- but we ALSO must make sure the instantiation matches up.  See
       -- test case bkpcabal04!
       ; if moduleUnitId (mi_module iface) /= thisPackage (hsc_dflags hsc_env)
            then return (RecompBecause "-this-unit-id changed", Nothing) else do {
       ; recomp <- checkFlagHash hsc_env iface
       ; if recompileRequired recomp then return (recomp, Nothing) else do {
       ; recomp <- checkOptimHash hsc_env iface
       ; if recompileRequired recomp then return (recomp, Nothing) else do {
       ; recomp <- checkHpcHash hsc_env iface
       ; if recompileRequired recomp then return (recomp, Nothing) else do {
       ; recomp <- checkMergedSignatures mod_summary iface
       ; if recompileRequired recomp then return (recomp, Nothing) else do {
       ; recomp <- checkHsig mod_summary iface
       ; if recompileRequired recomp then return (recomp, Nothing) else do {
       ; recomp <- checkHie mod_summary
       ; if recompileRequired recomp then return (recomp, Nothing) else do {
       ; recomp <- checkDependencies hsc_env mod_summary iface
       ; if recompileRequired recomp then return (recomp, Just iface) else do {
       ; recomp <- checkPlugins hsc_env iface
       ; if recompileRequired recomp then return (recomp, Nothing) else do {

       -- Source code unchanged and no errors yet... carry on
       -- First put the dependent-module info, read from the old
       -- interface, into the envt, so that when we look for
       -- interfaces we look for the right one (.hi or .hi-boot)
       -- It's just temporary because either the usage check will succeed
       -- (in which case we are done with this module) or it'll fail (in which
       -- case we'll compile the module from scratch anyhow).
       -- We do this regardless of compilation mode, although in --make mode
       -- all the dependent modules should be in the HPT already, so it's
       -- quite redundant
       ; updateEps_ $ \eps  -> eps { eps_is_boot = mod_deps }
       ; recomp <- checkList [checkModUsage this_pkg u | u <- mi_usages iface]
       ; return (recomp, Just iface)
    this_pkg = thisPackage (hsc_dflags hsc_env)
    -- This is a bit of a hack really
    mod_deps :: ModuleNameEnv (ModuleName, IsBootInterface)
    mod_deps = mkModDeps (dep_mods (mi_deps iface))

-- | Check if any plugins are requesting recompilation
checkPlugins :: HscEnv -> ModIface -> IfG RecompileRequired
checkPlugins hsc iface = liftIO $ do
  new_fingerprint <- fingerprintPlugins hsc
  let old_fingerprint = mi_plugin_hash (mi_final_exts iface)
  pr <- mconcat <$> mapM pluginRecompile' (plugins (hsc_dflags hsc))
  return $
    pluginRecompileToRecompileRequired old_fingerprint new_fingerprint pr

fingerprintPlugins :: HscEnv -> IO Fingerprint
fingerprintPlugins hsc_env = do
  fingerprintPlugins' $ plugins (hsc_dflags hsc_env)

fingerprintPlugins' :: [PluginWithArgs] -> IO Fingerprint
fingerprintPlugins' plugins = do
  res <- mconcat <$> mapM pluginRecompile' plugins
  return $ case res of
      NoForceRecompile ->  fingerprintString "NoForceRecompile"
      ForceRecompile   -> fingerprintString "ForceRecompile"
      -- is the chance of collision worth worrying about?
      -- An alternative is to fingerprintFingerprints [fingerprintString
      -- "maybeRecompile", fp]
      (MaybeRecompile fp) -> fp

    :: Fingerprint -> Fingerprint -> PluginRecompile -> RecompileRequired
pluginRecompileToRecompileRequired old_fp new_fp pr
  | old_fp == new_fp =
    case pr of
      NoForceRecompile  -> UpToDate

      -- we already checked the fingerprint above so a mismatch is not possible
      -- here, remember that: `fingerprint (MaybeRecomp x) == x`.
      MaybeRecompile _  -> UpToDate

      -- when we have an impure plugin in the stack we have to unconditionally
      -- recompile since it might integrate all sorts of crazy IO results into
      -- its compilation output.
      ForceRecompile    -> RecompBecause "Impure plugin forced recompilation"

  | old_fp `elem` magic_fingerprints ||
    new_fp `elem` magic_fingerprints
    -- The fingerprints do not match either the old or new one is a magic
    -- fingerprint. This happens when non-pure plugins are added for the first
    -- time or when we go from one recompilation strategy to another: (force ->
    -- no-force, maybe-recomp -> no-force, no-force -> maybe-recomp etc.)
    -- For example when we go from from ForceRecomp to NoForceRecomp
    -- recompilation is triggered since the old impure plugins could have
    -- changed the build output which is now back to normal.
    = RecompBecause "Plugins changed"

  | otherwise =
    let reason = "Plugin fingerprint changed" in
    case pr of
      -- even though a plugin is forcing recompilation the fingerprint changed
      -- which would cause recompilation anyways so we report the fingerprint
      -- change instead.
      ForceRecompile   -> RecompBecause reason

