Clean up boot vs non-boot disambiguating types

We often have (ModuleName, Bool) or (Module, Bool) pairs for "extended"
module names (without or with a unit id) disambiguating boot and normal
modules. We think this is important enough across the compiler that it
deserves a new nominal product type. We do this with synnoyms and a
functor named with a `Gen` prefix, matching other newly created
definitions.

It was also requested that we keep custom `IsBoot` / `NotBoot` sum type.
So we have it too. This means changing many the many bools to use that
instead.

Updates `haddock` submodule.

23 files changed, 316 insertions, 196 deletions

diff --git a/compiler/GHC.hs b/compiler/GHC.hs
index 759dda29e6..442fd0a323 100644
--- a/compiler/GHC.hs
+++ b/compiler/GHC.hs
@@ -936,7 +936,7 @@ getModSummary mod = do
    mg <- liftM hsc_mod_graph getSession
    let mods_by_name = [ ms | ms <- mgModSummaries mg
                       , ms_mod_name ms == mod
-                      , not (isBootSummary ms) ]
+                      , isBootSummary ms == NotBoot ]
    case mods_by_name of
      [] -> do dflags <- getDynFlags
               liftIO $ throwIO $ mkApiErr dflags (text "Module not part of module graph")
diff --git a/compiler/GHC/Driver/Backpack.hs b/compiler/GHC/Driver/Backpack.hs
index 7bae489f22..5d5be6c1ff 100644
--- a/compiler/GHC/Driver/Backpack.hs
+++ b/compiler/GHC/Driver/Backpack.hs
@@ -772,7 +772,7 @@ hsModuleToModSummary pn hsc_src modname
     hie_timestamp <- liftIO $ modificationTimeIfExists (ml_hie_file location)
 
     -- Also copied from 'getImports'
-    let (src_idecls, ord_idecls) = partition (ideclSource.unLoc) imps
+    let (src_idecls, ord_idecls) = partition ((== IsBoot) . ideclSource . unLoc) imps
 
              -- GHC.Prim doesn't exist physically, so don't go looking for it.
         ordinary_imps = filter ((/= moduleName gHC_PRIM) . unLoc . ideclName . unLoc)
diff --git a/compiler/GHC/Driver/Make.hs b/compiler/GHC/Driver/Make.hs
index 43c988c4c2..a3d2c0b1bb 100644
--- a/compiler/GHC/Driver/Make.hs
+++ b/compiler/GHC/Driver/Make.hs
@@ -21,14 +21,14 @@ module GHC.Driver.Make (
 
         ms_home_srcimps, ms_home_imps,
 
-        IsBoot(..),
         summariseModule,
         hscSourceToIsBoot,
         findExtraSigImports,
         implicitRequirements,
 
         noModError, cyclicModuleErr,
-        moduleGraphNodes, SummaryNode
+        moduleGraphNodes, SummaryNode,
+        IsBootInterface(..)
     ) where
 
 #include "HsVersions.h"
@@ -378,7 +378,7 @@ load' how_much mHscMessage mod_graph = do
     -- (see msDeps)
     let all_home_mods =
           mkUniqSet [ ms_mod_name s
-                    | s <- mgModSummaries mod_graph, not (isBootSummary s)]
+                    | s <- mgModSummaries mod_graph, isBootSummary s == NotBoot]
     -- TODO: Figure out what the correct form of this assert is. It's violated
     -- when you have HsBootMerge nodes in the graph: then you'll have hs-boot
     -- files without corresponding hs files.
@@ -930,23 +930,26 @@ buildCompGraph (scc:sccs) = case scc of
         return ((ms,mvar,log_queue):rest, cycle)
     CyclicSCC mss -> return ([], Just mss)
 
--- A Module and whether it is a boot module.
-type BuildModule = (Module, IsBoot)
-
--- | 'Bool' indicating if a module is a boot module or not.  We need to treat
--- boot modules specially when building compilation graphs, since they break
--- cycles.  Regular source files and signature files are treated equivalently.
-data IsBoot = NotBoot | IsBoot
-    deriving (Ord, Eq, Show, Read)
-
--- | Tests if an 'HscSource' is a boot file, primarily for constructing
--- elements of 'BuildModule'.
-hscSourceToIsBoot :: HscSource -> IsBoot
+-- | A Module and whether it is a boot module.
+--
+-- We need to treat boot modules specially when building compilation graphs,
+-- since they break cycles. Regular source files and signature files are treated
+-- equivalently.
+type BuildModule = ModuleWithIsBoot
+
+-- | Tests if an 'HscSource' is a boot file, primarily for constructing elements
+-- of 'BuildModule'. We conflate signatures and modules because they are bound
+-- in the same namespace; only boot interfaces can be disambiguated with
+-- `import {-# SOURCE #-}`.
+hscSourceToIsBoot :: HscSource -> IsBootInterface
 hscSourceToIsBoot HsBootFile = IsBoot
 hscSourceToIsBoot _ = NotBoot
 
 mkBuildModule :: ModSummary -> BuildModule
-mkBuildModule ms = (ms_mod ms, if isBootSummary ms then IsBoot else NotBoot)
+mkBuildModule ms = GWIB
+  { gwib_mod = ms_mod ms
+  , gwib_isBoot = isBootSummary ms
+  }
 
 -- | The entry point to the parallel upsweep.
 --
@@ -1014,12 +1017,12 @@ parUpsweep n_jobs mHscMessage old_hpt stable_mods cleanup sccs = do
     -- NB: For convenience, the last module of each loop (aka the module that
     -- finishes the loop) is prepended to the beginning of the loop.
     let graph = map fstOf3 (reverse comp_graph)
-        boot_modules = mkModuleSet [ms_mod ms | ms <- graph, isBootSummary ms]
+        boot_modules = mkModuleSet [ms_mod ms | ms <- graph, isBootSummary ms == IsBoot]
         comp_graph_loops = go graph boot_modules
           where
-            remove ms bm
-              | isBootSummary ms = delModuleSet bm (ms_mod ms)
-              | otherwise = bm
+            remove ms bm = case isBootSummary ms of
+              IsBoot -> delModuleSet bm (ms_mod ms)
+              NotBoot -> bm
             go [] _ = []
             go mg@(ms:mss) boot_modules
               | Just loop <- getModLoop ms mg (`elemModuleSet` boot_modules)
@@ -1193,9 +1196,13 @@ parUpsweep_one mod home_mod_map comp_graph_loops lcl_dflags mHscMessage cleanup
     let home_src_imps = map unLoc $ ms_home_srcimps mod
 
     -- All the textual imports of this module.
-    let textual_deps = Set.fromList $ mapFst (mkModule (thisPackage lcl_dflags)) $
-                            zip home_imps     (repeat NotBoot) ++
-                            zip home_src_imps (repeat IsBoot)
+    let textual_deps = Set.fromList $
+            zipWith f home_imps     (repeat NotBoot) ++
+            zipWith f home_src_imps (repeat IsBoot)
+          where f mn isBoot = GWIB
+                  { gwib_mod = mkModule (thisPackage lcl_dflags) mn
+                  , gwib_isBoot = isBoot
+                  }
 
     -- Dealing with module loops
     -- ~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -1301,8 +1308,8 @@ parUpsweep_one mod home_mod_map comp_graph_loops lcl_dflags mHscMessage cleanup
                     -- SCCs include the loop closer, so we have to filter
                     -- it out.
                     Just loop -> typecheckLoop lcl_dflags lcl_hsc_env' $
-                                 filter (/= moduleName (fst this_build_mod)) $
-                                 map (moduleName . fst) loop
+                                 filter (/= moduleName (gwib_mod this_build_mod)) $
+                                 map (moduleName . gwib_mod) loop
 
                 -- Compile the module.
                 mod_info <- upsweep_mod lcl_hsc_env'' mHscMessage old_hpt stable_mods
@@ -1315,7 +1322,7 @@ parUpsweep_one mod home_mod_map comp_graph_loops lcl_dflags mHscMessage cleanup
                 let this_mod = ms_mod_name mod
 
                 -- Prune the old HPT unless this is an hs-boot module.
-                unless (isBootSummary mod) $
+                unless (isBootSummary mod == IsBoot) $
                     atomicModifyIORef' old_hpt_var $ \old_hpt ->
                         (delFromHpt old_hpt this_mod, ())
 
@@ -1331,7 +1338,7 @@ parUpsweep_one mod home_mod_map comp_graph_loops lcl_dflags mHscMessage cleanup
                     hsc_env'' <- case finish_loop of
                         Nothing   -> return hsc_env'
                         Just loop -> typecheckLoop lcl_dflags hsc_env' $
-                                     map (moduleName . fst) loop
+                                     map (moduleName . gwib_mod) loop
                     return (hsc_env'', localize_hsc_env hsc_env'')
 
