Refactor module dependencies code

* moved deps related code into GHC.Unit.Module.Deps
* refactored Deps module to not export Dependencies constructor to help
  maintaining invariants

11 files changed, 212 insertions, 192 deletions

diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index a6f1a50382..3b28a0b17e 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -55,7 +55,6 @@ module GHC.Driver.Session (
         DynLibLoader(..),
         fFlags, fLangFlags, xFlags,
         wWarningFlags,
-        dynFlagDependencies,
         makeDynFlagsConsistent,
         positionIndependent,
         optimisationFlags,
@@ -1574,10 +1573,6 @@ lang_set dflags lang =
 setLanguage :: Language -> DynP ()
 setLanguage l = upd (`lang_set` Just l)
 
--- | Some modules have dependencies on others through the DynFlags rather than textual imports
-dynFlagDependencies :: DynFlags -> [ModuleName]
-dynFlagDependencies = pluginModNames
-
 -- | Is the -fpackage-trust mode on
 packageTrustOn :: DynFlags -> Bool
 packageTrustOn = gopt Opt_PackageTrust
diff --git a/compiler/GHC/HsToCore.hs b/compiler/GHC/HsToCore.hs
index 88d1133963..2824b1fafe 100644
--- a/compiler/GHC/HsToCore.hs
+++ b/compiler/GHC/HsToCore.hs
@@ -86,6 +86,7 @@ import GHC.Types.HpcInfo
 import GHC.Unit
 import GHC.Unit.Module.ModGuts
 import GHC.Unit.Module.ModIface
+import GHC.Unit.Module.Deps
 
 import Data.List (partition)
 import Data.IORef
diff --git a/compiler/GHC/HsToCore/Usage.hs b/compiler/GHC/HsToCore/Usage.hs
index c9c4214fd0..21bbc6eec1 100644
--- a/compiler/GHC/HsToCore/Usage.hs
+++ b/compiler/GHC/HsToCore/Usage.hs
@@ -4,7 +4,7 @@
 
 module GHC.HsToCore.Usage (
     -- * Dependency/fingerprinting code (used by GHC.Iface.Make)
-    mkUsageInfo, mkUsedNames, mkDependencies
+    mkUsageInfo, mkUsedNames,
     ) where
 
 import GHC.Prelude
@@ -23,7 +23,6 @@ import GHC.Utils.Panic
 import GHC.Types.Name
 import GHC.Types.Name.Set ( NameSet, allUses )
 import GHC.Types.Unique.Set
-import GHC.Types.Unique.FM
 
 import GHC.Unit
 import GHC.Unit.External
@@ -33,10 +32,9 @@ import GHC.Unit.Module.Deps
 
 import GHC.Data.Maybe
 
-import Data.List (sortBy, sort, partition)
+import Data.List (sortBy)
 import Data.Map (Map)
 import qualified Data.Map as Map
-import qualified Data.Set as Set
 
 import GHC.Linker.Types
 import GHC.Linker.Loader ( getLoaderState )
@@ -61,53 +59,6 @@ its dep_orphs. This was the cause of #14128.
 
 -}
 
--- | Extract information from the rename and typecheck phases to produce
--- a dependencies information for the module being compiled.
---
--- The fourth argument is a list of plugin modules.
-mkDependencies :: HomeUnit -> Module -> ImportAvails -> [Module] -> Dependencies
-mkDependencies home_unit mod imports plugin_mods =
-  let (home_plugins, external_plugins) = partition (isHomeUnit home_unit . moduleUnit) plugin_mods
-      plugin_units = map (toUnitId . moduleUnit) external_plugins
-      all_direct_mods = foldr (\mn m -> addToUFM m mn (GWIB mn NotBoot))
-                              (imp_direct_dep_mods imports)
-                              (map moduleName home_plugins)
-
-      direct_mods = modDepsElts (delFromUFM all_direct_mods (moduleName mod))
-            -- M.hi-boot can be in the imp_dep_mods, but we must remove
-            -- it before recording the modules on which this one depends!
-            -- (We want to retain M.hi-boot in imp_dep_mods so that
-            --  loadHiBootInterface can see if M's direct imports depend
-            --  on M.hi-boot, and hence that we should do the hi-boot consistency
-            --  check.)
-
-      dep_orphs = filter (/= mod) (imp_orphs imports)
-            -- We must also remove self-references from imp_orphs. See
-            -- Note [Module self-dependency]
-
-      direct_pkgs = foldr Set.insert (imp_dep_direct_pkgs imports) plugin_units
-
-      -- Set the packages required to be Safe according to Safe Haskell.
-      -- See Note [Tracking Trust Transitively] in GHC.Rename.Names
-      trust_pkgs  = imp_trust_pkgs imports
-
-      -- If there's a non-boot import, then it shadows the boot import
-      -- coming from the dependencies
-      source_mods = modDepsElts (imp_boot_mods imports)
-
-      sig_mods = filter (/= (moduleName mod)) $ imp_sig_mods imports
-
-  in Deps { dep_direct_mods  = direct_mods
-          , dep_direct_pkgs  = direct_pkgs
-          , dep_sig_mods     = sort sig_mods
-          , dep_trusted_pkgs = trust_pkgs
-          , dep_boot_mods    = source_mods
-          , dep_orphs        = dep_orphs
-          , dep_finsts       = sortBy stableModuleCmp (imp_finsts imports)
-            -- sort to get into canonical order
-            -- NB. remember to use lexicographic ordering
-          }
-
 mkUsedNames :: TcGblEnv -> NameSet
 mkUsedNames TcGblEnv{ tcg_dus = dus } = allUses dus
 