      _                -> RecompBecause reason

   magic_fingerprints =
       [ fingerprintString "NoForceRecompile"
       , fingerprintString "ForceRecompile"

-- | Check if an hsig file needs recompilation because its
-- implementing module has changed.
checkHsig :: ModSummary -> ModIface -> IfG RecompileRequired
checkHsig mod_summary iface = do
    dflags <- getDynFlags
    let outer_mod = ms_mod mod_summary
        inner_mod = canonicalizeHomeModule dflags (moduleName outer_mod)
    MASSERT( moduleUnitId outer_mod == thisPackage dflags )
    case inner_mod == mi_semantic_module iface of
        True -> up_to_date (text "implementing module unchanged")
        False -> return (RecompBecause "implementing module changed")

-- | Check if @.hie@ file is out of date or missing.
checkHie :: ModSummary -> IfG RecompileRequired
checkHie mod_summary = do
    dflags <- getDynFlags
    let hie_date_opt = ms_hie_date mod_summary
        hs_date = ms_hs_date mod_summary
    pure $ case gopt Opt_WriteHie dflags of
               False -> UpToDate
               True -> case hie_date_opt of
                           Nothing -> RecompBecause "HIE file is missing"
                           Just hie_date
                               | hie_date < hs_date
                               -> RecompBecause "HIE file is out of date"
                               | otherwise
                               -> UpToDate

-- | Check the flags haven't changed
checkFlagHash :: HscEnv -> ModIface -> IfG RecompileRequired
checkFlagHash hsc_env iface = do
    let old_hash = mi_flag_hash (mi_final_exts iface)
    new_hash <- liftIO $ fingerprintDynFlags (hsc_dflags hsc_env)
                                             (mi_module iface)
    case old_hash == new_hash of
        True  -> up_to_date (text "Module flags unchanged")
        False -> out_of_date_hash "flags changed"
                     (text "  Module flags have changed")
                     old_hash new_hash

-- | Check the optimisation flags haven't changed
checkOptimHash :: HscEnv -> ModIface -> IfG RecompileRequired
checkOptimHash hsc_env iface = do
    let old_hash = mi_opt_hash (mi_final_exts iface)
    new_hash <- liftIO $ fingerprintOptFlags (hsc_dflags hsc_env)
    if | old_hash == new_hash
         -> up_to_date (text "Optimisation flags unchanged")
       | gopt Opt_IgnoreOptimChanges (hsc_dflags hsc_env)
         -> up_to_date (text "Optimisation flags changed; ignoring")
       | otherwise
         -> out_of_date_hash "Optimisation flags changed"
                     (text "  Optimisation flags have changed")
                     old_hash new_hash

-- | Check the HPC flags haven't changed
checkHpcHash :: HscEnv -> ModIface -> IfG RecompileRequired
checkHpcHash hsc_env iface = do
    let old_hash = mi_hpc_hash (mi_final_exts iface)
    new_hash <- liftIO $ fingerprintHpcFlags (hsc_dflags hsc_env)
    if | old_hash == new_hash
         -> up_to_date (text "HPC flags unchanged")
       | gopt Opt_IgnoreHpcChanges (hsc_dflags hsc_env)
         -> up_to_date (text "HPC flags changed; ignoring")
       | otherwise
         -> out_of_date_hash "HPC flags changed"
                     (text "  HPC flags have changed")
                     old_hash new_hash

-- Check that the set of signatures we are merging in match.
-- If the -unit-id flags change, this can change too.
checkMergedSignatures :: ModSummary -> ModIface -> IfG RecompileRequired
checkMergedSignatures mod_summary iface = do
    dflags <- getDynFlags
    let old_merged = sort [ mod | UsageMergedRequirement{ usg_mod = mod } <- mi_usages iface ]
        new_merged = case Map.lookup (ms_mod_name mod_summary)
                                     (requirementContext (pkgState dflags)) of
                        Nothing -> []
                        Just r -> sort $ map (indefModuleToModule dflags) r
    if old_merged == new_merged
        then up_to_date (text "signatures to merge in unchanged" $$ ppr new_merged)
        else return (RecompBecause "signatures to merge in changed")

