wip: unify -c --make

12 files changed, 159 insertions, 143 deletions

diff --git a/compiler/GHC/Driver/Dependencies.hs b/compiler/GHC/Driver/Dependencies.hs
index bbfc22730a..a913f00ba0 100644
--- a/compiler/GHC/Driver/Dependencies.hs
+++ b/compiler/GHC/Driver/Dependencies.hs
@@ -51,7 +51,7 @@ import GHC.Types.Unique.DSet
 import qualified GHC.Data.Maybe as Maybes
 import GHC.Linker.Types
 import GHC.Data.List.SetOps
-import Data.List ( isSuffixOf, nub, isPrefixOf, intercalate )
+import Data.List ( isSuffixOf, nub, isPrefixOf, intercalate, sortBy )
 import GHC.Runtime.Interpreter
 import qualified GHC.Data.ShortText as ST
 import qualified GHC.Unit as Packages
@@ -68,15 +68,16 @@ import GHC.Platform
 import Data.IORef
 import GHC.Platform.Ways
 import GHC.Driver.Phases
+import Data.Ord
 
 
 -- | Used in OneShot mode to work out what the HPT looks like
-findHomeModules :: HscEnv -> HomeUnit -> [ModuleNameWithIsBoot] -> IO (ModuleNameEnv ModuleNameWithIsBoot)
+findHomeModules :: HscEnv -> HomeUnit -> [ModuleNameWithIsBoot] -> IO (ModuleNameEnv (ModIface, ModuleNameWithIsBoot))
 findHomeModules hsc_env home_unit mns = go mns mempty
   where
     go [] seen = return seen
     go (mn:mods) seen
-      | Just mn' <- lookupUFM seen (gwib_mod mn)
+      | Just (_, mn') <- lookupUFM seen (gwib_mod mn)
         -- Already seen the module before
       , gwib_isBoot mn' == gwib_isBoot mn = go mods seen
       | otherwise = do
@@ -86,7 +87,7 @@ findHomeModules hsc_env home_unit mns = go mns mempty
          mb_iface <- computeInterface hsc_env (text "need mi_direct_deps for") (gwib_isBoot mn) mod
          -- loadInterface should work but if you load A.hi-boot before trying to load A.hi, then the A.hi-boot
          -- interface will be returned, which is not what you want.
-         --mb_iface <- loadInterface  (text "need mi_direct_deps for") mod (ImportByUser (gwib_isBoot mn))
+--         mb_iface <- loadInterface  (text "need mi_direct_deps for") mod (ImportByUser (gwib_isBoot mn))
          case mb_iface of
            -- This case should **never** happen because there has to already be
            -- interface files for all the dependencies already in OneShot mode.
@@ -94,10 +95,10 @@ findHomeModules hsc_env home_unit mns = go mns mempty
            Failed _err -> go mods seen
            Succeeded (imported_iface, _) ->
             let new_names = dep_direct_mods $ mi_deps imported_iface
-                comb m@(GWIB { gwib_isBoot = NotBoot }) _ = m
-                comb (GWIB { gwib_isBoot = IsBoot }) x  = x
+                comb m@(_, (GWIB { gwib_isBoot = NotBoot })) _ = m
+                comb (_, (GWIB { gwib_isBoot = IsBoot })) x  = x
             in go (new_names ++ mods)
-                  (addToUFM_C comb seen (gwib_mod mn) mn)
+                  (addToUFM_C comb seen (gwib_mod mn) (imported_iface, mn))
 