                 -- Clean up any intermediate files.
@@ -1491,8 +1498,9 @@ upsweep mHscMessage old_hpt stable_mods cleanup sccs = do
                         -- main Haskell source file.  Deleting it
                         -- would force the real module to be recompiled
                         -- every time.
-                    old_hpt1 | isBootSummary mod = old_hpt
-                             | otherwise = delFromHpt old_hpt this_mod
+                    old_hpt1 = case isBootSummary mod of
+                      IsBoot -> old_hpt
+                      NotBoot -> delFromHpt old_hpt this_mod
 
                     done' = extendMG done mod
 
@@ -1596,10 +1604,10 @@ upsweep_mod hsc_env mHscMessage old_hpt (stable_obj, stable_bco) summary mod_ind
 
             mb_old_iface
                 = case old_hmi of
-                     Nothing                              -> Nothing
-                     Just hm_info | isBootSummary summary -> Just iface
-                                  | not (mi_boot iface)   -> Just iface
-                                  | otherwise             -> Nothing
+                     Nothing                                        -> Nothing
+                     Just hm_info | isBootSummary summary == IsBoot -> Just iface
+                                  | mi_boot iface == NotBoot        -> Just iface
+                                  | otherwise                       -> Nothing
                                    where
                                      iface = hm_iface hm_info
 
@@ -1823,7 +1831,7 @@ reTypecheckLoop hsc_env ms graph
   | Just loop <- getModLoop ms mss appearsAsBoot
   -- SOME hs-boot files should still
   -- get used, just not the loop-closer.
-  , let non_boot = filter (\l -> not (isBootSummary l &&
+  , let non_boot = filter (\l -> not (isBootSummary l == IsBoot &&
                                  ms_mod l == ms_mod ms)) loop
   = typecheckLoop (hsc_dflags hsc_env) hsc_env (map ms_mod_name non_boot)
   | otherwise
@@ -1874,7 +1882,7 @@ getModLoop
   -> (Module -> Bool) -- check if a module appears as a boot module in 'graph'
   -> Maybe [ModSummary]
 getModLoop ms graph appearsAsBoot
-  | not (isBootSummary ms)
+  | isBootSummary ms == NotBoot
   , appearsAsBoot this_mod
   , let mss = reachableBackwards (ms_mod_name ms) graph
   = Just mss
@@ -1974,14 +1982,23 @@ moduleGraphNodes drop_hs_boot_nodes summaries =
     numbered_summaries = zip summaries [1..]
 
     lookup_node :: HscSource -> ModuleName -> Maybe SummaryNode
-    lookup_node hs_src mod = Map.lookup (mod, hscSourceToIsBoot hs_src) node_map
+    lookup_node hs_src mod = Map.lookup
+      GWIB
+        { gwib_mod = mod
+        , gwib_isBoot = hscSourceToIsBoot hs_src
+        }
+      node_map
 
     lookup_key :: HscSource -> ModuleName -> Maybe Int
     lookup_key hs_src mod = fmap summaryNodeKey (lookup_node hs_src mod)
 
     node_map :: NodeMap SummaryNode
-    node_map = Map.fromList [ ((moduleName (ms_mod s),
-                                hscSourceToIsBoot (ms_hsc_src s)), node)
+    node_map = Map.fromList [ ( GWIB
+                                  { gwib_mod = moduleName $ ms_mod s
+                                  , gwib_isBoot = hscSourceToIsBoot $ ms_hsc_src s
+                                  }
+                              , node
+                              )
                             | node <- nodes
                             , let s = summaryNodeSummary node ]
 
@@ -1990,7 +2007,7 @@ moduleGraphNodes drop_hs_boot_nodes summaries =
     nodes = [ DigraphNode s key out_keys
             | (s, key) <- numbered_summaries
              -- Drop the hi-boot ones if told to do so
-            , not (isBootSummary s && drop_hs_boot_nodes)
+            , not (isBootSummary s == IsBoot && drop_hs_boot_nodes)
             , let out_keys = out_edge_keys hs_boot_key (map unLoc (ms_home_srcimps s)) ++
                              out_edge_keys HsSrcFile   (map unLoc (ms_home_imps s)) ++
                              (-- see [boot-edges] below
@@ -2015,17 +2032,20 @@ moduleGraphNodes drop_hs_boot_nodes summaries =
     out_edge_keys :: HscSource -> [ModuleName] -> [Int]
     out_edge_keys hi_boot ms = mapMaybe (lookup_key hi_boot) ms
         -- If we want keep_hi_boot_nodes, then we do lookup_key with
-        -- IsBoot; else NotBoot
+        -- IsBoot; else False
 
 -- The nodes of the graph are keyed by (mod, is boot?) pairs
 -- NB: hsig files show up as *normal* nodes (not boot!), since they don't
 -- participate in cycles (for now)
-type NodeKey   = (ModuleName, IsBoot)
+type NodeKey   = ModuleNameWithIsBoot
 type NodeMap a = Map.Map NodeKey a
 
 msKey :: ModSummary -> NodeKey
 msKey (ModSummary { ms_mod = mod, ms_hsc_src = boot })
-    = (moduleName mod, hscSourceToIsBoot boot)
+    = GWIB
+        { gwib_mod = moduleName mod
+        , gwib_isBoot = hscSourceToIsBoot boot
+        }
 
 mkNodeMap :: [ModSummary] -> NodeMap ModSummary
 mkNodeMap summaries = Map.fromList [ (msKey s, s) | s <- summaries]
@@ -2143,7 +2163,7 @@ downsweep hsc_env old_summaries excl_mods allow_dup_roots
              dup_roots :: [[ModSummary]]        -- Each at least of length 2
              dup_roots = filterOut isSingleton $ map rights $ nodeMapElts root_map
 
-        loop :: [(Located ModuleName,IsBoot)]
+        loop :: [GenWithIsBoot (Located ModuleName)]
                         -- Work list: process these modules
              -> NodeMap [Either ErrorMessages ModSummary]
                         -- Visited set; the range is a list because
@@ -2152,7 +2172,7 @@ downsweep hsc_env old_summaries excl_mods allow_dup_roots
              -> IO (NodeMap [Either ErrorMessages ModSummary])
                         -- The result is the completed NodeMap
         loop [] done = return done
-        loop ((wanted_mod, is_boot) : ss) done
+        loop (s : ss) done
           | Just summs <- Map.lookup key done
           = if isSingleton summs then
                 loop ss done
@@ -2170,7 +2190,12 @@ downsweep hsc_env old_summaries excl_mods allow_dup_roots
                        loop (calcDeps s) (Map.insert key [Right s] done)
                      loop ss new_map
           where
-            key = (unLoc wanted_mod, is_boot)
+            GWIB { gwib_mod = L loc mod, gwib_isBoot = is_boot } = s
+            wanted_mod = L loc mod
+            key = GWIB
+                    { gwib_mod = unLoc wanted_mod
+                    , gwib_isBoot = is_boot
+                    }
 
 -- | Update the every ModSummary that is depended on
 -- by a module that needs template haskell. We enable codegen to
@@ -2206,7 +2231,7 @@ enableCodeGenForUnboxedTuplesOrSums =
     condition ms =
       unboxed_tuples_or_sums (ms_hspp_opts ms) &&
       not (gopt Opt_ByteCode (ms_hspp_opts ms)) &&
-      not (isBootSummary ms)
+      (isBootSummary ms == NotBoot)
     unboxed_tuples_or_sums d =
       xopt LangExt.UnboxedTuples d || xopt LangExt.UnboxedSums d
     should_modify (ModSummary { ms_hspp_opts = dflags }) =
@@ -2281,10 +2306,11 @@ enableCodeGenWhen condition should_modify staticLife dynLife target nodemap =
                   -- If a module imports a boot module, msDeps helpfully adds a
                   -- dependency to that non-boot module in it's result. This
                   -- means we don't have to think about boot modules here.
-                  | (L _ mn, NotBoot) <- msDeps ms
-                  , dep_ms <-
-                      toList (Map.lookup (mn, NotBoot) nodemap) >>= toList >>=
-                      toList
+                  | dep <- msDeps ms
+                  , NotBoot == gwib_isBoot dep
+                  , dep_ms_0 <- toList $ Map.lookup (unLoc <$> dep) nodemap
+                  , dep_ms_1 <- toList $ dep_ms_0
+                  , dep_ms <- toList $ dep_ms_1
                   ]
                 new_marked_mods = Set.insert ms_mod marked_mods
             in foldl' go new_marked_mods deps
@@ -2302,10 +2328,16 @@ mkRootMap summaries = Map.insertListWith (flip (++))
 -- modules always contains B.hs if it contains B.hs-boot.
 -- Remember, this pass isn't doing the topological sort.  It's
 -- just gathering the list of all relevant ModSummaries
-msDeps :: ModSummary -> [(Located ModuleName, IsBoot)]
-msDeps s =
-    concat [ [(m,IsBoot), (m,NotBoot)] | m <- ms_home_srcimps s ]
-        ++ [ (m,NotBoot) | m <- ms_home_imps s ]
+msDeps :: ModSummary -> [GenWithIsBoot (Located ModuleName)]
+msDeps s = [ d
+           | m <- ms_home_srcimps s
+           , d <- [ GWIB { gwib_mod = m, gwib_isBoot = IsBoot }
+                  , GWIB { gwib_mod = m, gwib_isBoot = NotBoot }
+                  ]
+           ]
+        ++ [ GWIB { gwib_mod = m, gwib_isBoot = NotBoot }
+           | m <- ms_home_imps s
+           ]
 