diff --git a/compiler/GHC/Iface/Load.hs b/compiler/GHC/Iface/Load.hs
index dc993aa261..c271613b3d 100644
--- a/compiler/GHC/Iface/Load.hs
+++ b/compiler/GHC/Iface/Load.hs
@@ -115,8 +115,6 @@ import GHC.Data.FastString
 
 import Control.Monad
 import Data.Map ( toList )
-import qualified Data.Set as Set
-import Data.Set (Set)
 import System.FilePath
 import System.Directory
 import GHC.Driver.Env.KnotVars
@@ -1184,32 +1182,6 @@ pprUsageImport usage usg_mod'
         safe | usg_safe usage = text "safe"
              | otherwise      = text " -/ "
 
--- | Pretty-print unit dependencies
-pprDeps :: UnitState -> Dependencies -> SDoc
-pprDeps unit_state (Deps { dep_direct_mods = dmods
-                         , dep_boot_mods = bmods
-                         , dep_orphs = orphs
-                         , dep_direct_pkgs = pkgs
-                         , dep_trusted_pkgs = tps
-                         , dep_finsts = finsts
-                         })
-  = pprWithUnitState unit_state $
-    vcat [text "direct module dependencies:"  <+> ppr_set ppr_mod dmods,
-          text "boot module dependencies:"    <+> ppr_set ppr bmods,
-          text "direct package dependencies:" <+> ppr_set ppr pkgs,
-          if null tps
-            then empty
-            else text "trusted package dependencies:" <+> ppr_set ppr tps,
-          text "orphans:" <+> fsep (map ppr orphs),
-          text "family instance modules:" <+> fsep (map ppr finsts)
-        ]
-  where
-    ppr_mod (GWIB mod IsBoot)  = ppr mod <+> text "[boot]"
-    ppr_mod (GWIB mod NotBoot) = ppr mod
-
-    ppr_set :: Outputable a => (a -> SDoc) -> Set a -> SDoc
-    ppr_set w = fsep . fmap w . Set.toAscList
-
 pprFixities :: [(OccName, Fixity)] -> SDoc
 pprFixities []    = Outputable.empty
 pprFixities fixes = text "fixities" <+> pprWithCommas pprFix fixes
diff --git a/compiler/GHC/Iface/Make.hs b/compiler/GHC/Iface/Make.hs
index ed113ef7fd..b28e777a88 100644
--- a/compiler/GHC/Iface/Make.hs
+++ b/compiler/GHC/Iface/Make.hs
@@ -82,7 +82,7 @@ import GHC.Data.FastString
 import GHC.Data.Maybe
 
 import GHC.HsToCore.Docs
-import GHC.HsToCore.Usage ( mkUsageInfo, mkUsedNames, mkDependencies )
+import GHC.HsToCore.Usage ( mkUsageInfo, mkUsedNames )
 
 import GHC.Unit
 import GHC.Unit.Module.Warnings
diff --git a/compiler/GHC/Iface/Recomp.hs b/compiler/GHC/Iface/Recomp.hs
index 0abee1a5c0..918460a236 100644
--- a/compiler/GHC/Iface/Recomp.hs
+++ b/compiler/GHC/Iface/Recomp.hs
@@ -973,11 +973,11 @@ addFingerprints hsc_env iface0
    (local_env, decls_w_hashes) <-
        foldM fingerprint_group (emptyOccEnv, []) groups
 
