Add flag to control whether self-recompilation information is written to interfacewip/22188

This patch adds the flag -fwrite-self-recomp-info which controls whether
interface files contain the information necessary to answer the
question:

  Do I need to recompile myself or is this current interface file
  suitable?

Why? Most packages are only built once either by a distribution or cabal
and then placed into an immutable store, after which we will never ask
this question. Therefore we can derive two benefits from omitting this
information.

* Primary motivation: It vastly reduces the surface area for creating
  non-deterministic interface files. See issue #10424 which motivated a
  proper fix to that issue. Distributions have long contained versions
  of GHC which just have broken self-recompilation checking (in order to
  get deterministic interface files).

* Secondary motivation: This reduces the size of interface files
  slightly.. the `mi_usages` field can be quite big but probably this
  isn't such a great benefit.

* Third motivation: Conceptually clarity about which parts of an
  interface file are used in order to **communicate** with subsequent
  packages about the **interface** for a module. And which parts are
  used to self-communicate during recompilation checking.

In addition to this, the change alerted me to the incorrect
implemenation of the reifyModule function. See #8489 for more discussion
about how to fix this if anyone was so inclined. For now I just added a
warning `-Wreify-module-missing-info` which triggers if the module you
are trying to reify doesn't have a suitable interface. Interfaces which
are unsuitable include:

  * The GHC.Prim interface, which is a fake interface
  * Interfaces compiled with -fno-write-self-recomp-info

The main tracking issue is #22188 but fixes issues such as #10424 in a
proper way.

24 files changed, 449 insertions, 156 deletions

diff --git a/compiler/GHC/Driver/Flags.hs b/compiler/GHC/Driver/Flags.hs
index f45397d887..fbae3c7a24 100644
--- a/compiler/GHC/Driver/Flags.hs
+++ b/compiler/GHC/Driver/Flags.hs
@@ -302,6 +302,7 @@ data GeneralFlag
    | Opt_OmitInterfacePragmas
    | Opt_ExposeAllUnfoldings
    | Opt_WriteInterface -- forces .hi files to be written even with -fno-code
+   | Opt_WriteSelfRecompInfo
    | Opt_WriteHie -- generate .hie files
 
    -- profiling opts
@@ -624,6 +625,7 @@ data WarningFlag =
    | Opt_WarnGADTMonoLocalBinds                      -- Since 9.4
    | Opt_WarnTypeEqualityOutOfScope                  -- Since 9.4
    | Opt_WarnTypeEqualityRequiresOperators           -- Since 9.4
+   | Opt_WarnReifyModuleMissingInfo                  -- Since 9.6
    deriving (Eq, Ord, Show, Enum)
 
 -- | Return the names of a WarningFlag
@@ -729,6 +731,7 @@ warnFlagNames wflag = case wflag of
   Opt_WarnGADTMonoLocalBinds                      -> "gadt-mono-local-binds" :| []
   Opt_WarnTypeEqualityOutOfScope                  -> "type-equality-out-of-scope" :| []
   Opt_WarnTypeEqualityRequiresOperators           -> "type-equality-requires-operators" :| []
+  Opt_WarnReifyModuleMissingInfo                  -> "reify-module-missing-info" :| []
 
 -- -----------------------------------------------------------------------------
 -- Standard sets of warning options
@@ -824,7 +827,8 @@ standardWarnings -- see Note [Documenting warning flags]
         Opt_WarnForallIdentifier,
         Opt_WarnUnicodeBidirectionalFormatCharacters,
         Opt_WarnGADTMonoLocalBinds,
-        Opt_WarnTypeEqualityRequiresOperators
+        Opt_WarnTypeEqualityRequiresOperators,
+        Opt_WarnReifyModuleMissingInfo
       ]
 
 -- | Things you get with -W
diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index 831267e2bf..0dd7900699 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -3327,7 +3327,8 @@ wWarningFlagsDeps = mconcat [
   warnSpec    Opt_WarnUnicodeBidirectionalFormatCharacters,
   warnSpec    Opt_WarnGADTMonoLocalBinds,
   warnSpec    Opt_WarnTypeEqualityOutOfScope,
-  warnSpec    Opt_WarnTypeEqualityRequiresOperators
+  warnSpec    Opt_WarnTypeEqualityRequiresOperators,
+  warnSpec    Opt_WarnReifyModuleMissingInfo
  ]
 