 -----------------------------------------------------------------------------
 -- Summarising modules
@@ -2392,7 +2424,7 @@ findSummaryBySourceFile summaries file
         (x:_) -> Just x
 
 checkSummaryTimestamp
-    :: HscEnv -> DynFlags -> Bool -> IsBoot
+    :: HscEnv -> DynFlags -> Bool -> IsBootInterface
     -> (UTCTime -> IO (Either e ModSummary))
     -> ModSummary -> ModLocation -> UTCTime
     -> IO (Either e ModSummary)
@@ -2433,7 +2465,7 @@ checkSummaryTimestamp
 summariseModule
           :: HscEnv
           -> NodeMap ModSummary -- Map of old summaries
-          -> IsBoot             -- IsBoot <=> a {-# SOURCE #-} import
+          -> IsBootInterface    -- True <=> a {-# SOURCE #-} import
           -> Located ModuleName -- Imported module to be summarised
           -> Bool               -- object code allowed?
           -> Maybe (StringBuffer, UTCTime)
@@ -2445,7 +2477,9 @@ summariseModule hsc_env old_summary_map is_boot (L loc wanted_mod)
   | wanted_mod `elem` excl_mods
   = return Nothing
 
-  | Just old_summary <- Map.lookup (wanted_mod, is_boot) old_summary_map
+  | Just old_summary <- Map.lookup
+      (GWIB { gwib_mod = wanted_mod, gwib_isBoot = is_boot })
+      old_summary_map
   = do          -- Find its new timestamp; all the
                 -- ModSummaries in the old map have valid ml_hs_files
         let location = ms_location old_summary
@@ -2491,8 +2525,9 @@ summariseModule hsc_env old_summary_map is_boot (L loc wanted_mod)
     just_found location mod = do
                 -- Adjust location to point to the hs-boot source file,
                 -- hi file, object file, when is_boot says so
-        let location' | IsBoot <- is_boot = addBootSuffixLocn location
-                      | otherwise         = location
+        let location' = case is_boot of
+              IsBoot -> addBootSuffixLocn location
+              NotBoot -> location
             src_fn = expectJust "summarise2" (ml_hs_file location')
 
                 -- Check that it exists
@@ -2514,10 +2549,10 @@ summariseModule hsc_env old_summary_map is_boot (L loc wanted_mod)
         -- case, we know if it's a boot or not because of the {-# SOURCE #-}
         -- annotation, but we don't know if it's a signature or a regular
         -- module until we actually look it up on the filesystem.
-        let hsc_src = case is_boot of
-                IsBoot -> HsBootFile
-                _ | isHaskellSigFilename src_fn -> HsigFile
-                  | otherwise -> HsSrcFile
+        let hsc_src
+              | is_boot == IsBoot = HsBootFile
+              | isHaskellSigFilename src_fn = HsigFile
+              | otherwise = HsSrcFile
 
         when (pi_mod_name /= wanted_mod) $
                 throwE $ unitBag $ mkPlainErrMsg pi_local_dflags pi_mod_name_loc $
@@ -2560,7 +2595,7 @@ data MakeNewModSummary
   = MakeNewModSummary
       { nms_src_fn :: FilePath
       , nms_src_timestamp :: UTCTime
-      , nms_is_boot :: IsBoot
+      , nms_is_boot :: IsBootInterface
       , nms_hsc_src :: HscSource
       , nms_location :: ModLocation
       , nms_mod :: Module
@@ -2604,10 +2639,11 @@ makeNewModSummary hsc_env MakeNewModSummary{..} = do
       , ms_obj_date = obj_timestamp
       }
 
-getObjTimestamp :: ModLocation -> IsBoot -> IO (Maybe UTCTime)
+getObjTimestamp :: ModLocation -> IsBootInterface -> IO (Maybe UTCTime)
 getObjTimestamp location is_boot
-  = if is_boot == IsBoot then return Nothing
-                         else modificationTimeIfExists (ml_obj_file location)
+  = case is_boot of
+      IsBoot -> return Nothing
+      NotBoot -> modificationTimeIfExists (ml_obj_file location)
 
 data PreprocessedImports
   = PreprocessedImports
@@ -2722,8 +2758,11 @@ cyclicModuleErr mss
     graph = [ DigraphNode ms (msKey ms) (get_deps ms) | ms <- mss]
 
     get_deps :: ModSummary -> [NodeKey]
-    get_deps ms = ([ (unLoc m, IsBoot)  | m <- ms_home_srcimps ms ] ++
-                   [ (unLoc m, NotBoot) | m <- ms_home_imps    ms ])
+    get_deps ms =
+      [ GWIB { gwib_mod = unLoc m, gwib_isBoot = IsBoot }
+      | m <- ms_home_srcimps ms ] ++
+      [ GWIB { gwib_mod = unLoc m, gwib_isBoot = NotBoot }
+      | m <- ms_home_imps    ms ]
 
     show_path []         = panic "show_path"
     show_path [m]        = text "module" <+> ppr_ms m
diff --git a/compiler/GHC/Driver/MakeFile.hs b/compiler/GHC/Driver/MakeFile.hs
index f0de5b75c8..6a50ec483f 100644
--- a/compiler/GHC/Driver/MakeFile.hs
+++ b/compiler/GHC/Driver/MakeFile.hs
@@ -247,8 +247,8 @@ processDeps dflags hsc_env excl_mods root hdl (AcyclicSCC node)
                     | (mb_pkg, L loc mod) <- idecls,
                       mod `notElem` excl_mods ]
 
-        ; do_imps True  (ms_srcimps node)
-        ; do_imps False (ms_imps node)
+        ; do_imps IsBoot (ms_srcimps node)
+        ; do_imps NotBoot (ms_imps node)
         }
 
 
diff --git a/compiler/GHC/Driver/Types.hs b/compiler/GHC/Driver/Types.hs
index c93dc7649f..6903b3608f 100644
--- a/compiler/GHC/Driver/Types.hs
+++ b/compiler/GHC/Driver/Types.hs
@@ -114,7 +114,7 @@ module GHC.Driver.Types (
         MonadThings(..),
 
         -- * Information on imports and exports
-        WhetherHasOrphans, IsBootInterface, Usage(..),
+        WhetherHasOrphans, IsBootInterface(..), Usage(..),
         Dependencies(..), noDependencies,
         updNameCache,
         IfaceExport,
@@ -745,12 +745,12 @@ hptInstances hsc_env want_this_module
     in (concat insts, concat famInsts)
 
 -- | Get rules from modules "below" this one (in the dependency sense)
-hptRules :: HscEnv -> [(ModuleName, IsBootInterface)] -> [CoreRule]
+hptRules :: HscEnv -> [ModuleNameWithIsBoot] -> [CoreRule]
 hptRules = hptSomeThingsBelowUs (md_rules . hm_details) False
 
 
 -- | Get annotations from modules "below" this one (in the dependency sense)
-hptAnns :: HscEnv -> Maybe [(ModuleName, IsBootInterface)] -> [Annotation]
+hptAnns :: HscEnv -> Maybe [ModuleNameWithIsBoot] -> [Annotation]
 hptAnns hsc_env (Just deps) = hptSomeThingsBelowUs (md_anns . hm_details) False hsc_env deps
 hptAnns hsc_env Nothing = hptAllThings (md_anns . hm_details) hsc_env
 
@@ -759,7 +759,7 @@ hptAllThings extract hsc_env = concatMap extract (eltsHpt (hsc_HPT hsc_env))
 
 -- | Get things from modules "below" this one (in the dependency sense)
 -- C.f Inst.hptInstances
-hptSomeThingsBelowUs :: (HomeModInfo -> [a]) -> Bool -> HscEnv -> [(ModuleName, IsBootInterface)] -> [a]
+hptSomeThingsBelowUs :: (HomeModInfo -> [a]) -> Bool -> HscEnv -> [ModuleNameWithIsBoot] -> [a]
 hptSomeThingsBelowUs extract include_hi_boot hsc_env deps
   | isOneShot (ghcMode (hsc_dflags hsc_env)) = []
 
@@ -768,8 +768,8 @@ hptSomeThingsBelowUs extract include_hi_boot hsc_env deps
     in
     [ thing
     |   -- Find each non-hi-boot module below me
-      (mod, is_boot_mod) <- deps
-    , include_hi_boot || not is_boot_mod
+      GWIB { gwib_mod = mod, gwib_isBoot = is_boot } <- deps
+    , include_hi_boot || (is_boot == NotBoot)
 
         -- unsavoury: when compiling the base package with --make, we
         -- sometimes try to look up RULES etc for GHC.Prim. GHC.Prim won't
@@ -1114,8 +1114,10 @@ data ModIface_ (phase :: ModIfacePhase)
 
 -- | Old-style accessor for whether or not the ModIface came from an hs-boot
 -- file.
-mi_boot :: ModIface -> Bool
-mi_boot iface = mi_hsc_src iface == HsBootFile
+mi_boot :: ModIface -> IsBootInterface
+mi_boot iface = if mi_hsc_src iface == HsBootFile
+    then IsBoot
+    else NotBoot
 
 -- | Lookups up a (possibly cached) fixity from a 'ModIface'. If one cannot be
 -- found, 'defaultFixity' is returned instead.
@@ -1141,7 +1143,7 @@ mi_free_holes iface =
         -> renameFreeHoles (mkUniqDSet cands) (instUnitInsts (moduleUnit indef))
     _   -> emptyUniqDSet
   where
-    cands = map fst (dep_mods (mi_deps iface))
+    cands = map gwib_mod $ dep_mods $ mi_deps iface
 