-   -- when calculating fingerprints, we always need to use canonical
-   -- ordering for lists of things.  In particular, the mi_deps has various
-   -- lists of modules and suchlike, so put these all in canonical order:
+   -- when calculating fingerprints, we always need to use canonical ordering
+   -- for lists of things. The mi_deps has various lists of modules and
+   -- suchlike, which are stored in canonical order:
    let sorted_deps :: Dependencies
-       sorted_deps = sortDependencies (mi_deps iface0)
+       sorted_deps = mi_deps iface0
 
    -- The export hash of a module depends on the orphan hashes of the
    -- orphan modules below us in the dependency tree.  This is the way
@@ -1193,16 +1193,6 @@ getOrphanHashes hsc_env mods = do
   mapM get_orph_hash mods
 
 
-sortDependencies :: Dependencies -> Dependencies
-sortDependencies d
- = Deps { dep_direct_mods  = dep_direct_mods d,
-          dep_direct_pkgs  = dep_direct_pkgs d,
-          dep_sig_mods     = sort (dep_sig_mods d),
-          dep_trusted_pkgs = dep_trusted_pkgs d,
-          dep_boot_mods    = dep_boot_mods d,
-          dep_orphs  = sortBy stableModuleCmp (dep_orphs d),
-          dep_finsts = sortBy stableModuleCmp (dep_finsts d) }
-
 {-
 ************************************************************************
 *                                                                      *
diff --git a/compiler/GHC/Iface/Rename.hs b/compiler/GHC/Iface/Rename.hs
index e2a89570d3..fdbe0dd55a 100644
--- a/compiler/GHC/Iface/Rename.hs
+++ b/compiler/GHC/Iface/Rename.hs
@@ -128,19 +128,18 @@ rnModExports hsc_env insts iface
     $ mapM rnAvailInfo (mi_exports iface)
 
 rnDependencies :: Rename Dependencies
-rnDependencies deps = do
-    orphs  <- rnDepModules dep_orphs deps
-    finsts <- rnDepModules dep_finsts deps
-    return deps { dep_orphs = orphs, dep_finsts = finsts }
+rnDependencies deps0 = do
+    deps1  <- dep_orphs_update deps0 (rnDepModules dep_orphs)
+    dep_finsts_update deps1 (rnDepModules dep_finsts)
 
-rnDepModules :: (Dependencies -> [Module]) -> Dependencies -> ShIfM [Module]
-rnDepModules sel deps = do
+rnDepModules :: (Dependencies -> [Module]) -> [Module] -> ShIfM [Module]
+rnDepModules sel mods = do
     hsc_env <- getTopEnv
     hmap <- getHoleSubst
     -- NB: It's not necessary to test if we're doing signature renaming,
     -- because ModIface will never contain module reference for itself
     -- in these dependencies.
-    fmap (nubSort . concat) . T.forM (sel deps) $ \mod -> do
+    fmap (nubSort . concat) . T.forM mods $ \mod -> do
         -- For holes, its necessary to "see through" the instantiation
         -- of the hole to get accurate family instance dependencies.
         -- For example, if B imports <A>, and <A> is instantiated with
diff --git a/compiler/GHC/Rename/Names.hs b/compiler/GHC/Rename/Names.hs
index b7dd191aa0..9c9781aa9d 100644
--- a/compiler/GHC/Rename/Names.hs
+++ b/compiler/GHC/Rename/Names.hs
@@ -477,13 +477,16 @@ calculateAvails home_unit iface mod_safe' want_boot imported_by =
       -- 'imp_finsts' if it defines an orphan or instance family; thus the
       -- orph_iface/has_iface tests.
 