 -- | These @-\<blah\>@ flags can all be reversed with @-no-\<blah\>@
@@ -3486,6 +3487,7 @@ fFlagsDeps = [
   flagSpec "use-rpaths"                       Opt_RPath,
   flagSpec "write-interface"                  Opt_WriteInterface,
   flagSpec "write-if-simplified-core"         Opt_WriteIfSimplifiedCore,
+  flagSpec "write-self-recomp-info"           Opt_WriteSelfRecompInfo,
   flagSpec "write-ide-info"                   Opt_WriteHie,
   flagSpec "unbox-small-strict-fields"        Opt_UnboxSmallStrictFields,
   flagSpec "unbox-strict-fields"              Opt_UnboxStrictFields,
@@ -3809,7 +3811,9 @@ defaultFlags settings
       Opt_DumpWithWays,
       Opt_CompactUnwind,
       Opt_ShowErrorContext,
-      Opt_SuppressStgReps
+      Opt_SuppressStgReps,
+      Opt_WriteSelfRecompInfo
+
     ]
 
     ++ [f | (ns,f) <- optLevelFlags, 0 `elem` ns]
diff --git a/compiler/GHC/HsToCore.hs b/compiler/GHC/HsToCore.hs
index 3c6ec71079..dd57847586 100644
--- a/compiler/GHC/HsToCore.hs
+++ b/compiler/GHC/HsToCore.hs
@@ -22,14 +22,12 @@ import GHC.Driver.Session
 import GHC.Driver.Config
 import GHC.Driver.Config.Core.Lint ( endPassHscEnvIO )
 import GHC.Driver.Config.HsToCore.Ticks
-import GHC.Driver.Config.HsToCore.Usage
 import GHC.Driver.Env
 import GHC.Driver.Backend
 import GHC.Driver.Plugins
 
 import GHC.Hs
 
-import GHC.HsToCore.Usage
 import GHC.HsToCore.Monad
 import GHC.HsToCore.Errors.Types
 import GHC.HsToCore.Expr
@@ -41,7 +39,7 @@ import GHC.HsToCore.Coverage
 import GHC.HsToCore.Docs
 
 import GHC.Tc.Types
-import GHC.Tc.Utils.Monad  ( finalSafeMode, fixSafeInstances, initIfaceLoad )
+import GHC.Tc.Utils.Monad  ( finalSafeMode, fixSafeInstances )
 import GHC.Tc.Module ( runTcInteractive )
 
 import GHC.Core.Type
@@ -98,6 +96,7 @@ import GHC.Unit.Module.Deps
 import Data.List (partition)
 import Data.IORef
 import Data.Traversable (for)
+import GHC.Iface.Make (mkRecompUsageInfo)
 
 {-
 ************************************************************************
@@ -125,12 +124,10 @@ deSugar hsc_env
                             tcg_fix_env      = fix_env,
                             tcg_inst_env     = inst_env,
                             tcg_fam_inst_env = fam_inst_env,
-                            tcg_merged       = merged,
                             tcg_warns        = warns,
                             tcg_anns         = anns,
                             tcg_binds        = binds,
                             tcg_imp_specs    = imp_specs,
-                            tcg_dependent_files = dependent_files,
                             tcg_ev_binds     = ev_binds,
                             tcg_th_foreign_files = th_foreign_files_var,
                             tcg_fords        = fords,
@@ -224,8 +221,7 @@ deSugar hsc_env
 
         ; endPassHscEnvIO hsc_env name_ppr_ctx CoreDesugarOpt ds_binds ds_rules_for_imps
 
-        ; let used_names = mkUsedNames tcg_env
-              pluginModules = map lpModule (loadedPlugins (hsc_plugins hsc_env))
+        ; let pluginModules = map lpModule (loadedPlugins (hsc_plugins hsc_env))
               home_unit = hsc_home_unit hsc_env
         ; let deps = mkDependencies home_unit
                                     (tcg_mod tcg_env)
@@ -233,17 +229,10 @@ deSugar hsc_env
                                     (map mi_module pluginModules)
 