 -- | Find all depedencies that we need to link, used for GHCi and
 -- interface file dependency calculation
diff --git a/compiler/GHC/Driver/Env.hs b/compiler/GHC/Driver/Env.hs
index 42abd7aab2..a92491e8e7 100644
--- a/compiler/GHC/Driver/Env.hs
+++ b/compiler/GHC/Driver/Env.hs
@@ -236,9 +236,6 @@ hptModulesBelow include_boot hsc_env mn = go mn Set.empty
 -- C.f Inst.hptInstances
 hptSomeThingsBelowUs :: (HomeModInfo -> [a]) -> Bool -> HscEnv -> [ModuleNameWithIsBoot] -> [a]
 hptSomeThingsBelowUs extract include_hi_boot hsc_env mod
-  | isOneShot (ghcMode (hsc_dflags hsc_env)) = []
-
-  | otherwise
   = let hpt = hsc_HPT hsc_env
     in
     [ thing
@@ -297,7 +294,7 @@ lookupType hsc_env name = do
                then mkHomeModule (hsc_home_unit hsc_env) (moduleName (nameModule name))
                else nameModule name
 
-       !ty = if isOneShot (ghcMode (hsc_dflags hsc_env))
+       !ty = if False -- isOneShot (ghcMode (hsc_dflags hsc_env))
                -- in one-shot, we don't use the HPT
                then lookupNameEnv pte name
                else case lookupHptByModule hpt mod of
diff --git a/compiler/GHC/Driver/Main.hs b/compiler/GHC/Driver/Main.hs
index a6b3b2f7c3..75a8353a43 100644
--- a/compiler/GHC/Driver/Main.hs
+++ b/compiler/GHC/Driver/Main.hs
@@ -83,7 +83,6 @@ module GHC.Driver.Main
     , getHscEnv
     , hscSimpleIface'
     , oneShotMsg
-    , dumpIfaceStats
     , ioMsgMaybe
     , showModuleIndex
     , hscAddSptEntries
@@ -118,7 +117,7 @@ import GHC.HsToCore
 
 import GHC.StgToByteCode    ( byteCodeGen, stgExprToBCOs )
 
-import GHC.IfaceToCore  ( typecheckIface )
+import GHC.IfaceToCore  ( typecheckIface, initModDetails, dumpIfaceStats )
 
 import GHC.Iface.Load   ( ifaceStats, writeIface )
 import GHC.Iface.Make
@@ -881,7 +880,7 @@ hscIncrementalCompile always_do_basic_recompilation_check m_tc_result
         -- We didn't need to do any typechecking; the old interface
         -- file on disk was good enough.
         Left (iface, linkable) -> do
-            details <- liftIO $ initModDetails hsc_env mod_summary iface
+            details <- liftIO $ initModDetails hsc_env (ms_mod_name mod_summary) iface
             return (HscUpToDate (HomeModInfo iface details linkable), hsc_env')
         -- We finished type checking.  (mb_old_hash is the hash of
         -- the interface that existed on disk; it's possible we had
@@ -891,64 +890,8 @@ hscIncrementalCompile always_do_basic_recompilation_check m_tc_result
             status <- finish mod_summary tc_result mb_old_hash
             return (status, hsc_env)
 
--- Knot tying!  See Note [Knot-tying typecheckIface]
--- See Note [ModDetails and --make mode]
-initModDetails :: HscEnv -> ModSummary -> ModIface -> IO ModDetails
-initModDetails hsc_env mod_summary iface =
-  fixIO $ \details' -> do
-    let act hpt  = addToHpt hpt (ms_mod_name mod_summary)
-                                (HomeModInfo iface details' Nothing)
-    let hsc_env' = hscUpdateHPT act hsc_env
-    -- NB: This result is actually not that useful
-    -- in one-shot mode, since we're not going to do
-    -- any further typechecking.  It's much more useful
-    -- in make mode, since this HMI will go into the HPT.
-    genModDetails hsc_env' iface
 