-      orphans | orph_iface = assertPpr (not (imp_sem_mod `elem` dep_orphs deps)) (ppr imp_sem_mod <+> ppr (dep_orphs deps)) $
-                             imp_sem_mod : dep_orphs deps
-              | otherwise  = dep_orphs deps
+      deporphs  = dep_orphs deps
+      depfinsts = dep_finsts deps
 
-      finsts | has_finsts = assertPpr (not (imp_sem_mod `elem` dep_finsts deps)) (ppr imp_sem_mod <+> ppr (dep_orphs deps)) $
-                            imp_sem_mod : dep_finsts deps
-             | otherwise  = dep_finsts deps
+      orphans | orph_iface = assertPpr (not (imp_sem_mod `elem` deporphs)) (ppr imp_sem_mod <+> ppr deporphs) $
+                             imp_sem_mod : deporphs
+              | otherwise  = deporphs
+
+      finsts | has_finsts = assertPpr (not (imp_sem_mod `elem` depfinsts)) (ppr imp_sem_mod <+> ppr depfinsts) $
+                            imp_sem_mod : depfinsts
+             | otherwise  = depfinsts
 
       -- Trusted packages are a lot like orphans.
       trusted_pkgs | mod_safe' = dep_trusted_pkgs deps
diff --git a/compiler/GHC/Tc/Types.hs b/compiler/GHC/Tc/Types.hs
index 34d2c2e578..f89949d1f8 100644
--- a/compiler/GHC/Tc/Types.hs
+++ b/compiler/GHC/Tc/Types.hs
@@ -42,7 +42,7 @@ module GHC.Tc.Types(
         -- Renamer types
         ErrCtxt, RecFieldEnv, pushErrCtxt, pushErrCtxtSameOrigin,
         ImportAvails(..), emptyImportAvails, plusImportAvails,
-        WhereFrom(..), mkModDeps, modDepsElts,
+        WhereFrom(..), mkModDeps,
 
         -- Typechecker types
         TcTypeEnv, TcBinderStack, TcBinder(..),
@@ -146,7 +146,7 @@ import GHC.Data.List.SetOps
 
 import GHC.Unit
 import GHC.Unit.Module.Warnings
-import GHC.Unit.Module.Imported
+import GHC.Unit.Module.Deps
 import GHC.Unit.Module.ModDetails
 
 import GHC.Utils.Error
@@ -1365,69 +1365,6 @@ peCategory NoDataKindsDC          = "data constructor"
 ************************************************************************
 -}
 
--- | 'ImportAvails' summarises what was imported from where, irrespective of
--- whether the imported things are actually used or not.  It is used:
---
---  * when processing the export list,
---
---  * when constructing usage info for the interface file,
---
---  * to identify the list of directly imported modules for initialisation
---    purposes and for optimised overlap checking of family instances,
---
---  * when figuring out what things are really unused
---
-data ImportAvails
-   = ImportAvails {
-        imp_mods :: ImportedMods,
-          --      = ModuleEnv [ImportedModsVal],
-          -- ^ Domain is all directly-imported modules
-          --
-          -- See the documentation on ImportedModsVal in
-          -- "GHC.Unit.Module.Imported" for the meaning of the fields.
-          --
-          -- We need a full ModuleEnv rather than a ModuleNameEnv here,
-          -- because we might be importing modules of the same name from
-          -- different packages. (currently not the case, but might be in the
-          -- future).
-
-        imp_direct_dep_mods :: ModuleNameEnv ModuleNameWithIsBoot,
-          -- ^ Home-package modules directly imported by the module being compiled.
-
-        imp_dep_direct_pkgs :: Set UnitId,
-          -- ^ Packages directly needed by the module being compiled
-
-        imp_trust_own_pkg :: Bool,
-          -- ^ Do we require that our own package is trusted?
-          -- This is to handle efficiently the case where a Safe module imports
-          -- a Trustworthy module that resides in the same package as it.
-          -- See Note [Trust Own Package] in "GHC.Rename.Names"
-
-        -- Transitive information below here
-
-        imp_trust_pkgs :: Set UnitId,
-          -- ^ This records the
-          -- packages the current module needs to trust for Safe Haskell
-          -- compilation to succeed. A package is required to be trusted if
-          -- we are dependent on a trustworthy module in that package.
-          -- See Note [Tracking Trust Transitively] in "GHC.Rename.Names"
-
-        imp_boot_mods :: ModuleNameEnv ModuleNameWithIsBoot,
-          -- ^ Domain is all modules which have hs-boot files, and whether
-          -- we should import the boot version of interface file. Only used
-          -- in one-shot mode to populate eps_is_boot.
-
-        imp_sig_mods :: [ModuleName],
-          -- ^ Signature modules below this one
-
-        imp_orphs :: [Module],
-          -- ^ Orphan modules below us in the import tree (and maybe including
-          -- us for imported modules)
-
-        imp_finsts :: [Module]
-          -- ^ Family instance modules below us in the import tree (and maybe
-          -- including us for imported modules)
-      }
 
 mkModDeps :: Set ModuleNameWithIsBoot
           -> ModuleNameEnv ModuleNameWithIsBoot
@@ -1449,13 +1386,6 @@ plusModDeps = plusUFM_C plus_mod_dep
       -- Reusing existing tuples saves 10% of allocations on test
       -- perf/compiler/MultiLayerModules
 