 -- | Given a set of free holes, and a unit identifier, rename
 -- the free holes according to the instantiation of the unit
@@ -2494,9 +2496,6 @@ type WhetherHasOrphans   = Bool
 -- | Does this module define family instances?
 type WhetherHasFamInst = Bool
 
--- | Did this module originate from a *-boot file?
-type IsBootInterface = Bool
-
 -- | Dependency information about ALL modules and packages below this one
 -- in the import hierarchy.
 --
@@ -2504,7 +2503,7 @@ type IsBootInterface = Bool
 --
 -- Invariant: none of the lists contain duplicates.
 data Dependencies
-  = Deps { dep_mods   :: [(ModuleName, IsBootInterface)]
+  = Deps { dep_mods   :: [ModuleNameWithIsBoot]
                         -- ^ All home-package modules transitively below this one
                         -- I.e. modules that this one imports, or that are in the
                         --      dep_mods of those directly-imported modules
@@ -2694,7 +2693,7 @@ type PackageCompleteMatchMap = CompleteMatchMap
 -- their interface files
 data ExternalPackageState
   = EPS {
-        eps_is_boot :: !(ModuleNameEnv (ModuleName, IsBootInterface)),
+        eps_is_boot :: !(ModuleNameEnv ModuleNameWithIsBoot),
                 -- ^ In OneShot mode (only), home-package modules
                 -- accumulate in the external package state, and are
                 -- sucked in lazily.  For these home-pkg modules
@@ -2872,19 +2871,19 @@ isTemplateHaskellOrQQNonBoot :: ModSummary -> Bool
 isTemplateHaskellOrQQNonBoot ms =
   (xopt LangExt.TemplateHaskell (ms_hspp_opts ms)
     || xopt LangExt.QuasiQuotes (ms_hspp_opts ms)) &&
-  not (isBootSummary ms)
+  (isBootSummary ms == NotBoot)
 
 -- | Add a ModSummary to ModuleGraph. Assumes that the new ModSummary is
 -- not an element of the ModuleGraph.
 extendMG :: ModuleGraph -> ModSummary -> ModuleGraph
 extendMG ModuleGraph{..} ms = ModuleGraph
   { mg_mss = ms:mg_mss
-  , mg_non_boot = if isBootSummary ms
-      then mg_non_boot
-      else extendModuleEnv mg_non_boot (ms_mod ms) ms
-  , mg_boot = if isBootSummary ms
-      then extendModuleSet mg_boot (ms_mod ms)
-      else mg_boot
+  , mg_non_boot = case isBootSummary ms of
+      IsBoot -> mg_non_boot
+      NotBoot -> extendModuleEnv mg_non_boot (ms_mod ms) ms
+  , mg_boot = case isBootSummary ms of
+      NotBoot -> mg_boot
+      IsBoot -> extendModuleSet mg_boot (ms_mod ms)
   , mg_needs_th_or_qq = mg_needs_th_or_qq || isTemplateHaskellOrQQNonBoot ms
   }
 
@@ -2985,8 +2984,8 @@ msDynObjFilePath :: ModSummary -> DynFlags -> FilePath
 msDynObjFilePath ms dflags = dynamicOutputFile dflags (msObjFilePath ms)
 
 -- | Did this 'ModSummary' originate from a hs-boot file?
-isBootSummary :: ModSummary -> Bool
-isBootSummary ms = ms_hsc_src ms == HsBootFile
+isBootSummary :: ModSummary -> IsBootInterface
+isBootSummary ms = if ms_hsc_src ms == HsBootFile then IsBoot else NotBoot
 
 instance Outputable ModSummary where
    ppr ms
diff --git a/compiler/GHC/Hs/ImpExp.hs b/compiler/GHC/Hs/ImpExp.hs
index 02eb9db1ca..2257352b63 100644
--- a/compiler/GHC/Hs/ImpExp.hs
+++ b/compiler/GHC/Hs/ImpExp.hs
@@ -18,7 +18,7 @@ module GHC.Hs.ImpExp where
 
 import GHC.Prelude
 
-import GHC.Unit.Module       ( ModuleName )
+import GHC.Unit.Module        ( ModuleName, IsBootInterface(..) )
 import GHC.Hs.Doc             ( HsDocString )
 import GHC.Types.Name.Occurrence ( HasOccName(..), isTcOcc, isSymOcc )
 import GHC.Types.Basic        ( SourceText(..), StringLiteral(..), pprWithSourceText )
@@ -83,7 +83,7 @@ data ImportDecl pass
                                  -- Note [Pragma source text] in GHC.Types.Basic
       ideclName      :: Located ModuleName, -- ^ Module name.
       ideclPkgQual   :: Maybe StringLiteral,  -- ^ Package qualifier.
-      ideclSource    :: Bool,          -- ^ True <=> {-\# SOURCE \#-} import
+      ideclSource    :: IsBootInterface,      -- ^ IsBoot <=> {-\# SOURCE \#-} import
       ideclSafe      :: Bool,          -- ^ True => safe import
       ideclQualified :: ImportDeclQualifiedStyle, -- ^ If/how the import is qualified.
       ideclImplicit  :: Bool,          -- ^ True => implicit import (of Prelude)
@@ -118,7 +118,7 @@ simpleImportDecl mn = ImportDecl {
       ideclSourceSrc = NoSourceText,
       ideclName      = noLoc mn,
       ideclPkgQual   = Nothing,
-      ideclSource    = False,
+      ideclSource    = NotBoot,
       ideclSafe      = False,
       ideclImplicit  = False,
       ideclQualified = NotQualified,
@@ -156,10 +156,10 @@ instance OutputableBndrId p
         pp_as Nothing   = empty
         pp_as (Just a)  = text "as" <+> ppr a
 
-        ppr_imp True  = case mSrcText of
+        ppr_imp IsBoot = case mSrcText of
                           NoSourceText   -> text "{-# SOURCE #-}"
                           SourceText src -> text src <+> text "#-}"
-        ppr_imp False = empty
+        ppr_imp NotBoot = empty
 
         pp_spec Nothing             = empty
         pp_spec (Just (False, (L _ ies))) = ppr_ies ies
diff --git a/compiler/GHC/HsToCore/Monad.hs b/compiler/GHC/HsToCore/Monad.hs
index 1914498f4e..02bd5cf91e 100644
--- a/compiler/GHC/HsToCore/Monad.hs
+++ b/compiler/GHC/HsToCore/Monad.hs
@@ -285,7 +285,7 @@ mkDsEnvs dflags mod rdr_env type_env fam_inst_env msg_var cc_st_var
   = let if_genv = IfGblEnv { if_doc       = text "mkDsEnvs",
                              if_rec_types = Just (mod, return type_env) }
         if_lenv = mkIfLclEnv mod (text "GHC error in desugarer lookup in" <+> ppr mod)
-                             False -- not boot!
+                             NotBoot
         real_span = realSrcLocSpan (mkRealSrcLoc (moduleNameFS (moduleName mod)) 1 1)
         completeMatchMap = mkCompleteMatchMap complete_matches
         gbl_env = DsGblEnv { ds_mod     = mod
diff --git a/compiler/GHC/HsToCore/Usage.hs b/compiler/GHC/HsToCore/Usage.hs
index fef8fb03c4..0c48b5744d 100644
--- a/compiler/GHC/HsToCore/Usage.hs
+++ b/compiler/GHC/HsToCore/Usage.hs
@@ -215,9 +215,10 @@ mkPluginUsage hsc_env pluginModule
   where
     dflags   = hsc_dflags hsc_env
     platform = targetPlatform dflags
-    pNm      = moduleName (mi_module pluginModule)
-    pPkg     = moduleUnit (mi_module pluginModule)
-    deps     = map fst (dep_mods (mi_deps pluginModule))
+    pNm      = moduleName $ mi_module pluginModule
+    pPkg     = moduleUnit $ mi_module pluginModule
+    deps     = map gwib_mod $
+      dep_mods $ mi_deps pluginModule
 
     -- Lookup object file for a plugin dependency,
     -- from the same package as the plugin.
diff --git a/compiler/GHC/Iface/Load.hs b/compiler/GHC/Iface/Load.hs
index 80c4505c8e..ef69e97605 100644
--- a/compiler/GHC/Iface/Load.hs
+++ b/compiler/GHC/Iface/Load.hs
@@ -366,7 +366,7 @@ loadSysInterface doc mod_name = loadInterfaceWithException doc mod_name ImportBy
 ------------------
 -- | Loads a user interface and throws an exception if it fails. The first parameter indicates
 -- whether we should import the boot variant of the module
-loadUserInterface :: Bool -> SDoc -> Module -> IfM lcl ModIface
+loadUserInterface :: IsBootInterface -> SDoc -> Module -> IfM lcl ModIface
 loadUserInterface is_boot doc mod_name
   = loadInterfaceWithException doc mod_name (ImportByUser is_boot)
 
@@ -485,7 +485,7 @@ loadInterface doc_str mod from
                               }
                }
 