-- If the direct imports of this module are resolved to targets that
-- are not among the dependencies of the previous interface file,
-- then we definitely need to recompile.  This catches cases like
--   - an exposed package has been upgraded
--   - we are compiling with different package flags
--   - a home module that was shadowing a package module has been removed
--   - a new home module has been added that shadows a package module
-- See bug #1372.
-- In addition, we also check if the union of dependencies of the imported
-- modules has any difference to the previous set of dependencies. We would need
-- to recompile in that case also since the `mi_deps` field of ModIface needs
-- to be updated to match that information. This is one of the invariants
-- of interface files (see https://gitlab.haskell.org/ghc/ghc/wikis/commentary/compiler/recompilation-avoidance#interface-file-invariants).
-- See bug #16511.
-- Returns (RecompBecause <textual reason>) if recompilation is required.
checkDependencies :: HscEnv -> ModSummary -> ModIface -> IfG RecompileRequired
checkDependencies hsc_env summary iface
 = do
   checkList $
     [ checkList (map dep_missing (ms_imps summary ++ ms_srcimps summary))
     , do
         (recomp, mnames_seen) <- runUntilRecompRequired $ map
           (ms_home_imps summary)
         case recomp of
           UpToDate -> do
               seen_home_deps = Set.unions $ map Set.fromList mnames_seen
             checkIfAllOldHomeDependenciesAreSeen seen_home_deps
           _ -> return recomp]
   prev_dep_mods = dep_mods (mi_deps iface)
   prev_dep_plgn = dep_plgins (mi_deps iface)
   prev_dep_pkgs = dep_pkgs (mi_deps iface)

   this_pkg = thisPackage (hsc_dflags hsc_env)

   dep_missing (mb_pkg, L _ mod) = do
     find_res <- liftIO $ findImportedModule hsc_env mod (mb_pkg)
     let reason = moduleNameString mod ++ " changed"
     case find_res of
        Found _ mod
          | pkg == this_pkg
           -> if moduleName mod `notElem` map fst prev_dep_mods ++ prev_dep_plgn
                 then do traceHiDiffs $
                           text "imported module " <> quotes (ppr mod) <>
                           text " not among previous dependencies"
                         return (RecompBecause reason)
                         return UpToDate
          | otherwise
           -> if toInstalledUnitId pkg `notElem` (map fst prev_dep_pkgs)
                 then do traceHiDiffs $
                           text "imported module " <> quotes (ppr mod) <>
                           text " is from package " <> quotes (ppr pkg) <>
                           text ", which is not among previous dependencies"
                         return (RecompBecause reason)
                         return UpToDate
           where pkg = moduleUnitId mod
        _otherwise  -> return (RecompBecause reason)

   old_deps = Set.fromList $ map fst $ filter (not . snd) prev_dep_mods
   isOldHomeDeps = flip Set.member old_deps
   checkForNewHomeDependency (L _ mname) = do
       mod = mkModule this_pkg mname
       str_mname = moduleNameString mname
       reason = str_mname ++ " changed"
     -- We only want to look at home modules to check if any new home dependency
     -- pops in and thus here, skip modules that are not home. Checking
     -- membership in old home dependencies suffice because the `dep_missing`
     -- check already verified that all imported home modules are present there.
     if not (isOldHomeDeps mname)
       then return (UpToDate, [])
       else do
         mb_result <- getFromModIface "need mi_deps for" mod $ \imported_iface -> do
           let mnames = mname:(map fst $ filter (not . snd) $
                 dep_mods $ mi_deps imported_iface)
           case find (not . isOldHomeDeps) mnames of
             Nothing -> return (UpToDate, mnames)
             Just new_dep_mname -> do
               traceHiDiffs $
                 text "imported home module " <> quotes (ppr mod) <>
                 text " has a new dependency " <> quotes (ppr new_dep_mname)
               return (RecompBecause reason, [])
         return $ fromMaybe (MustCompile, []) mb_result

   -- Performs all recompilation checks in the list until a check that yields
   -- recompile required is encountered. Returns the list of the results of
   -- all UpToDate checks.
   runUntilRecompRequired []             = return (UpToDate, [])
   runUntilRecompRequired (check:checks) = do
     (recompile, value) <- check
     if recompileRequired recompile
       then return (recompile, [])
       else do
         (recomp, values) <- runUntilRecompRequired checks
         return (recomp, value:values)

   checkIfAllOldHomeDependenciesAreSeen seen_deps = do
     let unseen_old_deps = Set.difference
     if not (null unseen_old_deps)
       then do
         let missing_dep = Set.elemAt 0 unseen_old_deps
         traceHiDiffs $
           text "missing old home dependency " <> quotes (ppr missing_dep)
         return $ RecompBecause "missing old dependency"
       else return UpToDate

needInterface :: Module -> (ModIface -> IfG RecompileRequired)
             -> IfG RecompileRequired
needInterface mod continue
  = do
      mb_recomp <- getFromModIface
        "need version info for"
      case mb_recomp of
        Nothing -> return MustCompile
        Just recomp -> return recomp