         ; used_th <- readIORef tc_splice_used
-        ; dep_files <- readIORef dependent_files
         ; safe_mode <- finalSafeMode dflags tcg_env
-        ; (needed_mods, needed_pkgs) <- readIORef (tcg_th_needed_deps tcg_env)
-
-        ; let uc = initUsageConfig hsc_env
-        ; let plugins = hsc_plugins hsc_env
-        ; let fc = hsc_FC hsc_env
-        ; let unit_env = hsc_unit_env hsc_env
-        ; usages <- initIfaceLoad hsc_env $
-                      mkUsageInfo uc plugins fc unit_env mod (imp_mods imports) used_names
-                        dep_files merged needed_mods needed_pkgs
+
+        ; usages <- mkRecompUsageInfo hsc_env tcg_env
+
         -- id_mod /= mod when we are processing an hsig, but hsigs
         -- never desugared and compiled (there's no code!)
         -- Consequently, this should hold for any ModGuts that make
diff --git a/compiler/GHC/Iface/Binary.hs b/compiler/GHC/Iface/Binary.hs
index 78045aa782..48dc54fa8b 100644
--- a/compiler/GHC/Iface/Binary.hs
+++ b/compiler/GHC/Iface/Binary.hs
@@ -45,7 +45,6 @@ import GHC.Types.Name.Cache
 import GHC.Types.SrcLoc
 import GHC.Platform
 import GHC.Settings.Constants
-import GHC.Utils.Fingerprint
 
 import Data.Array
 import Data.Array.IO
@@ -75,7 +74,7 @@ readBinIfaceHeader
   -> CheckHiWay
   -> TraceBinIFace
   -> FilePath
-  -> IO (Fingerprint, BinHandle)
+  -> IO BinHandle
 readBinIfaceHeader profile _name_cache checkHiWay traceBinIFace hi_path = do
     let platform = profilePlatform profile
 
@@ -117,8 +116,7 @@ readBinIfaceHeader profile _name_cache checkHiWay traceBinIFace hi_path = do
     when (checkHiWay == CheckHiWay) $
         errorOnMismatch "mismatched interface file profile tag" tag check_tag
 
-    src_hash <- get bh
-    pure (src_hash, bh)
+    pure bh
 
 -- | Read an interface file.
 readBinIface
@@ -129,7 +127,7 @@ readBinIface
   -> FilePath
   -> IO ModIface
 readBinIface profile name_cache checkHiWay traceBinIface hi_path = do
-    (src_hash, bh) <- readBinIfaceHeader profile name_cache checkHiWay traceBinIface hi_path
+    bh <- readBinIfaceHeader profile name_cache checkHiWay traceBinIface hi_path
 
     extFields_p <- get bh
 
@@ -140,7 +138,6 @@ readBinIface profile name_cache checkHiWay traceBinIface hi_path = do
 
     return mod_iface
       { mi_ext_fields = extFields
-      , mi_src_hash = src_hash
       }
 
 -- | This performs a get action after reading the dictionary and symbol
@@ -182,7 +179,6 @@ writeBinIface profile traceBinIface hi_path mod_iface = do
     put_ bh (show hiVersion)
     let tag = profileBuildTag profile
     put_  bh tag
-    put_  bh (mi_src_hash mod_iface)
 