-        ; let bad_boot = mi_boot iface && fmap fst (if_rec_types gbl_env) == Just mod
+        ; let bad_boot = mi_boot iface == IsBoot && fmap fst (if_rec_types gbl_env) == Just mod
                             -- Warn warn against an EPS-updating import
                             -- of one's own boot file! (one-shot only)
                             -- See Note [Loading your own hi-boot file]
@@ -690,7 +690,7 @@ moduleFreeHolesPrecise doc_str mod
             Just ifhs  -> Just (renameFreeHoles ifhs insts)
             _otherwise -> Nothing
     readAndCache imod insts = do
-        mb_iface <- findAndReadIface (text "moduleFreeHolesPrecise" <+> doc_str) imod mod False
+        mb_iface <- findAndReadIface (text "moduleFreeHolesPrecise" <+> doc_str) imod mod NotBoot
         case mb_iface of
             Succeeded (iface, _) -> do
                 let ifhs = mi_free_holes iface
@@ -706,23 +706,25 @@ wantHiBootFile :: DynFlags -> ExternalPackageState -> Module -> WhereFrom
 wantHiBootFile dflags eps mod from
   = case from of
        ImportByUser usr_boot
-          | usr_boot && not this_package
+          | usr_boot == IsBoot && not this_package
           -> Failed (badSourceImport mod)
           | otherwise -> Succeeded usr_boot
 
        ImportByPlugin
-          -> Succeeded False
+          -> Succeeded NotBoot
 
        ImportBySystem
           | not this_package   -- If the module to be imported is not from this package
-          -> Succeeded False   -- don't look it up in eps_is_boot, because that is keyed
+          -> Succeeded NotBoot -- don't look it up in eps_is_boot, because that is keyed
                                -- on the ModuleName of *home-package* modules only.
                                -- We never import boot modules from other packages!
 
           | otherwise
           -> case lookupUFM (eps_is_boot eps) (moduleName mod) of
-                Just (_, is_boot) -> Succeeded is_boot
-                Nothing           -> Succeeded False
+                Just (GWIB { gwib_isBoot = is_boot }) ->
+                  Succeeded is_boot
+                Nothing ->
+                  Succeeded NotBoot
                      -- The boot-ness of the requested interface,
                      -- based on the dependencies in directly-imported modules
   where
@@ -899,7 +901,7 @@ findAndReadIface :: SDoc
         -- sometimes it's ok to fail... see notes with loadInterface
 findAndReadIface doc_str mod wanted_mod_with_insts hi_boot_file
   = do traceIf (sep [hsep [text "Reading",
-                           if hi_boot_file
+                           if hi_boot_file == IsBoot
                              then text "[boot]"
                              else Outputable.empty,
                            text "interface for",
@@ -1219,11 +1221,11 @@ pprDeps (Deps { dep_mods = mods, dep_pkgs = pkgs, dep_orphs = orphs,
           text "family instance modules:" <+> fsep (map ppr finsts)
         ]
   where
-    ppr_mod (mod_name, boot) = ppr mod_name <+> ppr_boot boot
+    ppr_mod (GWIB { gwib_mod = mod_name, gwib_isBoot = boot }) = ppr mod_name <+> ppr_boot boot
     ppr_pkg (pkg,trust_req)  = ppr pkg <>
                                (if trust_req then text "*" else Outputable.empty)
-    ppr_boot True  = text "[boot]"
-    ppr_boot False = Outputable.empty
+    ppr_boot IsBoot  = text "[boot]"
+    ppr_boot NotBoot = Outputable.empty
 
 pprFixities :: [(OccName, Fixity)] -> SDoc
 pprFixities []    = Outputable.empty
diff --git a/compiler/GHC/Iface/Recomp.hs b/compiler/GHC/Iface/Recomp.hs
index 68103fc1f4..03223c5712 100644
--- a/compiler/GHC/Iface/Recomp.hs
+++ b/compiler/GHC/Iface/Recomp.hs
@@ -252,7 +252,7 @@ checkVersions hsc_env mod_summary iface
   where
     this_pkg = thisPackage (hsc_dflags hsc_env)
     -- This is a bit of a hack really
-    mod_deps :: ModuleNameEnv (ModuleName, IsBootInterface)
+    mod_deps :: ModuleNameEnv ModuleNameWithIsBoot
     mod_deps = mkModDeps (dep_mods (mi_deps iface))
 
 -- | Check if any plugins are requesting recompilation
@@ -455,7 +455,7 @@ checkDependencies hsc_env summary iface
      case find_res of
         Found _ mod
           | pkg == this_pkg
-           -> if moduleName mod `notElem` map fst prev_dep_mods ++ prev_dep_plgn
+           -> if moduleName mod `notElem` map gwib_mod prev_dep_mods ++ prev_dep_plgn
                  then do traceHiDiffs $
                            text "imported module " <> quotes (ppr mod) <>
                            text " not among previous dependencies"
@@ -474,7 +474,9 @@ checkDependencies hsc_env summary iface
            where pkg = moduleUnit mod
         _otherwise  -> return (RecompBecause reason)
 
-   old_deps = Set.fromList $ map fst $ filter (not . snd) prev_dep_mods
+   projectNonBootNames = map gwib_mod . filter ((== NotBoot) . gwib_isBoot)
+   old_deps = Set.fromList
+     $ projectNonBootNames prev_dep_mods
    isOldHomeDeps = flip Set.member old_deps
    checkForNewHomeDependency (L _ mname) = do
      let
@@ -489,7 +491,7 @@ checkDependencies hsc_env summary iface
        then return (UpToDate, [])
        else do
          mb_result <- getFromModIface "need mi_deps for" mod $ \imported_iface -> do
-           let mnames = mname:(map fst $ filter (not . snd) $
+           let mnames = mname:(map gwib_mod $ filter ((== NotBoot) . gwib_isBoot) $
                  dep_mods $ mi_deps imported_iface)
            case find (not . isOldHomeDeps) mnames of
              Nothing -> return (UpToDate, mnames)
@@ -1073,7 +1075,7 @@ getOrphanHashes hsc_env mods = do
 
 sortDependencies :: Dependencies -> Dependencies
 sortDependencies d
- = Deps { dep_mods   = sortBy (compare `on` (moduleNameFS.fst)) (dep_mods d),
+ = Deps { dep_mods   = sortBy (compare `on` (moduleNameFS . gwib_mod)) (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),
diff --git a/compiler/GHC/IfaceToCore.hs b/compiler/GHC/IfaceToCore.hs
index 48652573f3..b84fe1619d 100644
--- a/compiler/GHC/IfaceToCore.hs
+++ b/compiler/GHC/IfaceToCore.hs
@@ -354,7 +354,7 @@ mergeIfaceDecls = plusOccEnv_C mergeIfaceDecl
 typecheckIfacesForMerging :: Module -> [ModIface] -> IORef TypeEnv -> IfM lcl (TypeEnv, [ModDetails])
 typecheckIfacesForMerging mod ifaces tc_env_var =
   -- cannot be boot (False)
-  initIfaceLcl mod (text "typecheckIfacesForMerging") False $ do
+  initIfaceLcl mod (text "typecheckIfacesForMerging") NotBoot $ do
     ignore_prags <- goptM Opt_IgnoreInterfacePragmas
     -- Build the initial environment
     -- NB: Don't include dfuns here, because we don't want to
@@ -506,7 +506,7 @@ tcHiBootIface hsc_src mod
                 -- it's been compiled once, and we don't need to check the boot iface
           then do { hpt <- getHpt
                  ; case lookupHpt hpt (moduleName mod) of
-                      Just info | mi_boot (hm_iface info)
+                      Just info | mi_boot (hm_iface info) == IsBoot
                                 -> mkSelfBootInfo (hm_iface info) (hm_details info)
                       _ -> return NoSelfBoot }
           else do
@@ -517,7 +517,7 @@ tcHiBootIface hsc_src mod
         -- that an hi-boot is necessary due to a circular import.
         { read_result <- findAndReadIface
                                 need (fst (getModuleInstantiation mod)) mod
-                                True    -- Hi-boot file
+                                IsBoot  -- Hi-boot file
 