getFromModIface :: String -> Module -> (ModIface -> IfG a)
              -> IfG (Maybe a)
getFromModIface doc_msg mod getter
  = do  -- Load the imported interface if possible
    let doc_str = sep [text doc_msg, ppr mod]
    traceHiDiffs (text "Checking innterface for module" <+> ppr mod)

    mb_iface <- loadInterface doc_str mod ImportBySystem
        -- Load the interface, but don't complain on failure;
        -- Instead, get an Either back which we can test

    case mb_iface of
      Failed _ -> do
        traceHiDiffs (sep [text "Couldn't load interface for module",
                           ppr mod])
        return Nothing
                  -- Couldn't find or parse a module mentioned in the
                  -- old interface file.  Don't complain: it might
                  -- just be that the current module doesn't need that
                  -- import and it's been deleted
      Succeeded iface -> Just <$> getter iface

-- | Given the usage information extracted from the old
-- M.hi file for the module being compiled, figure out
-- whether M needs to be recompiled.
checkModUsage :: UnitId -> Usage -> IfG RecompileRequired
checkModUsage _this_pkg UsagePackageModule{
                                usg_mod = mod,
                                usg_mod_hash = old_mod_hash }
  = needInterface mod $ \iface -> do
    let reason = moduleNameString (moduleName mod) ++ " changed"
    checkModuleFingerprint reason old_mod_hash (mi_mod_hash (mi_final_exts iface))
        -- We only track the ABI hash of package modules, rather than
        -- individual entity usages, so if the ABI hash changes we must
        -- recompile.  This is safe but may entail more recompilation when
        -- a dependent package has changed.

checkModUsage _ UsageMergedRequirement{ usg_mod = mod, usg_mod_hash = old_mod_hash }
  = needInterface mod $ \iface -> do
    let reason = moduleNameString (moduleName mod) ++ " changed (raw)"
    checkModuleFingerprint reason old_mod_hash (mi_mod_hash (mi_final_exts iface))

checkModUsage this_pkg UsageHomeModule{
                                usg_mod_name = mod_name,
                                usg_mod_hash = old_mod_hash,
                                usg_exports = maybe_old_export_hash,
                                usg_entities = old_decl_hash }
  = do
    let mod = mkModule this_pkg mod_name
    needInterface mod $ \iface -> do

        new_mod_hash    = mi_mod_hash (mi_final_exts iface)
        new_decl_hash   = mi_hash_fn  (mi_final_exts iface)
        new_export_hash = mi_exp_hash (mi_final_exts iface)

        reason = moduleNameString mod_name ++ " changed"

        -- CHECK MODULE
    recompile <- checkModuleFingerprint reason old_mod_hash new_mod_hash
    if not (recompileRequired recompile)
      then return UpToDate
      else do

        checkMaybeHash reason maybe_old_export_hash new_export_hash
            (text "  Export list changed") $ do

        recompile <- checkList [ checkEntityUsage reason new_decl_hash u
                               | u <- old_decl_hash]
        if recompileRequired recompile
          then return recompile     -- This one failed, so just bail out now
          else up_to_date (text "  Great!  The bits I use are up to date")

checkModUsage _this_pkg UsageFile{ usg_file_path = file,
                                   usg_file_hash = old_hash } =
  liftIO $
    handleIO handle $ do
      new_hash <- getFileHash file
      if (old_hash /= new_hash)
         then return recomp
         else return UpToDate
   recomp = RecompBecause (file ++ " changed")
   handle =
#if defined(DEBUG)
       \e -> pprTrace "UsageFile" (text (show e)) $ return recomp
       \_ -> return recomp -- if we can't find the file, just recompile, don't fail

checkModuleFingerprint :: String -> Fingerprint -> Fingerprint
                       -> IfG RecompileRequired
checkModuleFingerprint reason old_mod_hash new_mod_hash
  | new_mod_hash == old_mod_hash
  = up_to_date (text "Module fingerprint unchanged")

  | otherwise
  = out_of_date_hash reason (text "  Module fingerprint has changed")
                     old_mod_hash new_mod_hash

checkMaybeHash :: String -> Maybe Fingerprint -> Fingerprint -> SDoc
               -> IfG RecompileRequired -> IfG RecompileRequired
checkMaybeHash reason maybe_old_hash new_hash doc continue
  | Just hash <- maybe_old_hash, hash /= new_hash
  = out_of_date_hash reason doc hash new_hash
  | otherwise
  = continue

checkEntityUsage :: String
                 -> (OccName -> Maybe (OccName, Fingerprint))
                 -> (OccName, Fingerprint)
                 -> IfG RecompileRequired
checkEntityUsage reason new_hash (name,old_hash)
  = case new_hash name of

        Nothing       ->        -- We used it before, but it ain't there now
                          out_of_date reason (sep [text "No longer exported:", ppr name])