-modDepsElts
-  :: ModuleNameEnv ModuleNameWithIsBoot
-  -> Set ModuleNameWithIsBoot
-modDepsElts = S.fromList . nonDetEltsUFM
-  -- It's OK to use nonDetEltsUFM here because sorting by module names
-  -- restores determinism
-
 emptyImportAvails :: ImportAvails
 emptyImportAvails = ImportAvails { imp_mods          = emptyModuleEnv,
                                    imp_direct_dep_mods = emptyUFM,
diff --git a/compiler/GHC/Unit/Module/Deps.hs b/compiler/GHC/Unit/Module/Deps.hs
index ebdd4b351f..c3c3340f41 100644
--- a/compiler/GHC/Unit/Module/Deps.hs
+++ b/compiler/GHC/Unit/Module/Deps.hs
@@ -1,8 +1,20 @@
 -- | Dependencies and Usage of a module
 module GHC.Unit.Module.Deps
-   ( Dependencies (..)
-   , Usage (..)
+   ( Dependencies
+   , mkDependencies
    , noDependencies
+   , dep_direct_mods
+   , dep_direct_pkgs
+   , dep_sig_mods
+   , dep_trusted_pkgs
+   , dep_orphs
+   , dep_finsts
+   , dep_boot_mods
+   , dep_orphs_update
+   , dep_finsts_update
+   , pprDeps
+   , Usage (..)
+   , ImportAvails (..)
    )
 where
 
@@ -10,12 +22,19 @@ import GHC.Prelude
 
 import GHC.Types.SafeHaskell
 import GHC.Types.Name
+import GHC.Types.Unique.FM
+
 import GHC.Unit.Module.Name
+import GHC.Unit.Module.Imported
 import GHC.Unit.Module
+import GHC.Unit.Home
+import GHC.Unit.State
 
 import GHC.Utils.Fingerprint
 import GHC.Utils.Binary
+import GHC.Utils.Outputable
 
+import Data.List (sortBy, sort, partition)
 import Data.Set (Set)
 import qualified Data.Set as Set
 
@@ -26,6 +45,8 @@ import qualified Data.Set as Set
 --
 -- Invariant: none of the lists contain duplicates.
 --
+-- Invariant: lists are ordered canonically (e.g. using stableModuleCmp)
+--
 -- See Note [Transitive Information in Dependencies]
 data Dependencies = Deps
    { dep_direct_mods :: Set ModuleNameWithIsBoot
@@ -34,9 +55,10 @@ data Dependencies = Deps
    , dep_direct_pkgs :: Set UnitId
       -- ^ All packages directly imported by this module
       -- I.e. packages to which this module's direct imports belong.
-      --
 
+    ------------------------------------
     -- Transitive information below here
+
    , dep_sig_mods :: ![ModuleName]
     -- ^ Transitive closure of hsig files in the home package
 
@@ -74,6 +96,71 @@ data Dependencies = Deps
         -- Equality used only for old/new comparison in GHC.Iface.Recomp.addFingerprints
         -- See 'GHC.Tc.Utils.ImportAvails' for details on dependencies.
 