 
-{-
-Note [ModDetails and --make mode]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-An interface file consists of two parts
-
-* The `ModIface` which ends up getting written to disk.
-  The `ModIface` is a completely acyclic tree, which can be serialised
-  and de-serialised completely straightforwardly.  The `ModIface` is
-  also the structure that is finger-printed for recompilation control.
-
-* The `ModDetails` which provides a more structured view that is suitable
-  for usage during compilation.  The `ModDetails` is heavily cyclic:
-  An `Id` contains a `Type`, which mentions a `TyCon` that contains kind
-  that mentions other `TyCons`; the `Id` also includes an unfolding that
-  in turn mentions more `Id`s;  And so on.
-
-The `ModIface` can be created from the `ModDetails` and the `ModDetails` from
-a `ModIface`.
-
-During tidying, just before interfaces are written to disk,
-the ModDetails is calculated and then converted into a ModIface (see GHC.Iface.Make.mkIface_).
-Then when GHC needs to restart typechecking from a certain point it can read the
-interface file, and regenerate the ModDetails from the ModIface (see GHC.IfaceToCore.typecheckIface).
-The key part about the loading is that the ModDetails is regenerated lazily
-from the ModIface, so that there's only a detailed in-memory representation
-for declarations which are actually used from the interface. This mode is
-also used when reading interface files from external packages.
-
-In the old --make mode implementation, the interface was written after compiling a module
-but the in-memory ModDetails which was used to compute the ModIface was retained.
-The result was that --make mode used much more memory than `-c` mode, because a large amount of
-information about a module would be kept in the ModDetails but never used.
-
-The new idea is that even in `--make` mode, when there is an in-memory `ModDetails`
-at hand, we re-create the `ModDetails` from the `ModIface`. Doing this means that
-we only have to keep the `ModIface` decls in memory and then lazily load
-detailed representations if needed. It turns out this makes a really big difference
-to memory usage, halving maximum memory used in some cases.
-
-See !5492 and #13586
--}
-
 -- Runs the post-typechecking frontend (desugar and simplify). We want to
 -- generate most of the interface as late as possible. This gets us up-to-date
 -- and good unfoldings and other info in the interface file.
@@ -1155,14 +1098,6 @@ hscMaybeWriteIface logger dflags is_simple iface old_iface mod_location = do
 -- NoRecomp handlers
 --------------------------------------------------------------
 
--- NB: this must be knot-tied appropriately, see hscIncrementalCompile
-genModDetails :: HscEnv -> ModIface -> IO ModDetails
-genModDetails hsc_env old_iface
-  = do
-    new_details <- {-# SCC "tcRnIface" #-}
-                   initIfaceLoad hsc_env (typecheckIface old_iface)
-    dumpIfaceStats hsc_env
-    return new_details
 
 --------------------------------------------------------------
 -- Progress displayers.
@@ -2273,23 +2208,6 @@ hscCompileCoreExpr' hsc_env srcspan ds_expr
          ; loadExpr (hscInterp hsc_env) hsc_env srcspan bcos }
 
 
-{- **********************************************************************
-%*                                                                      *
-        Statistics on reading interfaces
-%*                                                                      *
-%********************************************************************* -}
-
-dumpIfaceStats :: HscEnv -> IO ()
-dumpIfaceStats hsc_env = do
-    eps <- hscEPS hsc_env
-    dumpIfSet logger dflags (dump_if_trace || dump_rn_stats)
-              "Interface statistics"
-              (ifaceStats eps)
-  where
-    dflags = hsc_dflags hsc_env
-    logger = hsc_logger hsc_env
-    dump_rn_stats = dopt Opt_D_dump_rn_stats dflags
-    dump_if_trace = dopt Opt_D_dump_if_trace dflags
 
 
 {- **********************************************************************
diff --git a/compiler/GHC/Driver/Make.hs b/compiler/GHC/Driver/Make.hs
index 24ffdc1635..1f564cbf17 100644
--- a/compiler/GHC/Driver/Make.hs
+++ b/compiler/GHC/Driver/Make.hs
@@ -135,6 +135,7 @@ import System.FilePath
 import System.IO        ( fixIO )
 
 import GHC.Conc ( getNumProcessors, getNumCapabilities, setNumCapabilities )
+import GHC.Driver.Ppr
 
 label_self :: String -> IO ()
 label_self thread_name = do
@@ -937,6 +938,10 @@ buildCompGraph (scc:sccs) = case scc of
 data BuildModule = BuildModule_Unit {-# UNPACK #-} !InstantiatedUnit | BuildModule_Module {-# UNPACK #-} !ModuleWithIsBoot
   deriving (Eq, Ord)
 