        Just (_, new_hash)      -- It's there, but is it up to date?
          | new_hash == old_hash -> do traceHiDiffs (text "  Up to date" <+> ppr name <+> parens (ppr new_hash))
                                       return UpToDate
          | otherwise            -> out_of_date_hash reason (text "  Out of date:" <+> ppr name)
                                                     old_hash new_hash

up_to_date :: SDoc -> IfG RecompileRequired
up_to_date  msg = traceHiDiffs msg >> return UpToDate

out_of_date :: String -> SDoc -> IfG RecompileRequired
out_of_date reason msg = traceHiDiffs msg >> return (RecompBecause reason)

out_of_date_hash :: String -> SDoc -> Fingerprint -> Fingerprint -> IfG RecompileRequired
out_of_date_hash reason msg old_hash new_hash
  = out_of_date reason (hsep [msg, ppr old_hash, text "->", ppr new_hash])

checkList :: [IfG RecompileRequired] -> IfG RecompileRequired
-- This helper is used in two places
checkList []             = return UpToDate
checkList (check:checks) = do recompile <- check
                              if recompileRequired recompile
                                then return recompile
                                else checkList checks

*                                                                      *
                Converting things to their Iface equivalents
*                                                                      *

tyThingToIfaceDecl :: TyThing -> IfaceDecl
tyThingToIfaceDecl (AnId id)      = idToIfaceDecl id
tyThingToIfaceDecl (ATyCon tycon) = snd (tyConToIfaceDecl emptyTidyEnv tycon)
tyThingToIfaceDecl (ACoAxiom ax)  = coAxiomToIfaceDecl ax
tyThingToIfaceDecl (AConLike cl)  = case cl of
    RealDataCon dc -> dataConToIfaceDecl dc -- for ppr purposes only
    PatSynCon ps   -> patSynToIfaceDecl ps

idToIfaceDecl :: Id -> IfaceDecl
-- The Id is already tidied, so that locally-bound names
-- (lambdas, for-alls) already have non-clashing OccNames
-- We can't tidy it here, locally, because it may have
-- free variables in its type or IdInfo
idToIfaceDecl id
  = IfaceId { ifName      = getName id,
              ifType      = toIfaceType (idType id),
              ifIdDetails = toIfaceIdDetails (idDetails id),
              ifIdInfo    = toIfaceIdInfo (idInfo id) }

dataConToIfaceDecl :: DataCon -> IfaceDecl
dataConToIfaceDecl dataCon
  = IfaceId { ifName      = getName dataCon,
              ifType      = toIfaceType (dataConUserType dataCon),
              ifIdDetails = IfVanillaId,
              ifIdInfo    = NoInfo }

coAxiomToIfaceDecl :: CoAxiom br -> IfaceDecl
-- We *do* tidy Axioms, because they are not (and cannot
-- conveniently be) built in tidy form
coAxiomToIfaceDecl ax@(CoAxiom { co_ax_tc = tycon, co_ax_branches = branches
                               , co_ax_role = role })
 = IfaceAxiom { ifName       = getName ax
              , ifTyCon      = toIfaceTyCon tycon
              , ifRole       = role
              , ifAxBranches = map (coAxBranchToIfaceBranch tycon
                                     (map coAxBranchLHS branch_list))
                                   branch_list }
   branch_list = fromBranches branches

-- 2nd parameter is the list of branch LHSs, in case of a closed type family,
-- for conversion from incompatible branches to incompatible indices.
-- For an open type family the list should be empty.
-- See Note [Storing compatibility] in CoAxiom
coAxBranchToIfaceBranch :: TyCon -> [[Type]] -> CoAxBranch -> IfaceAxBranch
coAxBranchToIfaceBranch tc lhs_s
                        (CoAxBranch { cab_tvs = tvs, cab_cvs = cvs
                                    , cab_eta_tvs = eta_tvs
                                    , cab_lhs = lhs, cab_roles = roles
                                    , cab_rhs = rhs, cab_incomps = incomps })

  = IfaceAxBranch { ifaxbTyVars  = toIfaceTvBndrs tvs
                  , ifaxbCoVars  = map toIfaceIdBndr cvs
                  , ifaxbEtaTyVars = toIfaceTvBndrs eta_tvs
                  , ifaxbLHS     = toIfaceTcArgs tc lhs
                  , ifaxbRoles   = roles
                  , ifaxbRHS     = toIfaceType rhs
                  , ifaxbIncomps = iface_incomps }
    iface_incomps = map (expectJust "iface_incomps"
                        . flip findIndex lhs_s
                        . eqTypes
                        . coAxBranchLHS) incomps