     extFields_p_p <- tellBin bh
     put_ bh extFields_p_p
diff --git a/compiler/GHC/Iface/Load.hs b/compiler/GHC/Iface/Load.hs
index bf7ae8e005..400782628b 100644
--- a/compiler/GHC/Iface/Load.hs
+++ b/compiler/GHC/Iface/Load.hs
@@ -1105,28 +1105,31 @@ pprModIfaceSimple unit_state iface =
 -- The UnitState is used to pretty-print units
 pprModIface :: UnitState -> ModIface -> SDoc
 pprModIface unit_state iface@ModIface{ mi_final_exts = exts }
- = vcat [ text "interface"
+ = vcat $ [ text "interface"
                 <+> ppr (mi_module iface) <+> pp_hsc_src (mi_hsc_src iface)
+                <+> whenIsSelfRecomp (text "[self-recomp]")
                 <+> (if mi_orphan exts then text "[orphan module]" else Outputable.empty)
                 <+> (if mi_finsts exts then text "[family instance module]" else Outputable.empty)
                 <+> (if mi_hpc iface then text "[hpc]" else Outputable.empty)
                 <+> integer hiVersion
-        , nest 2 (text "interface hash:" <+> ppr (mi_iface_hash exts))
         , nest 2 (text "ABI hash:" <+> ppr (mi_mod_hash exts))
         , nest 2 (text "export-list hash:" <+> ppr (mi_exp_hash exts))
+        , whenIsSelfRecomp $ vcat
+                [ nest 2 (text "interface hash:" <+> ppr (mi_iface_hash exts))
+                , nest 2 (text "flag hash:" <+> ppr (mi_flag_hash exts))
+                , nest 2 (text "opt_hash:" <+> ppr (mi_opt_hash exts))
+                , nest 2 (text "hpc_hash:" <+> ppr (mi_hpc_hash exts))
+                , nest 2 (text "plugin_hash:" <+> ppr (mi_plugin_hash exts))
+                ]
         , nest 2 (text "orphan hash:" <+> ppr (mi_orphan_hash exts))
-        , nest 2 (text "flag hash:" <+> ppr (mi_flag_hash exts))
-        , nest 2 (text "opt_hash:" <+> ppr (mi_opt_hash exts))
-        , nest 2 (text "hpc_hash:" <+> ppr (mi_hpc_hash exts))
-        , nest 2 (text "plugin_hash:" <+> ppr (mi_plugin_hash exts))
-        , nest 2 (text "src_hash:" <+> ppr (mi_src_hash iface))
         , nest 2 (text "sig of:" <+> ppr (mi_sig_of iface))
         , nest 2 (text "used TH splices:" <+> ppr (mi_used_th iface))
+        , whenIsSelfRecomp $ nest 2 (text "src_hash:" <+> ppr (mi_src_hash iface))
         , nest 2 (text "where")
         , text "exports:"
         , nest 2 (vcat (map pprExport (mi_exports iface)))
         , pprDeps unit_state (mi_deps iface)
-        , vcat (map pprUsage (mi_usages iface))
+        , whenIsSelfRecomp $ vcat (map pprUsage (mi_usages iface))
         , vcat (map pprIfaceAnnotation (mi_anns iface))
         , pprFixities (mi_fixities iface)
         , vcat [ppr ver $$ nest 2 (ppr decl) | (ver,decl) <- mi_decls iface]
@@ -1145,6 +1148,9 @@ pprModIface unit_state iface@ModIface{ mi_final_exts = exts }
         , text "extensible fields:" $$ nest 2 (pprExtensibleFields (mi_ext_fields iface))
         ]
   where
+    whenIsSelfRecomp action =
+      if (isSelfRecompilationInterface iface) then action else empty
+
     pp_hsc_src HsBootFile = text "[boot]"
     pp_hsc_src HsigFile = text "[hsig]"
     pp_hsc_src HsSrcFile = Outputable.empty
diff --git a/compiler/GHC/Iface/Make.hs b/compiler/GHC/Iface/Make.hs
index ac55220cbf..f55e943dbe 100644
--- a/compiler/GHC/Iface/Make.hs
+++ b/compiler/GHC/Iface/Make.hs
@@ -13,6 +13,7 @@ module GHC.Iface.Make
    ( mkPartialIface
    , mkFullIface
    , mkIfaceTc
+   , mkRecompUsageInfo
    , mkIfaceExports
    , coAxiomToIfaceDecl
    , tyThingToIfaceDecl -- Converting things to their Iface equivalents
@@ -203,14 +204,11 @@ mkIfaceTc hsc_env safe_mode mod_details mod_summary mb_program
                       tcg_imports = imports,
                       tcg_rdr_env = rdr_env,
                       tcg_fix_env = fix_env,
-                      tcg_merged = merged,
                       tcg_warns = warns,
                       tcg_hpc = other_hpc_info,
-                      tcg_th_splice_used = tc_splice_used,
-                      tcg_dependent_files = dependent_files
+                      tcg_th_splice_used = tc_splice_used
                     }
   = do
-          let used_names = mkUsedNames tc_result
           let pluginModules = map lpModule (loadedPlugins (hsc_plugins hsc_env))
           let home_unit = hsc_home_unit hsc_env
           let deps = mkDependencies home_unit
@@ -219,47 +217,58 @@ mkIfaceTc hsc_env safe_mode mod_details mod_summary mb_program
                                     (map mi_module pluginModules)
           let hpc_info = emptyHpcInfo other_hpc_info
           used_th <- readIORef tc_splice_used
-          dep_files <- (readIORef dependent_files)
-          (needed_links, needed_pkgs) <- readIORef (tcg_th_needed_deps tc_result)
-          let uc = initUsageConfig hsc_env
-              plugins = hsc_plugins hsc_env
-              fc = hsc_FC hsc_env
-              unit_env = hsc_unit_env hsc_env
-          -- Do NOT use semantic module here; this_mod in mkUsageInfo
-          -- is used solely to decide if we should record a dependency
-          -- or not.  When we instantiate a signature, the semantic
-          -- module is something we want to record dependencies for,
-          -- but if you pass that in here, we'll decide it's the local
-          -- module and does not need to be recorded as a dependency.
-          -- See Note [Identity versus semantic module]
-          usages <- initIfaceLoad hsc_env $ mkUsageInfo uc plugins fc unit_env this_mod (imp_mods imports) used_names
-                      dep_files merged needed_links needed_pkgs
 