+instance Outputable BuildModule where
+  ppr (BuildModule_Unit u) = ppr u
+  ppr (BuildModule_Module m) = ppr m
+
 
 mkBuildModule :: ModuleGraphNode -> BuildModule
 mkBuildModule = \case
diff --git a/compiler/GHC/Driver/Pipeline.hs b/compiler/GHC/Driver/Pipeline.hs
index 08fb0c9889..8f8300e2ea 100644
--- a/compiler/GHC/Driver/Pipeline.hs
+++ b/compiler/GHC/Driver/Pipeline.hs
@@ -16,6 +16,7 @@
 --
 -----------------------------------------------------------------------------
 
+{-# LANGUAGE TupleSections #-}
 module GHC.Driver.Pipeline (
         -- Run a series of compilation steps in a pipeline, for a
         -- collection of source files.
@@ -83,7 +84,7 @@ import qualified GHC.LanguageExtensions as LangExt
 import GHC.Settings
 
 import GHC.Data.Bag            ( unitBag )
-import GHC.Data.FastString     ( mkFastString )
+import GHC.Data.FastString     ( mkFastString, FastString )
 import GHC.Data.StringBuffer   ( hGetStringBuffer, hPutStringBuffer )
 import GHC.Data.Maybe          ( expectJust )
 
@@ -117,6 +118,8 @@ import Data.Version
 import Data.Either      ( partitionEithers )
 
 import Data.Time        ( getCurrentTime )
+import GHC.Driver.Dependencies
+import GHC.Types.Unique.FM
 
 -- ---------------------------------------------------------------------------
 -- Pre-process
@@ -265,7 +268,7 @@ compileOne' m_tc_result mHscMessage
             final_iface <- mkFullIface hsc_env' partial_iface Nothing
             -- Reconstruct the `ModDetails` from the just-constructed `ModIface`
             -- See Note [ModDetails and --make mode]
-            hmi_details <- liftIO $ initModDetails hsc_env' summary final_iface
+            hmi_details <- liftIO $ initModDetails hsc_env' (ms_mod_name summary) final_iface
             liftIO $ hscMaybeWriteIface logger dflags True final_iface mb_old_iface_hash (ms_location summary)
 
             (hasStub, comp_bc, spt_entries) <- hscInteractive hsc_env' cgguts mod_location
@@ -297,7 +300,7 @@ compileOne' m_tc_result mHscMessage
             o_time <- getModificationUTCTime object_filename
             let !linkable = LM o_time this_mod [DotO object_filename]
             -- See Note [ModDetails and --make mode]
-            details <- initModDetails hsc_env' summary iface
+            details <- initModDetails hsc_env' (ms_mod_name summary) iface
             return $! HomeModInfo iface details (Just linkable)
 
  where dflags0     = ms_hspp_opts summary
@@ -1083,6 +1086,30 @@ llvmOptions dflags =
                 ArchRISCV64 -> "lp64d"
                 _           -> ""
 
+-- | Populate the HPT with the transively discovered interfaces from the imports
+-- given
+preloadInterfaces :: HscEnv -> [(Maybe FastString, ModuleNameWithIsBoot)] -> IO (HscEnv -> HscEnv)
+preloadInterfaces hsc_env imps = do
+  let fc = hsc_FC hsc_env
+  let dflags = hsc_dflags hsc_env
+  let units = hsc_units hsc_env
+  let home_unit = hsc_home_unit hsc_env
+  let find mod maybe_pkg = liftIO $ findImportedModule fc units home_unit dflags mod maybe_pkg
+  let do_one (mb_pkg, GWIB mod_name is_boot) = do
+        res <- find mod_name mb_pkg
+        case res of
+          Found _ m | isHomeModule home_unit m -> return $ Just $ GWIB mod_name is_boot
+          _ -> return Nothing
+      mk_hmi mod_iface mod_name = do
+        (gwib_mod mod_name,) . (\md -> HomeModInfo mod_iface md Nothing) <$> initModDetails hsc_env (gwib_mod mod_name) mod_iface
+  direct_home_mods <- mapMaybeM do_one imps
+  interfaces <- liftIO $ findHomeModules hsc_env (hsc_home_unit hsc_env) direct_home_mods
+  hmis <- liftIO $ mapM (uncurry mk_hmi) (eltsUFM interfaces)
+  let new_hpt = addListToHpt emptyHomePackageTable hmis
+  let upd = hscUpdateHPT (const new_hpt)
+  pprTraceM "preloadInterfaces" (ppr (fmap snd interfaces))
+  return upd
+
 -- -----------------------------------------------------------------------------
 -- | Each phase in the pipeline returns the next phase to execute, and the
 -- name of the file in which the output was placed.
@@ -1248,6 +1275,11 @@ runPhase (RealPhase (Hsc src_flavour)) input_fn
               Right (src_imps,imps,L _ mod_name) -> return
                   (Just buf, mod_name, imps, src_imps)
 