tyConToIfaceDecl :: TidyEnv -> TyCon -> (TidyEnv, IfaceDecl)
-- We *do* tidy TyCons, because they are not (and cannot
-- conveniently be) built in tidy form
-- The returned TidyEnv is the one after tidying the tyConTyVars
tyConToIfaceDecl env tycon
  | Just clas <- tyConClass_maybe tycon
  = classToIfaceDecl env clas

  | Just syn_rhs <- synTyConRhs_maybe tycon
  = ( tc_env1
    , IfaceSynonym { ifName    = getName tycon,
                     ifRoles   = tyConRoles tycon,
                     ifSynRhs  = if_syn_type syn_rhs,
                     ifBinders = if_binders,
                     ifResKind = if_res_kind

  | Just fam_flav <- famTyConFlav_maybe tycon
  = ( tc_env1
    , IfaceFamily { ifName    = getName tycon,
                    ifResVar  = if_res_var,
                    ifFamFlav = to_if_fam_flav fam_flav,
                    ifBinders = if_binders,
                    ifResKind = if_res_kind,
                    ifFamInj  = tyConInjectivityInfo tycon

  | isAlgTyCon tycon
  = ( tc_env1
    , IfaceData { ifName    = getName tycon,
                  ifBinders = if_binders,
                  ifResKind = if_res_kind,
                  ifCType   = tyConCType tycon,
                  ifRoles   = tyConRoles tycon,
                  ifCtxt    = tidyToIfaceContext tc_env1 (tyConStupidTheta tycon),
                  ifCons    = ifaceConDecls (algTyConRhs tycon),
                  ifGadtSyntax = isGadtSyntaxTyCon tycon,
                  ifParent  = parent })

  | otherwise  -- FunTyCon, PrimTyCon, promoted TyCon/DataCon
  -- We only convert these TyCons to IfaceTyCons when we are
  -- just about to pretty-print them, not because we are going
  -- to put them into interface files
  = ( env
    , IfaceData { ifName       = getName tycon,
                  ifBinders    = if_binders,
                  ifResKind    = if_res_kind,
                  ifCType      = Nothing,
                  ifRoles      = tyConRoles tycon,
                  ifCtxt       = [],
                  ifCons       = IfDataTyCon [],
                  ifGadtSyntax = False,
                  ifParent     = IfNoParent })
    -- NOTE: Not all TyCons have `tyConTyVars` field. Forcing this when `tycon`
    -- is one of these TyCons (FunTyCon, PrimTyCon, PromotedDataCon) will cause
    -- an error.
    (tc_env1, tc_binders) = tidyTyConBinders env (tyConBinders tycon)
    tc_tyvars      = binderVars tc_binders
    if_binders     = toIfaceTyCoVarBinders tc_binders
                     -- No tidying of the binders; they are already tidy
    if_res_kind    = tidyToIfaceType tc_env1 (tyConResKind tycon)
    if_syn_type ty = tidyToIfaceType tc_env1 ty
    if_res_var     = getOccFS `fmap` tyConFamilyResVar_maybe tycon

    parent = case tyConFamInstSig_maybe tycon of
               Just (tc, ty, ax) -> IfDataInstance (coAxiomName ax)
                                                   (toIfaceTyCon tc)
                                                   (tidyToIfaceTcArgs tc_env1 tc ty)
               Nothing           -> IfNoParent

    to_if_fam_flav OpenSynFamilyTyCon             = IfaceOpenSynFamilyTyCon
    to_if_fam_flav AbstractClosedSynFamilyTyCon   = IfaceAbstractClosedSynFamilyTyCon
    to_if_fam_flav (DataFamilyTyCon {})           = IfaceDataFamilyTyCon
    to_if_fam_flav (BuiltInSynFamTyCon {})        = IfaceBuiltInSynFamTyCon
    to_if_fam_flav (ClosedSynFamilyTyCon Nothing) = IfaceClosedSynFamilyTyCon Nothing
    to_if_fam_flav (ClosedSynFamilyTyCon (Just ax))
      = IfaceClosedSynFamilyTyCon (Just (axn, ibr))
      where defs = fromBranches $ coAxiomBranches ax
            lhss = map coAxBranchLHS defs
            ibr  = map (coAxBranchToIfaceBranch tycon lhss) defs
            axn  = coAxiomName ax

    ifaceConDecls (NewTyCon { data_con = con })    = IfNewTyCon  (ifaceConDecl con)
    ifaceConDecls (DataTyCon { data_cons = cons }) = IfDataTyCon (map ifaceConDecl cons)
    ifaceConDecls (TupleTyCon { data_con = con })  = IfDataTyCon [ifaceConDecl con]
    ifaceConDecls (SumTyCon { data_cons = cons })  = IfDataTyCon (map ifaceConDecl cons)
    ifaceConDecls AbstractTyCon                    = IfAbstractTyCon
        -- The AbstractTyCon case happens when a TyCon has been trimmed
        -- during tidying.
        -- Furthermore, tyThingToIfaceDecl is also used in TcRnDriver
        -- for GHCi, when browsing a module, in which case the
        -- AbstractTyCon and TupleTyCon cases are perfectly sensible.
        -- (Tuple declarations are not serialised into interface files.)