+          usage <- mkRecompUsageInfo hsc_env tc_result
           docs <- extractDocs (ms_hspp_opts mod_summary) tc_result
 
           let partial_iface = mkIface_ hsc_env
                    this_mod (fromMaybe [] mb_program) hsc_src
                    used_th deps rdr_env
                    fix_env warns hpc_info
-                   (imp_trust_own_pkg imports) safe_mode usages
+                   (imp_trust_own_pkg imports) safe_mode usage
                    docs mod_summary
                    mod_details
 
           mkFullIface hsc_env partial_iface Nothing Nothing
 
+mkRecompUsageInfo :: HscEnv -> TcGblEnv -> IO (Maybe [Usage])
+mkRecompUsageInfo hsc_env tc_result = do
+  let dflags = hsc_dflags hsc_env
+  if not (gopt Opt_WriteSelfRecompInfo dflags)
+    then return Nothing
+    else do
+     let used_names = mkUsedNames tc_result
+     dep_files <- (readIORef (tcg_dependent_files tc_result))
+     (needed_links, needed_pkgs) <- readIORef (tcg_th_needed_deps tc_result)
+     let uc = initUsageConfig hsc_env
+         plugins = hsc_plugins hsc_env
+         fc = hsc_FC hsc_env
+         unit_env = hsc_unit_env hsc_env
+
+     -- Do NOT use semantic module here; this_mod in mkUsageInfo
+     -- is used solely to decide if we should record a dependency
+     -- or not.  When we instantiate a signature, the semantic
+     -- module is something we want to record dependencies for,
+     -- but if you pass that in here, we'll decide it's the local
+     -- module and does not need to be recorded as a dependency.
+     -- See Note [Identity versus semantic module]
+     usages <- initIfaceLoad hsc_env $ mkUsageInfo uc plugins fc unit_env (tcg_mod tc_result) (imp_mods (tcg_imports tc_result)) used_names
+                 dep_files (tcg_merged tc_result) needed_links needed_pkgs
+     return (Just usages)
+
 mkIface_ :: HscEnv -> Module -> CoreProgram -> HscSource
          -> Bool -> Dependencies -> GlobalRdrEnv
          -> NameEnv FixItem -> Warnings GhcRn -> HpcInfo
          -> Bool
          -> SafeHaskellMode
-         -> [Usage]
+         -> Maybe [Usage]
          -> Maybe Docs
          -> ModSummary
          -> ModDetails
          -> PartialModIface
 mkIface_ hsc_env
          this_mod core_prog hsc_src used_th deps rdr_env fix_env src_warns
-         hpc_info pkg_trust_req safe_mode usages
+         hpc_info pkg_trust_req safe_mode musages
          docs mod_summary
          ModDetails{  md_insts     = insts,
                       md_fam_insts = fam_insts,
@@ -307,6 +316,9 @@ mkIface_ hsc_env
         trust_info  = setSafeMode safe_mode
         annotations = map mkIfaceAnnotation anns
         icomplete_matches = map mkIfaceCompleteMatch complete_matches
+        self_recomp = case musages of
+                        Nothing -> NoSelfRecomp
+                        Just usages -> ModIfaceSelfRecomp (ms_hs_hash mod_summary) usages
 