+        PipeState{hsc_env=hsc_env'} <- getPipeState
+        let mns = [(mb_pkg, GWIB (unLoc mn) NotBoot) | (mb_pkg, mn) <- imps]
+                  ++ [(mb_pkg, GWIB (unLoc mn) IsBoot) | (mb_pkg, mn) <- src_imps]
+        upd_hsc <- liftIO $ preloadInterfaces hsc_env' mns
+
   -- Take -o into account if present
   -- Very like -ohi, but we must *only* do this if we aren't linking
   -- (If we're linking then the -o applies to the linked thing, not to
@@ -1265,7 +1297,6 @@ runPhase (RealPhase (Hsc src_flavour)) input_fn
         o_mod <- liftIO $ modificationTimeIfExists o_file
         dyn_o_mod <- liftIO $ modificationTimeIfExists dyn_o_file
 
-        PipeState{hsc_env=hsc_env'} <- getPipeState
 
   -- Tell the finder cache about this module
         mod <- liftIO $ do
@@ -1295,7 +1326,7 @@ runPhase (RealPhase (Hsc src_flavour)) input_fn
   -- run the compiler!
         let msg hsc_env _ what _ = oneShotMsg hsc_env what
         (result, plugin_hsc_env) <-
-          liftIO $ hscIncrementalCompile True Nothing (Just msg) hsc_env'
+          liftIO $ hscIncrementalCompile True Nothing (Just msg) (upd_hsc hsc_env')
                             mod_summary Nothing Nothing (1,1)
 
         -- In the rest of the pipeline use the loaded plugins
diff --git a/compiler/GHC/Iface/Load.hs b/compiler/GHC/Iface/Load.hs
index 03627641aa..7b8069abb6 100644
--- a/compiler/GHC/Iface/Load.hs
+++ b/compiler/GHC/Iface/Load.hs
@@ -940,8 +940,7 @@ findAndReadIface logger name_cache fc hooks unit_state home_unit dflags doc_str
                   -- Found file, so read it
                   let file_path = addBootSuffix_maybe hi_boot_file (ml_hi_file loc)
                   -- See Note [Home module load error]
-                  if isHomeInstalledModule home_unit mod &&
-                     not (isOneShot (ghcMode dflags))
+                  if isHomeInstalledModule home_unit mod
                       then return (Failed (homeModError mod loc))
                       else do
                         r <- read_file logger name_cache unit_state dflags wanted_mod file_path
diff --git a/compiler/GHC/Iface/Recomp.hs b/compiler/GHC/Iface/Recomp.hs
index 059619baa9..b2f3e00af9 100644
--- a/compiler/GHC/Iface/Recomp.hs
+++ b/compiler/GHC/Iface/Recomp.hs
@@ -282,7 +282,7 @@ checkVersions hsc_env mod_summary iface
         --  ; pprTraceM "home_mods" (ppr home_mods)
         --  ; pprTraceM "home_mods" (ppr (dep_mods (mi_deps iface)))
         --  ; pprTraceM "home_mods" (ppr (Set.difference  (Set.fromList (eltsUFM home_mods)) (Set.fromList (dep_mods (mi_deps iface)))))
-          ; updateEps_ $ \eps  -> eps { eps_is_boot = home_mods }
+          ; updateEps_ $ \eps  -> eps { eps_is_boot = fmap snd $ home_mods }
        }
        ; recomp <- checkList [checkModUsage (homeUnitAsUnit home_unit) u
                              | u <- mi_usages iface]
diff --git a/compiler/GHC/IfaceToCore.hs b/compiler/GHC/IfaceToCore.hs
index b3644633af..477ead3dfe 100644
--- a/compiler/GHC/IfaceToCore.hs
+++ b/compiler/GHC/IfaceToCore.hs
@@ -21,7 +21,8 @@ module GHC.IfaceToCore (
         tcIfaceAnnotations, tcIfaceCompleteMatches,
         tcIfaceExpr,    -- Desired by HERMIT (#7683)
         tcIfaceGlobal,
-        tcIfaceOneShot
+        tcIfaceOneShot,
+        initModDetails, genModDetails, dumpIfaceStats
  ) where
 