    ifaceConDecl data_con
        = IfCon   { ifConName    = dataConName data_con,
                    ifConInfix   = dataConIsInfix data_con,
                    ifConWrapper = isJust (dataConWrapId_maybe data_con),
                    ifConExTCvs  = map toIfaceBndr ex_tvs',
                    ifConUserTvBinders = map toIfaceForAllBndr user_bndrs',
                    ifConEqSpec  = map (to_eq_spec . eqSpecPair) eq_spec,
                    ifConCtxt    = tidyToIfaceContext con_env2 theta,
                    ifConArgTys  = map (tidyToIfaceType con_env2) arg_tys,
                    ifConFields  = dataConFieldLabels data_con,
                    ifConStricts = map (toIfaceBang con_env2)
                                       (dataConImplBangs data_con),
                    ifConSrcStricts = map toIfaceSrcBang
                                          (dataConSrcBangs data_con)}
          (univ_tvs, ex_tvs, eq_spec, theta, arg_tys, _)
            = dataConFullSig data_con
          user_bndrs = dataConUserTyVarBinders data_con

          -- Tidy the univ_tvs of the data constructor to be identical
          -- to the tyConTyVars of the type constructor.  This means
          -- (a) we don't need to redundantly put them into the interface file
          -- (b) when pretty-printing an Iface data declaration in H98-style syntax,
          --     we know that the type variables will line up
          -- The latter (b) is important because we pretty-print type constructors
          -- by converting to IfaceSyn and pretty-printing that
          con_env1 = (fst tc_env1, mkVarEnv (zipEqual "ifaceConDecl" univ_tvs tc_tyvars))
                     -- A bit grimy, perhaps, but it's simple!

          (con_env2, ex_tvs') = tidyVarBndrs con_env1 ex_tvs
          user_bndrs' = map (tidyUserTyCoVarBinder con_env2) user_bndrs
          to_eq_spec (tv,ty) = (tidyTyVar con_env2 tv, tidyToIfaceType con_env2 ty)

          -- By this point, we have tidied every universal and existential
          -- tyvar. Because of the dcUserTyCoVarBinders invariant
          -- (see Note [DataCon user type variable binders]), *every*
          -- user-written tyvar must be contained in the substitution that
          -- tidying produced. Therefore, tidying the user-written tyvars is a
          -- simple matter of looking up each variable in the substitution,
          -- which tidyTyCoVarOcc accomplishes.
          tidyUserTyCoVarBinder :: TidyEnv -> TyCoVarBinder -> TyCoVarBinder
          tidyUserTyCoVarBinder env (Bndr tv vis) =
            Bndr (tidyTyCoVarOcc env tv) vis

classToIfaceDecl :: TidyEnv -> Class -> (TidyEnv, IfaceDecl)
classToIfaceDecl env clas
  = ( env1
    , IfaceClass { ifName   = getName tycon,
                   ifRoles  = tyConRoles (classTyCon clas),
                   ifBinders = toIfaceTyCoVarBinders tc_binders,
                   ifBody   = body,
                   ifFDs    = map toIfaceFD clas_fds })
    (_, clas_fds, sc_theta, _, clas_ats, op_stuff)
      = classExtraBigSig clas
    tycon = classTyCon clas

    body | isAbstractTyCon tycon = IfAbstractClass
         | otherwise
         = IfConcreteClass {
                ifClassCtxt   = tidyToIfaceContext env1 sc_theta,
                ifATs    = map toIfaceAT clas_ats,
                ifSigs   = map toIfaceClassOp op_stuff,
                ifMinDef = fmap getOccFS (classMinimalDef clas)

    (env1, tc_binders) = tidyTyConBinders env (tyConBinders tycon)

    toIfaceAT :: ClassATItem -> IfaceAT
    toIfaceAT (ATI tc def)
      = IfaceAT if_decl (fmap (tidyToIfaceType env2 . fst) def)
        (env2, if_decl) = tyConToIfaceDecl env1 tc

    toIfaceClassOp (sel_id, def_meth)
        = ASSERT( sel_tyvars == binderVars tc_binders )
          IfaceClassOp (getName sel_id)
                       (tidyToIfaceType env1 op_ty)
                       (fmap toDmSpec def_meth)
                -- Be careful when splitting the type, because of things
                -- like         class Foo a where
                --                op :: (?x :: String) => a -> a
                -- and          class Baz a where
                --                op :: (Ord a) => a -> a
          (sel_tyvars, rho_ty) = splitForAllTys (idType sel_id)
          op_ty                = funResultTy rho_ty