         ; case read_result of {
             Succeeded (iface, _path) -> do { tc_iface <- initIfaceTcRn $ typecheckIface iface
@@ -533,14 +533,15 @@ tcHiBootIface hsc_src mod
         -- disappeared.
     do  { eps <- getEps
         ; case lookupUFM (eps_is_boot eps) (moduleName mod) of
-            Nothing -> return NoSelfBoot -- The typical case
-
-            Just (_, False) -> failWithTc moduleLoop
-                -- Someone below us imported us!
-                -- This is a loop with no hi-boot in the way
-
-            Just (_mod, True) -> failWithTc (elaborate err)
-                -- The hi-boot file has mysteriously disappeared.
+            -- The typical case
+            Nothing -> return NoSelfBoot
+            -- error cases
+            Just (GWIB { gwib_isBoot = is_boot }) -> case is_boot of
+              IsBoot -> failWithTc (elaborate err)
+              -- The hi-boot file has mysteriously disappeared.
+              NotBoot -> failWithTc moduleLoop
+              -- Someone below us imported us!
+              -- This is a loop with no hi-boot in the way
     }}}}
   where
     need = text "Need the hi-boot interface for" <+> ppr mod
@@ -1480,8 +1481,9 @@ tcIdInfo ignore_prags toplvl name ty info = do
     lcl_env <- getLclEnv
     -- Set the CgInfo to something sensible but uninformative before
     -- we start; default assumption is that it has CAFs
-    let init_info | if_boot lcl_env = vanillaIdInfo `setUnfoldingInfo` BootUnfolding
-                  | otherwise       = vanillaIdInfo
+    let init_info = if if_boot lcl_env == IsBoot
+                      then vanillaIdInfo `setUnfoldingInfo` BootUnfolding
+                      else vanillaIdInfo
 
     let needed = needed_prags info
     foldlM tcPrag init_info needed
diff --git a/compiler/GHC/Parser.y b/compiler/GHC/Parser.y
index a9bb4fa87d..50459c673e 100644
--- a/compiler/GHC/Parser.y
+++ b/compiler/GHC/Parser.y
@@ -51,7 +51,7 @@ import qualified Prelude
 import GHC.Hs
 
 import GHC.Driver.Phases  ( HscSource(..) )
-import GHC.Driver.Types   ( IsBootInterface, WarningTxt(..) )
+import GHC.Driver.Types   ( IsBootInterface(..), WarningTxt(..) )
 import GHC.Driver.Session
 import GHC.Driver.Backpack.Syntax
 import GHC.Unit.Info
@@ -722,8 +722,8 @@ unitdecl :: { LHsUnitDecl PackageName }
              -- XXX not accurate
              { sL1 $2 $ DeclD
                  (case snd $3 of
-                   False -> HsSrcFile
-                   True  -> HsBootFile)
+                   NotBoot -> HsSrcFile
+                   IsBoot  -> HsBootFile)
                  $4
                  (Just $ sL1 $2 (HsModule (Just $4) $6 (fst $ snd $8) (snd $ snd $8) $5 $1)) }
         | maybedocheader 'signature' modid maybemodwarning maybeexports 'where' body
@@ -735,8 +735,8 @@ unitdecl :: { LHsUnitDecl PackageName }
         -- will prevent us from parsing both forms.
         | maybedocheader 'module' maybe_src modid
              { sL1 $2 $ DeclD (case snd $3 of
-                   False -> HsSrcFile
-                   True  -> HsBootFile) $4 Nothing }
+                   NotBoot -> HsSrcFile
+                   IsBoot  -> HsBootFile) $4 Nothing }
         | maybedocheader 'signature' modid
              { sL1 $2 $ DeclD HsigFile $3 Nothing }
         | 'dependency' unitid mayberns
@@ -985,8 +985,8 @@ importdecl :: { LImportDecl GhcPs }
 
 maybe_src :: { (([AddAnn],SourceText),IsBootInterface) }
         : '{-# SOURCE' '#-}'        { (([mo $1,mc $2],getSOURCE_PRAGs $1)
-                                      , True) }
-        | {- empty -}               { (([],NoSourceText),False) }
+                                      , IsBoot) }
+        | {- empty -}               { (([],NoSourceText),NotBoot) }
 
 maybe_safe :: { ([AddAnn],Bool) }
         : 'safe'                                { ([mj AnnSafe $1],True) }
diff --git a/compiler/GHC/Parser/Header.hs b/compiler/GHC/Parser/Header.hs
index f6be2a2487..bfdeb71631 100644
--- a/compiler/GHC/Parser/Header.hs
+++ b/compiler/GHC/Parser/Header.hs
@@ -91,7 +91,7 @@ getImports dflags buf filename source_filename = do
                 main_loc = srcLocSpan (mkSrcLoc (mkFastString source_filename)
                                        1 1)
                 mod = mb_mod `orElse` L main_loc mAIN_NAME
-                (src_idecls, ord_idecls) = partition (ideclSource.unLoc) imps
+                (src_idecls, ord_idecls) = partition ((== IsBoot) . ideclSource . unLoc) imps
 
                -- GHC.Prim doesn't exist physically, so don't go looking for it.
                 ordinary_imps = filter ((/= moduleName gHC_PRIM) . unLoc
@@ -135,7 +135,7 @@ mkPrelImports this_mod loc implicit_prelude import_decls
                                ideclSourceSrc = NoSourceText,
                                ideclName      = L loc pRELUDE_NAME,
                                ideclPkgQual   = Nothing,
-                               ideclSource    = False,
+                               ideclSource    = NotBoot,
                                ideclSafe      = False,  -- Not a safe import
                                ideclQualified = NotQualified,
                                ideclImplicit  = True,   -- Implicit!
diff --git a/compiler/GHC/Rename/Env.hs b/compiler/GHC/Rename/Env.hs
index e6fa48c004..89d1e66311 100644
--- a/compiler/GHC/Rename/Env.hs
+++ b/compiler/GHC/Rename/Env.hs
@@ -1347,7 +1347,7 @@ lookupQualifiedNameGHCi rdr_name
       , is_ghci
       , gopt Opt_ImplicitImportQualified dflags   -- Enables this GHCi behaviour
       , not (safeDirectImpsReq dflags)            -- See Note [Safe Haskell and GHCi]
-      = do { res <- loadSrcInterface_maybe doc mod False Nothing
+      = do { res <- loadSrcInterface_maybe doc mod NotBoot Nothing
            ; case res of
                 Succeeded iface
                   -> return [ name
diff --git a/compiler/GHC/Rename/Names.hs b/compiler/GHC/Rename/Names.hs
index 9e7f1a4216..354954f19c 100644
--- a/compiler/GHC/Rename/Names.hs
+++ b/compiler/GHC/Rename/Names.hs
@@ -176,7 +176,7 @@ rnImports imports = do
     -- module to import from its implementor
     let this_mod = tcg_mod tcg_env
     let (source, ordinary) = partition is_source_import imports
-        is_source_import d = ideclSource (unLoc d)
+        is_source_import d = ideclSource (unLoc d) == IsBoot
     stuff1 <- mapAndReportM (rnImportDecl this_mod) ordinary
     stuff2 <- mapAndReportM (rnImportDecl this_mod) source
     -- Safe Haskell: See Note [Tracking Trust Transitively]
@@ -323,7 +323,7 @@ rnImportDecl this_mod
 
     -- Compiler sanity check: if the import didn't say
     -- {-# SOURCE #-} we should not get a hi-boot file
-    WARN( not want_boot && mi_boot iface, ppr imp_mod_name ) do
+    WARN( (want_boot == NotBoot) && (mi_boot iface == IsBoot), ppr imp_mod_name ) do
 
     -- Issue a user warning for a redundant {- SOURCE -} import
     -- NB that we arrange to read all the ordinary imports before
@@ -334,7 +334,7 @@ rnImportDecl this_mod
     -- the non-boot module depends on the compilation order, which
     -- is not deterministic.  The hs-boot test can show this up.
     dflags <- getDynFlags
-    warnIf (want_boot && not (mi_boot iface) && isOneShot (ghcMode dflags))
+    warnIf ((want_boot == IsBoot) && (mi_boot iface == NotBoot) && isOneShot (ghcMode dflags))
            (warnRedundantSourceImport imp_mod_name)
     when (mod_safe && not (safeImportsOn dflags)) $
         addErr (text "safe import can't be used as Safe Haskell isn't on!"
@@ -460,7 +460,10 @@ calculateAvails dflags iface mod_safe' want_boot imported_by =
             -- know if any of them depended on CM.hi-boot, in
             -- which case we should do the hi-boot consistency
             -- check.  See GHC.Iface.Load.loadHiBootInterface
-            ((moduleName imp_mod,want_boot):dep_mods deps,dep_pkgs deps,ptrust)
+            ( GWIB { gwib_mod = moduleName imp_mod, gwib_isBoot = want_boot } : dep_mods deps
+            , dep_pkgs deps
+            , ptrust
+            )
 
          | otherwise =
             -- Imported module is from another package
@@ -1698,20 +1701,23 @@ qualImportItemErr rdr
   = hang (text "Illegal qualified name in import item:")
        2 (ppr rdr)
 