 #include "HsVersions.h"
@@ -56,7 +57,7 @@ import GHC.Core
 import GHC.Core.Unify( RoughMatchTc(..) )
 import GHC.Core.Utils
 import GHC.Core.Unfold.Make
-import GHC.Core.Lint
+import GHC.Core.Lint hiding (dumpIfSet)
 import GHC.Core.Make
 import GHC.Core.Class
 import GHC.Core.TyCon
@@ -108,6 +109,7 @@ import qualified GHC.Data.BooleanFormula as BF
 
 import Control.Monad
 import GHC.Parser.Annotation
+import System.IO
 
 {-
 This module takes
@@ -558,27 +560,9 @@ tcHiBootIface hsc_src mod
         ; case read_result of {
             Succeeded (iface, _path) -> do { tc_iface <- initIfaceTcRn $ typecheckIface iface
                                            ; mkSelfBootInfo iface tc_iface } ;
-            Failed err               ->
-
-        -- There was no hi-boot file. But if there is circularity in
-        -- the module graph, there really should have been one.
-        -- Since we've read all the direct imports by now,
-        -- eps_is_boot will record if any of our imports mention the
-        -- current module, which either means a module loop (not
-        -- a SOURCE import) or that our hi-boot file has mysteriously
-        -- disappeared.
-    do  { eps <- getEps
-        ; case lookupUFM (eps_is_boot eps) (moduleName mod) of
-            -- 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
-    }}}}
+            Failed err               -> return NoSelfBoot }
+
+    }}
   where
     need = text "Need the hi-boot interface for" <+> ppr mod
                  <+> text "to compare against the Real Thing"
@@ -613,6 +597,89 @@ mkSelfBootInfo iface mds
     isIfaceTyCon IfaceAxiom{}   = False
     isIfaceTyCon IfacePatSyn{}  = False
 