+
+-- | Extract information from the rename and typecheck phases to produce
+-- a dependencies information for the module being compiled.
+--
+-- The fourth argument is a list of plugin modules.
+mkDependencies :: HomeUnit -> Module -> ImportAvails -> [Module] -> Dependencies
+mkDependencies home_unit mod imports plugin_mods =
+  let (home_plugins, external_plugins) = partition (isHomeUnit home_unit . moduleUnit) plugin_mods
+      plugin_units = Set.fromList (map (toUnitId . moduleUnit) external_plugins)
+      all_direct_mods = foldr (\mn m -> addToUFM m mn (GWIB mn NotBoot))
+                              (imp_direct_dep_mods imports)
+                              (map moduleName home_plugins)
+
+      modDepsElts = Set.fromList . nonDetEltsUFM
+        -- It's OK to use nonDetEltsUFM here because sorting by module names
+        -- restores determinism
+
+      direct_mods = modDepsElts (delFromUFM all_direct_mods (moduleName mod))
+            -- M.hi-boot can be in the imp_dep_mods, but we must remove
+            -- it before recording the modules on which this one depends!
+            -- (We want to retain M.hi-boot in imp_dep_mods so that
+            --  loadHiBootInterface can see if M's direct imports depend
+            --  on M.hi-boot, and hence that we should do the hi-boot consistency
+            --  check.)
+
+      dep_orphs = filter (/= mod) (imp_orphs imports)
+            -- We must also remove self-references from imp_orphs. See
+            -- Note [Module self-dependency]
+
+      direct_pkgs = foldr Set.insert (imp_dep_direct_pkgs imports) plugin_units
+
+      -- Set the packages required to be Safe according to Safe Haskell.
+      -- See Note [Tracking Trust Transitively] in GHC.Rename.Names
+      trust_pkgs  = imp_trust_pkgs imports
+
+      -- If there's a non-boot import, then it shadows the boot import
+      -- coming from the dependencies
+      source_mods = modDepsElts (imp_boot_mods imports)
+
+      sig_mods = filter (/= (moduleName mod)) $ imp_sig_mods imports
+
+  in Deps { dep_direct_mods  = direct_mods
+          , dep_direct_pkgs  = direct_pkgs
+          , dep_sig_mods     = sort sig_mods
+          , dep_trusted_pkgs = trust_pkgs
+          , dep_boot_mods    = source_mods
+          , dep_orphs        = sortBy stableModuleCmp dep_orphs
+          , dep_finsts       = sortBy stableModuleCmp (imp_finsts imports)
+            -- sort to get into canonical order
+            -- NB. remember to use lexicographic ordering
+          }
+
+-- | Update module dependencies containing orphans (used by Backpack)
+dep_orphs_update :: Monad m => Dependencies -> ([Module] -> m [Module]) -> m Dependencies
+dep_orphs_update deps f = do
+  r <- f (dep_orphs deps)
+  pure (deps { dep_orphs = sortBy stableModuleCmp r })
+
+-- | Update module dependencies containing family instances (used by Backpack)
+dep_finsts_update :: Monad m => Dependencies -> ([Module] -> m [Module]) -> m Dependencies
+dep_finsts_update deps f = do
+  r <- f (dep_finsts deps)
+  pure (deps { dep_finsts = sortBy stableModuleCmp r })
+
+
 instance Binary Dependencies where
     put_ bh deps = do put_ bh (dep_direct_mods deps)
                       put_ bh (dep_direct_pkgs deps)
@@ -109,6 +196,32 @@ noDependencies = Deps
   , dep_finsts       = []
   }
 
+-- | Pretty-print unit dependencies
+pprDeps :: UnitState -> Dependencies -> SDoc
+pprDeps unit_state (Deps { dep_direct_mods = dmods
+                         , dep_boot_mods = bmods
+                         , dep_orphs = orphs
+                         , dep_direct_pkgs = pkgs
+                         , dep_trusted_pkgs = tps
+                         , dep_finsts = finsts
+                         })
+  = pprWithUnitState unit_state $
+    vcat [text "direct module dependencies:"  <+> ppr_set ppr_mod dmods,
+          text "boot module dependencies:"    <+> ppr_set ppr bmods,
+          text "direct package dependencies:" <+> ppr_set ppr pkgs,
+          if null tps
+            then empty
+            else text "trusted package dependencies:" <+> ppr_set ppr tps,
+          text "orphans:" <+> fsep (map ppr orphs),
+          text "family instance modules:" <+> fsep (map ppr finsts)
+        ]
+  where
+    ppr_mod (GWIB mod IsBoot)  = ppr mod <+> text "[boot]"
+    ppr_mod (GWIB mod NotBoot) = ppr mod
+
+    ppr_set :: Outputable a => (a -> SDoc) -> Set a -> SDoc
+    ppr_set w = fsep . fmap w . Set.toAscList
+
 -- | Records modules for which changes may force recompilation of this module
 -- See wiki: https://gitlab.haskell.org/ghc/ghc/wikis/commentary/compiler/recompilation-avoidance
 --
@@ -327,3 +440,67 @@ Question: does this happen even across packages?
 No: if I need to load the interface for module X from package P I always look for p:X.hi.
 