     ModIface {
           mi_module      = this_mod,
@@ -317,7 +329,6 @@ mkIface_ hsc_env
                             else Just semantic_mod,
           mi_hsc_src     = hsc_src,
           mi_deps        = deps,
-          mi_usages      = usages,
           mi_exports     = mkIfaceExports exports,
 
           -- Sort these lexicographically, so that
@@ -340,7 +351,7 @@ mkIface_ hsc_env
           mi_docs        = docs,
           mi_final_exts  = (),
           mi_ext_fields  = emptyExtensibleFields,
-          mi_src_hash = ms_hs_hash mod_summary
+          mi_self_recomp_info = self_recomp
           }
   where
      cmp_rule     = lexicalCompareFS `on` ifRuleName
diff --git a/compiler/GHC/Iface/Recomp.hs b/compiler/GHC/Iface/Recomp.hs
index 886bc12192..8abf38d537 100644
--- a/compiler/GHC/Iface/Recomp.hs
+++ b/compiler/GHC/Iface/Recomp.hs
@@ -171,6 +171,7 @@ data RecompReason
   = UnitDepRemoved UnitId
   | ModulePackageChanged FastString
   | SourceFileChanged
+  | NoSelfRecompInfo
   | ThisUnitIdChanged
   | ImpurePlugin
   | PluginsChanged
@@ -203,6 +204,7 @@ instance Outputable RecompReason where
     UnitDepRemoved uid       -> ppr uid <+> text "removed"
     ModulePackageChanged s   -> ftext s <+> text "package changed"
     SourceFileChanged        -> text "Source file changed"
+    NoSelfRecompInfo         -> text "Old interface lacks recompilation info"
     ThisUnitIdChanged        -> text "-this-unit-id changed"
     ImpurePlugin             -> text "Impure plugin forced recompilation"
     PluginsChanged           -> text "Plugins changed"
@@ -373,6 +375,8 @@ checkVersions hsc_env mod_summary iface
        -- but we ALSO must make sure the instantiation matches up.  See
        -- test case bkpcabal04!
        ; hsc_env <- getTopEnv
+       ; if not (isSelfRecompilationInterface iface)
+            then return $ outOfDateItemBecause NoSelfRecompInfo Nothing else do {
        ; if mi_src_hash iface /= ms_hs_hash mod_summary
             then return $ outOfDateItemBecause SourceFileChanged Nothing else do {
        ; if not (isHomeModule home_unit (mi_module iface))
@@ -407,7 +411,7 @@ checkVersions hsc_env mod_summary iface
                              | u <- mi_usages iface]
        ; case recomp of (NeedsRecompile reason) -> return $ OutOfDateItem reason (Just iface) ; _ -> do {
        ; return $ UpToDateItem iface
-    }}}}}}}
+    }}}}}}}}
   where
     logger = hsc_logger hsc_env
     dflags = hsc_dflags hsc_env
@@ -1210,18 +1214,6 @@ addFingerprints hsc_env iface0
        sorted_extra_decls :: Maybe [IfaceBindingX IfaceMaybeRhs IfaceTopBndrInfo]
        sorted_extra_decls = sortOn binding_key <$> mi_extra_decls iface0
 
-   -- the flag hash depends on:
-   --   - (some of) dflags
-   -- it returns two hashes, one that shouldn't change
-   -- the abi hash and one that should
-   flag_hash <- fingerprintDynFlags hsc_env this_mod putNameLiterally
-
-   opt_hash <- fingerprintOptFlags dflags putNameLiterally
-
-   hpc_hash <- fingerprintHpcFlags dflags putNameLiterally
-
-   plugin_hash <- fingerprintPlugins (hsc_plugins hsc_env)
-
    -- the ABI hash depends on:
    --   - decls
    --   - export list
@@ -1233,29 +1225,18 @@ addFingerprints hsc_env iface0
                        export_hash,  -- includes orphan_hash
                        mi_warns iface0)
 