+-- Knot tying!  See Note [Knot-tying typecheckIface]
+-- See Note [ModDetails and --make mode]
+initModDetails :: HscEnv -> ModuleName -> ModIface -> IO ModDetails
+initModDetails hsc_env mod_name iface =
+  fixIO $ \details' -> do
+    let act hpt  = addToHpt hpt mod_name
+                                (HomeModInfo iface details' Nothing)
+    let hsc_env' = hscUpdateHPT act hsc_env
+    -- NB: This result is actually not that useful
+    -- in one-shot mode, since we're not going to do
+    -- any further typechecking.  It's much more useful
+    -- in make mode, since this HMI will go into the HPT.
+    genModDetails hsc_env' iface
+
+-- NB: this must be knot-tied appropriately, see hscIncrementalCompile
+genModDetails :: HscEnv -> ModIface -> IO ModDetails
+genModDetails hsc_env old_iface
+  = do
+    new_details <- {-# SCC "tcRnIface" #-}
+                   initIfaceLoad hsc_env (typecheckIface old_iface)
+    dumpIfaceStats hsc_env
+    return new_details
+
+{- **********************************************************************
+%*                                                                      *
+        Statistics on reading interfaces
+%*                                                                      *
+%********************************************************************* -}
+
+dumpIfaceStats :: HscEnv -> IO ()
+dumpIfaceStats hsc_env = do
+    eps <- hscEPS hsc_env
+    dumpIfSet logger dflags (dump_if_trace || dump_rn_stats)
+              "Interface statistics"
+              (ifaceStats eps)
+  where
+    dflags = hsc_dflags hsc_env
+    logger = hsc_logger hsc_env
+    dump_rn_stats = dopt Opt_D_dump_rn_stats dflags
+    dump_if_trace = dopt Opt_D_dump_if_trace dflags
+{-
+Note [ModDetails and --make mode]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+An interface file consists of two parts
+
+* The `ModIface` which ends up getting written to disk.
+  The `ModIface` is a completely acyclic tree, which can be serialised
+  and de-serialised completely straightforwardly.  The `ModIface` is
+  also the structure that is finger-printed for recompilation control.
+
+* The `ModDetails` which provides a more structured view that is suitable
+  for usage during compilation.  The `ModDetails` is heavily cyclic:
+  An `Id` contains a `Type`, which mentions a `TyCon` that contains kind
+  that mentions other `TyCons`; the `Id` also includes an unfolding that
+  in turn mentions more `Id`s;  And so on.
+
+The `ModIface` can be created from the `ModDetails` and the `ModDetails` from
+a `ModIface`.
+
+During tidying, just before interfaces are written to disk,
+the ModDetails is calculated and then converted into a ModIface (see GHC.Iface.Make.mkIface_).
+Then when GHC needs to restart typechecking from a certain point it can read the
+interface file, and regenerate the ModDetails from the ModIface (see GHC.IfaceToCore.typecheckIface).
+The key part about the loading is that the ModDetails is regenerated lazily
+from the ModIface, so that there's only a detailed in-memory representation
+for declarations which are actually used from the interface. This mode is
+also used when reading interface files from external packages.
+
+In the old --make mode implementation, the interface was written after compiling a module
+but the in-memory ModDetails which was used to compute the ModIface was retained.
+The result was that --make mode used much more memory than `-c` mode, because a large amount of
+information about a module would be kept in the ModDetails but never used.
+
+The new idea is that even in `--make` mode, when there is an in-memory `ModDetails`
+at hand, we re-create the `ModDetails` from the `ModIface`. Doing this means that
+we only have to keep the `ModIface` decls in memory and then lazily load
+detailed representations if needed. It turns out this makes a really big difference
+to memory usage, halving maximum memory used in some cases.
+
+See !5492 and #13586
+-}
+
 {-
 ************************************************************************
 *                                                                      *
diff --git a/compiler/GHC/Rename/Names.hs b/compiler/GHC/Rename/Names.hs
index 3037a4e408..b8adcd6a92 100644
--- a/compiler/GHC/Rename/Names.hs
+++ b/compiler/GHC/Rename/Names.hs
@@ -268,7 +268,6 @@ Running generateModules from #14693 with DEPTH=16, WIDTH=30 finishes in
 -}
 
 
-
 -- | Given a located import declaration @decl@ from @this_mod@,
 -- calculate the following pieces of information:
 --
diff --git a/compiler/GHC/Tc/Module.hs b/compiler/GHC/Tc/Module.hs
index c31e7925d7..6fac7fe845 100644
--- a/compiler/GHC/Tc/Module.hs
+++ b/compiler/GHC/Tc/Module.hs
@@ -18,6 +18,7 @@
 -- | Typechecking a whole module
 --
 -- https://gitlab.haskell.org/ghc/ghc/wikis/commentary/compiler/type-checker
+{-# LANGUAGE TupleSections #-}
 module GHC.Tc.Module (
         tcRnStmt, tcRnExpr, TcRnExprMode(..), tcRnType,
         tcRnImportDecls,
@@ -162,6 +163,7 @@ import GHC.Unit.Module.ModSummary
 import GHC.Unit.Module.ModIface
 import GHC.Unit.Module.ModDetails
 import GHC.Unit.Module.Deps
+import GHC.Unit.Finder.Types
 