    toDmSpec :: (Name, DefMethSpec Type) -> DefMethSpec IfaceType
    toDmSpec (_, VanillaDM)       = VanillaDM
    toDmSpec (_, GenericDM dm_ty) = GenericDM (tidyToIfaceType env1 dm_ty)

    toIfaceFD (tvs1, tvs2) = (map (tidyTyVar env1) tvs1
                             ,map (tidyTyVar env1) tvs2)


tidyTyConBinder :: TidyEnv -> TyConBinder -> (TidyEnv, TyConBinder)
-- If the type variable "binder" is in scope, don't re-bind it
-- In a class decl, for example, the ATD binders mention
-- (amd must mention) the class tyvars
tidyTyConBinder env@(_, subst) tvb@(Bndr tv vis)
 = case lookupVarEnv subst tv of
     Just tv' -> (env,  Bndr tv' vis)
     Nothing  -> tidyTyCoVarBinder env tvb

tidyTyConBinders :: TidyEnv -> [TyConBinder] -> (TidyEnv, [TyConBinder])
tidyTyConBinders = mapAccumL tidyTyConBinder

tidyTyVar :: TidyEnv -> TyVar -> FastString
tidyTyVar (_, subst) tv = toIfaceTyVar (lookupVarEnv subst tv `orElse` tv)

instanceToIfaceInst :: ClsInst -> IfaceClsInst
instanceToIfaceInst (ClsInst { is_dfun = dfun_id, is_flag = oflag
                             , is_cls_nm = cls_name, is_cls = cls
                             , is_tcs = mb_tcs
                             , is_orphan = orph })
  = ASSERT( cls_name == className cls )
    IfaceClsInst { ifDFun    = dfun_name,
                ifOFlag   = oflag,
                ifInstCls = cls_name,
                ifInstTys = map do_rough mb_tcs,
                ifInstOrph = orph }
    do_rough Nothing  = Nothing
    do_rough (Just n) = Just (toIfaceTyCon_name n)

    dfun_name = idName dfun_id

famInstToIfaceFamInst :: FamInst -> IfaceFamInst
famInstToIfaceFamInst (FamInst { fi_axiom    = axiom,
                                 fi_fam      = fam,
                                 fi_tcs      = roughs })
  = IfaceFamInst { ifFamInstAxiom    = coAxiomName axiom
                 , ifFamInstFam      = fam
                 , ifFamInstTys      = map do_rough roughs
                 , ifFamInstOrph     = orph }
    do_rough Nothing  = Nothing
    do_rough (Just n) = Just (toIfaceTyCon_name n)

    fam_decl = tyConName $ coAxiomTyCon axiom
    mod = ASSERT( isExternalName (coAxiomName axiom) )
          nameModule (coAxiomName axiom)
    is_local name = nameIsLocalOrFrom mod name

    lhs_names = filterNameSet is_local (orphNamesOfCoCon axiom)

    orph | is_local fam_decl
         = NotOrphan (nameOccName fam_decl)
         | otherwise
         = chooseOrphanAnchor lhs_names

coreRuleToIfaceRule :: CoreRule -> IfaceRule
coreRuleToIfaceRule (BuiltinRule { ru_fn = fn})
  = pprTrace "toHsRule: builtin" (ppr fn) $
    bogusIfaceRule fn

coreRuleToIfaceRule (Rule { ru_name = name, ru_fn = fn,
                            ru_act = act, ru_bndrs = bndrs,
                            ru_args = args, ru_rhs = rhs,
                            ru_orphan = orph, ru_auto = auto })
  = IfaceRule { ifRuleName  = name, ifActivation = act,
                ifRuleBndrs = map toIfaceBndr bndrs,
                ifRuleHead  = fn,
                ifRuleArgs  = map do_arg args,
                ifRuleRhs   = toIfaceExpr rhs,
                ifRuleAuto  = auto,
                ifRuleOrph  = orph }
        -- For type args we must remove synonyms from the outermost
        -- level.  Reason: so that when we read it back in we'll
        -- construct the same ru_rough field as we have right now;
        -- see tcIfaceRule
    do_arg (Type ty)     = IfaceType (toIfaceType (deNoteType ty))
    do_arg (Coercion co) = IfaceCo   (toIfaceCoercion co)
    do_arg arg           = toIfaceExpr arg

bogusIfaceRule :: Name -> IfaceRule
bogusIfaceRule id_name
  = IfaceRule { ifRuleName = fsLit "bogus", ifActivation = NeverActive,
        ifRuleBndrs = [], ifRuleHead = id_name, ifRuleArgs = [],
        ifRuleRhs = IfaceExt id_name, ifRuleOrph = IsOrphan,
        ifRuleAuto = True }