+pprImpDeclSpec :: ModIface -> ImpDeclSpec -> SDoc
+pprImpDeclSpec iface decl_spec =
+  quotes (ppr (is_mod decl_spec)) <+> case mi_boot iface of
+    IsBoot -> text "(hi-boot interface)"
+    NotBoot -> Outputable.empty
+
 badImportItemErrStd :: ModIface -> ImpDeclSpec -> IE GhcPs -> SDoc
 badImportItemErrStd iface decl_spec ie
-  = sep [text "Module", quotes (ppr (is_mod decl_spec)), source_import,
+  = sep [text "Module", pprImpDeclSpec iface decl_spec,
          text "does not export", quotes (ppr ie)]
-  where
-    source_import | mi_boot iface = text "(hi-boot interface)"
-                  | otherwise     = Outputable.empty
 
 badImportItemErrDataCon :: OccName -> ModIface -> ImpDeclSpec -> IE GhcPs
                         -> SDoc
 badImportItemErrDataCon dataType_occ iface decl_spec ie
   = vcat [ text "In module"
-             <+> quotes (ppr (is_mod decl_spec))
-             <+> source_import <> colon
+             <+> pprImpDeclSpec iface decl_spec
+             <> colon
          , nest 2 $ quotes datacon
              <+> text "is a data constructor of"
              <+> quotes dataType
@@ -1728,8 +1734,6 @@ badImportItemErrDataCon dataType_occ iface decl_spec ie
     datacon_occ = rdrNameOcc $ ieName ie
     datacon = parenSymOcc datacon_occ (ppr datacon_occ)
     dataType = parenSymOcc dataType_occ (ppr dataType_occ)
-    source_import | mi_boot iface = text "(hi-boot interface)"
-                  | otherwise     = Outputable.empty
     parens_sp d = parens (space <> d <> space)  -- T( f,g )
 
 badImportItemErr :: ModIface -> ImpDeclSpec -> IE GhcPs -> [AvailInfo] -> SDoc
diff --git a/compiler/GHC/Runtime/Linker.hs b/compiler/GHC/Runtime/Linker.hs
index b936bde303..d6b916ff39 100644
--- a/compiler/GHC/Runtime/Linker.hs
+++ b/compiler/GHC/Runtime/Linker.hs
@@ -68,6 +68,7 @@ import Control.Monad
 
 import qualified Data.Set as Set
 import Data.Char (isSpace)
+import Data.Function ((&))
 import Data.IORef
 import Data.List (intercalate, isPrefixOf, isSuffixOf, nub, partition)
 import Data.Maybe
@@ -670,21 +671,23 @@ getLinkDeps hsc_env hpt pls replace_osuf span mods
     follow_deps (mod:mods) acc_mods acc_pkgs
         = do
           mb_iface <- initIfaceCheck (text "getLinkDeps") hsc_env $
-                        loadInterface msg mod (ImportByUser False)
+                        loadInterface msg mod (ImportByUser NotBoot)
           iface <- case mb_iface of
                     Maybes.Failed err      -> throwGhcExceptionIO (ProgramError (showSDoc dflags err))
                     Maybes.Succeeded iface -> return iface
 
-          when (mi_boot iface) $ link_boot_mod_error mod
+          when (mi_boot iface == IsBoot) $ link_boot_mod_error mod
 
           let
             pkg = moduleUnit mod
             deps  = mi_deps iface
 
             pkg_deps = dep_pkgs deps
-            (boot_deps, mod_deps) = partitionWith is_boot (dep_mods deps)
-                    where is_boot (m,True)  = Left m
-                          is_boot (m,False) = Right m
+            (boot_deps, mod_deps) = flip partitionWith (dep_mods deps) $
+              \ (GWIB { gwib_mod = m, gwib_isBoot = is_boot }) ->
+                m & case is_boot of
+                  IsBoot -> Left
+                  NotBoot -> Right
 
             boot_deps' = filter (not . (`elementOfUniqDSet` acc_mods)) boot_deps
             acc_mods'  = addListToUniqDSet acc_mods (moduleName mod : mod_deps)
diff --git a/compiler/GHC/Tc/Module.hs b/compiler/GHC/Tc/Module.hs
index 94402c0989..267a36cd89 100644
--- a/compiler/GHC/Tc/Module.hs
+++ b/compiler/GHC/Tc/Module.hs
@@ -321,7 +321,7 @@ tcRnImports hsc_env import_decls
   = do  { (rn_imports, rdr_env, imports, hpc_info) <- rnImports import_decls ;
 
         ; this_mod <- getModule
-        ; let { dep_mods :: ModuleNameEnv (ModuleName, IsBootInterface)
+        ; let { dep_mods :: ModuleNameEnv ModuleNameWithIsBoot
               ; dep_mods = imp_dep_mods imports
 
                 -- We want instance declarations from all home-package
@@ -1973,7 +1973,7 @@ runTcInteractive hsc_env thing_inside
        ; let getOrphans m mb_pkg = fmap (\iface -> mi_module iface
                                           : dep_orphs (mi_deps iface))
                                  (loadSrcInterface (text "runTcInteractive") m
-                                                   False mb_pkg)
+                                                   NotBoot mb_pkg)
 
        ; !orphs <- fmap (force . concat) . forM (ic_imports icxt) $ \i ->
             case i of                   -- force above: see #15111
diff --git a/compiler/GHC/Tc/Types.hs b/compiler/GHC/Tc/Types.hs
index 6e60efd4d5..4da234ea08 100644
--- a/compiler/GHC/Tc/Types.hs
+++ b/compiler/GHC/Tc/Types.hs
@@ -262,7 +262,7 @@ data IfLclEnv
         -- Whether or not the IfaceDecl came from a boot
         -- file or not; we'll use this to choose between
         -- NoUnfolding and BootUnfolding
-        if_boot :: Bool,
+        if_boot :: IsBootInterface,
 
         -- The field is used only for error reporting
         -- if (say) there's a Lint error in it
@@ -1340,7 +1340,7 @@ data ImportAvails
           -- different packages. (currently not the case, but might be in the
           -- future).
 
-        imp_dep_mods :: ModuleNameEnv (ModuleName, IsBootInterface),
+        imp_dep_mods :: ModuleNameEnv ModuleNameWithIsBoot,
           -- ^ Home-package modules needed by the module being compiled
           --
           -- It doesn't matter whether any of these dependencies
@@ -1381,15 +1381,15 @@ data ImportAvails
           -- including us for imported modules)
       }
 
-mkModDeps :: [(ModuleName, IsBootInterface)]
-          -> ModuleNameEnv (ModuleName, IsBootInterface)
+mkModDeps :: [ModuleNameWithIsBoot]
+          -> ModuleNameEnv ModuleNameWithIsBoot
 mkModDeps deps = foldl' add emptyUFM deps
-               where
-                 add env elt@(m,_) = addToUFM env m elt
+  where
+    add env elt = addToUFM env (gwib_mod elt) elt
 
 modDepsElts
-  :: ModuleNameEnv (ModuleName, IsBootInterface)
-  -> [(ModuleName, IsBootInterface)]
+  :: ModuleNameEnv ModuleNameWithIsBoot
+  -> [ModuleNameWithIsBoot]
 modDepsElts = sort . nonDetEltsUFM
   -- It's OK to use nonDetEltsUFM here because sorting by module names
   -- restores determinism
@@ -1426,9 +1426,10 @@ plusImportAvails
                    imp_orphs         = orphs1 `unionLists` orphs2,
                    imp_finsts        = finsts1 `unionLists` finsts2 }
   where
-    plus_mod_dep r1@(m1, boot1) r2@(m2, boot2)
-      | ASSERT2( m1 == m2, (ppr m1 <+> ppr m2) $$ (ppr boot1 <+> ppr boot2) )
-        boot1 = r2
+    plus_mod_dep r1@(GWIB { gwib_mod = m1, gwib_isBoot = boot1 })
+                 r2@(GWIB {gwib_mod = m2, gwib_isBoot = boot2})
+      | ASSERT2( m1 == m2, (ppr m1 <+> ppr m2) $$ (ppr (boot1 == IsBoot) <+> ppr (boot2 == IsBoot)))
+        boot1 == IsBoot = r2
       | otherwise = r1
       -- If either side can "see" a non-hi-boot interface, use that
       -- Reusing existing tuples saves 10% of allocations on test
@@ -1451,8 +1452,8 @@ data WhereFrom
                                         -- See Note [Care with plugin imports] in GHC.Iface.Load
 
 instance Outputable WhereFrom where
-  ppr (ImportByUser is_boot) | is_boot     = text "{- SOURCE -}"
-                             | otherwise   = empty
+  ppr (ImportByUser IsBoot)                = text "{- SOURCE -}"
+  ppr (ImportByUser NotBoot)               = empty
   ppr ImportBySystem                       = text "{- SYSTEM -}"
   ppr ImportByPlugin                       = text "{- PLUGIN -}"
 
diff --git a/compiler/GHC/Tc/Utils/Backpack.hs b/compiler/GHC/Tc/Utils/Backpack.hs
index d28dad8f70..98458b884b 100644
--- a/compiler/GHC/Tc/Utils/Backpack.hs
+++ b/compiler/GHC/Tc/Utils/Backpack.hs
@@ -549,7 +549,7 @@ mergeSignatures
             im = fst (getModuleInstantiation m)
         in fmap fst
          . withException
-         $ findAndReadIface (text "mergeSignatures") im m False
+         $ findAndReadIface (text "mergeSignatures") im m NotBoot
 