 import GHC.Data.FastString
 import GHC.Data.Maybe
@@ -176,6 +178,9 @@ import qualified Data.Set as S
 import Control.DeepSeq
 import Control.Monad
 import GHC.Driver.Dependencies
+import GHC.Unit.Finder
+import GHC.Driver.Ppr
+import GHC.Unit.Home.ModInfo
 
 #include "HsVersions.h"
 
@@ -347,6 +352,12 @@ tcRnModuleTcRnM hsc_env mod_sum
         }
       }
 
+
+modNameBootWithPackage :: LImportDecl GhcPs -> (Maybe FastString, ModuleNameWithIsBoot)
+modNameBootWithPackage (L _ decl) =
+  ( sl_fs <$> ideclPkgQual decl
+  , GWIB (unLoc $ ideclName decl) (ideclSource decl))
+
 implicitPreludeWarn :: SDoc
 implicitPreludeWarn
   = text "Module `Prelude' implicitly imported"
@@ -385,7 +396,7 @@ tcRnImports hsc_env import_decls
                 --  ; pprTraceM "home_mods" (ppr home_mods)
                 --  ; pprTraceM "home_mods" (ppr (dep_mods (mi_deps iface)))
                 --  ; pprTraceM "home_mods" (ppr (Set.difference  (Set.fromList (eltsUFM home_mods)) (Set.fromList (dep_mods (mi_deps iface)))))
-                  ; updateEps_ $ \eps  -> eps { eps_is_boot = home_mods }
+                  ; updateEps_ $ \eps  -> eps { eps_is_boot = fmap snd home_mods }
                }
 
                 -- Update the gbl env
diff --git a/compiler/GHC/Unit/External.hs b/compiler/GHC/Unit/External.hs
index 177a9db2ba..6e3e774932 100644
--- a/compiler/GHC/Unit/External.hs
+++ b/compiler/GHC/Unit/External.hs
@@ -62,8 +62,7 @@ initExternalUnitCache = ExternalUnitCache <$> newIORef initExternalPackageState
 
 initExternalPackageState :: ExternalPackageState
 initExternalPackageState = EPS
-  { eps_is_boot          = emptyUFM
-  , eps_PIT              = emptyPackageIfaceTable
+  { eps_PIT              = emptyPackageIfaceTable
   , eps_free_holes       = emptyInstalledModuleEnv
   , eps_PTE              = emptyTypeEnv
   , eps_inst_env         = emptyInstEnv
@@ -89,19 +88,6 @@ initExternalPackageState = EPS
 -- their interface files
 data ExternalPackageState
   = EPS {
-        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
-                -- (only) we need to record which are boot modules.
-                -- We set this field after loading all the
-                -- explicitly-imported interfaces, but before doing
-                -- anything else
-                --
-                -- The 'ModuleName' part is not necessary, but it's useful for
-                -- debug prints, and it's convenient because this field comes
-                -- direct from 'GHC.Tc.Utils.imp_dep_mods'
-
         eps_PIT :: !PackageIfaceTable,
                 -- ^ The 'ModIface's for modules in external packages
                 -- whose interfaces we have opened.
diff --git a/compiler/GHC/Unit/Home/ModInfo.hs b/compiler/GHC/Unit/Home/ModInfo.hs
index fd97689972..81b94eadfd 100644
--- a/compiler/GHC/Unit/Home/ModInfo.hs
+++ b/compiler/GHC/Unit/Home/ModInfo.hs
@@ -30,6 +30,8 @@ import GHC.Types.Unique
 import GHC.Types.Unique.DFM
 
 import GHC.Utils.Outputable
+import GHC.Driver.Ppr
+import GHC.Data.Maybe
 
 -- | Information about modules in the package being compiled
 data HomeModInfo = HomeModInfo