 -}
+
+-- | 'ImportAvails' summarises what was imported from where, irrespective of
+-- whether the imported things are actually used or not.  It is used:
+--
+--  * when processing the export list,
+--
+--  * when constructing usage info for the interface file,
+--
+--  * to identify the list of directly imported modules for initialisation
+--    purposes and for optimised overlap checking of family instances,
+--
+--  * when figuring out what things are really unused
+--
+data ImportAvails
+   = ImportAvails {
+        imp_mods :: ImportedMods,
+          --      = ModuleEnv [ImportedModsVal],
+          -- ^ Domain is all directly-imported modules
+          --
+          -- See the documentation on ImportedModsVal in
+          -- "GHC.Unit.Module.Imported" for the meaning of the fields.
+          --
+          -- We need a full ModuleEnv rather than a ModuleNameEnv here,
+          -- because we might be importing modules of the same name from
+          -- different packages. (currently not the case, but might be in the
+          -- future).
+
+        imp_direct_dep_mods :: ModuleNameEnv ModuleNameWithIsBoot,
+          -- ^ Home-package modules directly imported by the module being compiled.
+
+        imp_dep_direct_pkgs :: Set UnitId,
+          -- ^ Packages directly needed by the module being compiled
+
+        imp_trust_own_pkg :: Bool,
+          -- ^ Do we require that our own package is trusted?
+          -- This is to handle efficiently the case where a Safe module imports
+          -- a Trustworthy module that resides in the same package as it.
+          -- See Note [Trust Own Package] in "GHC.Rename.Names"
+
+        -- Transitive information below here
+
+        imp_trust_pkgs :: Set UnitId,
+          -- ^ This records the
+          -- packages the current module needs to trust for Safe Haskell
+          -- compilation to succeed. A package is required to be trusted if
+          -- we are dependent on a trustworthy module in that package.
+          -- See Note [Tracking Trust Transitively] in "GHC.Rename.Names"
+
+        imp_boot_mods :: ModuleNameEnv ModuleNameWithIsBoot,
+          -- ^ Domain is all modules which have hs-boot files, and whether
+          -- we should import the boot version of interface file. Only used
+          -- in one-shot mode to populate eps_is_boot.
+
+        imp_sig_mods :: [ModuleName],
+          -- ^ Signature modules below this one
+
+        imp_orphs :: [Module],
+          -- ^ Orphan modules below us in the import tree (and maybe including
+          -- us for imported modules)
+
+        imp_finsts :: [Module]
+          -- ^ Family instance modules below us in the import tree (and maybe
+          -- including us for imported modules)
+      }
diff --git a/compiler/GHC/Unit/Module/ModSummary.hs b/compiler/GHC/Unit/Module/ModSummary.hs
index ba59655033..c584385aef 100644
--- a/compiler/GHC/Unit/Module/ModSummary.hs
+++ b/compiler/GHC/Unit/Module/ModSummary.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE TupleSections #-}
 
 -- | A ModSummary is a node in the compilation manager's dependency graph
 -- (ModuleGraph)
@@ -10,6 +11,7 @@ module GHC.Unit.Module.ModSummary
    , ms_installed_mod
    , ms_mod_name
    , ms_imps
+   , ms_plugin_imps
    , ms_mnwib
    , ms_home_srcimps
    , ms_home_imps
@@ -114,11 +116,11 @@ ms_mod_name = moduleName . ms_mod
 
 -- | Textual imports, plus plugin imports but not SOURCE imports.
 ms_imps :: ModSummary -> [(Maybe FastString, Located ModuleName)]
-ms_imps ms =
-  ms_textual_imps ms ++
-  map mk_additional_import (dynFlagDependencies (ms_hspp_opts ms))
-  where
-    mk_additional_import mod_nm = (Nothing, noLoc mod_nm)
+ms_imps ms = ms_textual_imps ms ++ ms_plugin_imps ms
+
+-- | Plugin imports
+ms_plugin_imps :: ModSummary -> [(Maybe FastString, Located ModuleName)]
+ms_plugin_imps ms = map ((Nothing,) . noLoc) (pluginModNames (ms_hspp_opts ms))
 
 home_imps :: [(Maybe FastString, Located ModuleName)] -> [Located ModuleName]
 home_imps imps = [ lmodname |  (mb_pkg, lmodname) <- imps,