     -- STEP 3: Get the unrenamed exports of all these interfaces,
     -- thin it according to the export list, and do shaping on them.
@@ -842,7 +842,7 @@ mergeSignatures
             -- supposed to include itself in its dep_orphs/dep_finsts.  See #13214
             iface' = iface { mi_final_exts = (mi_final_exts iface){ mi_orphan = False, mi_finsts = False } }
             avails = plusImportAvails (tcg_imports tcg_env) $
-                        calculateAvails dflags iface' False False ImportedBySystem
+                        calculateAvails dflags iface' False NotBoot ImportedBySystem
         return tcg_env {
             tcg_inst_env = inst_env,
             tcg_insts    = insts,
@@ -929,7 +929,7 @@ checkImplements impl_mod req_mod@(Module uid mod_name) =
 
     dflags <- getDynFlags
     let avails = calculateAvails dflags
-                    impl_iface False{- safe -} False{- boot -} ImportedBySystem
+                    impl_iface False{- safe -} NotBoot ImportedBySystem
         fix_env = mkNameEnv [ (gre_name rdr_elt, FixItem occ f)
                             | (occ, f) <- mi_fixities impl_iface
                             , rdr_elt <- lookupGlobalRdrEnv impl_gr occ ]
@@ -953,7 +953,7 @@ checkImplements impl_mod req_mod@(Module uid mod_name) =
     -- instantiation is correct.
     let sig_mod = mkModule (VirtUnit uid) mod_name
         isig_mod = fst (getModuleInstantiation sig_mod)
-    mb_isig_iface <- findAndReadIface (text "checkImplements 2") isig_mod sig_mod False
+    mb_isig_iface <- findAndReadIface (text "checkImplements 2") isig_mod sig_mod NotBoot
     isig_iface <- case mb_isig_iface of
         Succeeded (iface, _) -> return iface
         Failed err -> failWithTc $
diff --git a/compiler/GHC/Tc/Utils/Monad.hs b/compiler/GHC/Tc/Utils/Monad.hs
index 85b3ad2e96..2fc741ce6f 100644
--- a/compiler/GHC/Tc/Utils/Monad.hs
+++ b/compiler/GHC/Tc/Utils/Monad.hs
@@ -1830,7 +1830,7 @@ setLocalRdrEnv rdr_env thing_inside
 ************************************************************************
 -}
 
-mkIfLclEnv :: Module -> SDoc -> Bool -> IfLclEnv
+mkIfLclEnv :: Module -> SDoc -> IsBootInterface -> IfLclEnv
 mkIfLclEnv mod loc boot
                    = IfLclEnv { if_mod     = mod,
                                 if_loc     = loc,
@@ -1887,14 +1887,14 @@ initIfaceCheck doc hsc_env do_this
                     }
       initTcRnIf 'i' hsc_env gbl_env () do_this
 
-initIfaceLcl :: Module -> SDoc -> Bool -> IfL a -> IfM lcl a
+initIfaceLcl :: Module -> SDoc -> IsBootInterface -> IfL a -> IfM lcl a
 initIfaceLcl mod loc_doc hi_boot_file thing_inside
   = setLclEnv (mkIfLclEnv mod loc_doc hi_boot_file) thing_inside
 
 -- | Initialize interface typechecking, but with a 'NameShape'
 -- to apply when typechecking top-level 'OccName's (see
 -- 'lookupIfaceTop')
-initIfaceLclWithSubst :: Module -> SDoc -> Bool -> NameShape -> IfL a -> IfM lcl a
+initIfaceLclWithSubst :: Module -> SDoc -> IsBootInterface -> NameShape -> IfL a -> IfM lcl a
 initIfaceLclWithSubst mod loc_doc hi_boot_file nsubst thing_inside
   = setLclEnv ((mkIfLclEnv mod loc_doc hi_boot_file) { if_nsubst = Just nsubst }) thing_inside
 
diff --git a/compiler/GHC/Unit/Module.hs b/compiler/GHC/Unit/Module.hs
index 7eed456311..14751d7003 100644
--- a/compiler/GHC/Unit/Module.hs
+++ b/compiler/GHC/Unit/Module.hs
@@ -9,13 +9,13 @@ These are Uniquable, hence we can build Maps with Modules as
 the keys.
 -}
 
-{-# LANGUAGE RecordWildCards #-}
-{-# LANGUAGE MultiParamTypeClasses #-}
-{-# LANGUAGE ExplicitNamespaces #-}
-{-# LANGUAGE TypeSynonymInstances #-}
-{-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE DeriveFunctor #-}
+{-# LANGUAGE ExplicitNamespaces #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE RecordWildCards #-}
+{-# LANGUAGE TypeSynonymInstances #-}
 
 module GHC.Unit.Module
     ( module GHC.Unit.Types
@@ -29,7 +29,6 @@ module GHC.Unit.Module
       -- * ModuleEnv
     , module GHC.Unit.Module.Env
 
-
       -- * Generalization
     , getModuleInstantiation
     , getUnitInstantiations
@@ -148,4 +147,3 @@ isHoleModule _                   = False
 -- | Create a hole Module
 mkHoleModule :: ModuleName -> GenModule (GenUnit u)
 mkHoleModule = Module HoleUnit
-
diff --git a/compiler/GHC/Unit/Module/Location.hs b/compiler/GHC/Unit/Module/Location.hs
index 540f2305d2..7518bd63e8 100644
--- a/compiler/GHC/Unit/Module/Location.hs
+++ b/compiler/GHC/Unit/Module/Location.hs
@@ -9,6 +9,7 @@ module GHC.Unit.Module.Location
 where
 
 import GHC.Prelude
+import GHC.Unit.Types
 import GHC.Utils.Outputable
 
 -- | Module Location
@@ -54,10 +55,10 @@ addBootSuffix :: FilePath -> FilePath
 addBootSuffix path = path ++ "-boot"
 
 -- | Add the @-boot@ suffix if the @Bool@ argument is @True@
-addBootSuffix_maybe :: Bool -> FilePath -> FilePath
-addBootSuffix_maybe is_boot path
- | is_boot   = addBootSuffix path
- | otherwise = path
+addBootSuffix_maybe :: IsBootInterface -> FilePath -> FilePath
+addBootSuffix_maybe is_boot path = case is_boot of
+  IsBoot -> addBootSuffix path
+  NotBoot -> path
 
 -- | Add the @-boot@ suffix to all file paths associated with the module
 addBootSuffixLocn :: ModLocation -> ModLocation
diff --git a/compiler/GHC/Unit/Types.hs b/compiler/GHC/Unit/Types.hs
index 7282b385b6..04db40a154 100644
--- a/compiler/GHC/Unit/Types.hs
+++ b/compiler/GHC/Unit/Types.hs
@@ -1,7 +1,8 @@
 {-# LANGUAGE TypeSynonymInstances #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE DeriveDataTypeable #-}
-{-# LANGUAGE DeriveFunctor #-}
+{-# LANGUAGE DeriveTraversable #-}
+{-# LANGUAGE NamedFieldPuns #-}
 
 -- | Unit & Module types
 --
@@ -63,6 +64,12 @@ module GHC.Unit.Types
    , interactiveUnitId
    , isInteractiveModule
    , wiredInUnitIds
+
+     -- * Boot modules
+   , IsBootInterface (..)
+   , GenWithIsBoot (..)
+   , ModuleNameWithIsBoot
+   , ModuleWithIsBoot
    )
 where
 
@@ -634,3 +641,64 @@ wiredInUnitIds =
    , thUnitId
    , thisGhcUnitId
    ]
+
+---------------------------------------------------------------------
+-- Boot Modules
+---------------------------------------------------------------------
+
+-- Note [Boot Module Naming]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~
+-- Why is this section here? After all, these modules are supposed to be about
+-- ways of referring to modules, not modules themselves. Well, the "bootness" of
+-- a module is in a way part of its name, because 'import {-# SOURCE #-} Foo'
+-- references the boot module in particular while 'import Foo' references the
+-- regular module. Backpack signatures live in the normal module namespace (no
+-- special import), so they don't matter here. When dealing with the modules
+-- themselves, however, one should use not 'IsBoot' or conflate signatures and
+-- modules in opposition to boot interfaces. Instead, one should use
+-- 'DriverPhases.HscSource'. See Note [HscSource types].
+
+-- | Indicates whether a module name is referring to a boot interface (hs-boot
+-- file) or regular module (hs file). We need to treat boot modules specially
+-- when building compilation graphs, since they break cycles. Regular source
+-- files and signature files are treated equivalently.
+data IsBootInterface = NotBoot | IsBoot
+  deriving (Eq, Ord, Show, Data)
+
+instance Binary IsBootInterface where
+  put_ bh ib = put_ bh $
+    case ib of
+      NotBoot -> False
+      IsBoot -> True
+  get bh = do
+    b <- get bh
+    return $ case b of
+      False -> NotBoot
+      True -> IsBoot
+
+-- | This data type just pairs a value 'mod' with an IsBootInterface flag. In
+-- practice, 'mod' is usually a @Module@ or @ModuleName@'.
+data GenWithIsBoot mod = GWIB
+  { gwib_mod :: mod
+  , gwib_isBoot :: IsBootInterface
+  } deriving ( Eq, Ord, Show
+             , Functor, Foldable, Traversable
+             )
+
+type ModuleNameWithIsBoot = GenWithIsBoot ModuleName
+
+type ModuleWithIsBoot = GenWithIsBoot Module
+
+instance Binary a => Binary (GenWithIsBoot a) where
+  put_ bh (GWIB { gwib_mod, gwib_isBoot }) = do
+    put_ bh gwib_mod
+    put_ bh gwib_isBoot
+  get bh = do
+    gwib_mod <- get bh
+    gwib_isBoot <- get bh
+    pure $ GWIB { gwib_mod, gwib_isBoot }
+
+instance Outputable a => Outputable (GenWithIsBoot a) where
+  ppr (GWIB  { gwib_mod, gwib_isBoot }) = hsep $ ppr gwib_mod : case gwib_isBoot of
+    IsBoot -> []
+    NotBoot -> [text "{-# SOURCE #-}"]