-   -- The interface hash depends on:
-   --   - the ABI hash, plus
-   --   - the source file hash,
-   --   - the module level annotations,
-   --   - usages
-   --   - deps (home and external packages, dependent files)
-   --   - hpc
-   iface_hash <- computeFingerprint putNameLiterally
-                      (mod_hash,
-                       mi_src_hash iface0,
-                       ann_fn (mkVarOccFS (fsLit "module")),  -- See mkIfaceAnnCache
-                       mi_usages iface0,
-                       sorted_deps,
-                       mi_hpc iface0)
+   -- the flag hash depends on:
+   --   - (some of) dflags
+   -- it returns two hashes, one that shouldn't change
+   -- the abi hash and one that should
+   self_recomp <- if gopt Opt_WriteSelfRecompInfo dflags
+                    then mkSelfRecomp mod_hash sorted_deps
+                    else return NoSelfRecompBackend
 
    let
     final_iface_exts = ModIfaceBackend
-      { mi_iface_hash  = iface_hash
+      { mi_self_recomp_backend_info = self_recomp
       , mi_mod_hash    = mod_hash
-      , mi_flag_hash   = flag_hash
-      , mi_opt_hash    = opt_hash
-      , mi_hpc_hash    = hpc_hash
-      , mi_plugin_hash = plugin_hash
       , mi_orphan      = not (   all ifRuleAuto orph_rules
                                    -- See Note [Orphans and auto-generated rules]
                               && null orph_insts
@@ -1280,6 +1261,38 @@ addFingerprints hsc_env iface0
     (non_orph_fis,   orph_fis)   = mkOrphMap ifFamInstOrph (mi_fam_insts iface0)
     ann_fn = mkIfaceAnnCache (mi_anns iface0)
 
+    mkSelfRecomp mod_hash sorted_deps = do
+
+      flag_hash <- fingerprintDynFlags hsc_env this_mod putNameLiterally
+
+      opt_hash <- fingerprintOptFlags dflags putNameLiterally
+
+      hpc_hash <- fingerprintHpcFlags dflags putNameLiterally
+
+      plugin_hash <- fingerprintPlugins (hsc_plugins hsc_env)
+
+      -- The interface hash depends on:
+      --   - the ABI hash, plus
+      --   - the source file hash,
+      --   - the module level annotations,
+      --   - usages
+      --   - deps (home and external packages, dependent files)
+      --   - hpc
+      iface_hash <- computeFingerprint putNameLiterally
+                         (mod_hash,
+                          mi_src_hash iface0,
+                          ann_fn (mkVarOccFS (fsLit "module")),  -- See mkIfaceAnnCache
+                          mi_usages iface0,
+                          sorted_deps,
+                          mi_hpc iface0)
+
+      return (ModIfaceSelfRecompBackend
+                                 { mi_sr_flag_hash = flag_hash
+                                 , mi_sr_hpc_hash = hpc_hash
+                                 , mi_sr_opt_hash = opt_hash
+                                 , mi_sr_iface_hash = iface_hash
+                                 , mi_sr_plugin_hash = plugin_hash })
+
 -- | Retrieve the orphan hashes 'mi_orphan_hash' for a list of modules
 -- (in particular, the orphan modules which are transitively imported by the
 -- current module).
diff --git a/compiler/GHC/Tc/Errors/Ppr.hs b/compiler/GHC/Tc/Errors/Ppr.hs
index 33b75e3eb1..014ffe33c2 100644
--- a/compiler/GHC/Tc/Errors/Ppr.hs
+++ b/compiler/GHC/Tc/Errors/Ppr.hs
@@ -1223,6 +1223,9 @@ instance Diagnostic TcRnMessage where
     TcRnSectionWithoutParentheses expr -> mkSimpleDecorated $
       hang (text "A section must be enclosed in parentheses")
          2 (text "thus:" <+> (parens (ppr expr)))
+    TcRnReifyModuleMissingInfo m -> mkSimpleDecorated $
+      vcat [ (ppr m) <+> text "can't be reified due to missing information in its interface file."
+           , text "Possible cause:" <+> ppr m <+> text "was compiled with -fno-write-self-recomp-info" ]
 
 
   diagnosticReason = \case
@@ -1628,6 +1631,8 @@ instance Diagnostic TcRnMessage where
       -> ErrorWithoutFlag
     TcRnIllegalTupleSection{}
       -> ErrorWithoutFlag
+    TcRnReifyModuleMissingInfo {} ->
+      WarningWithFlag Opt_WarnReifyModuleMissingInfo
 
   diagnosticHints = \case
     TcRnUnknownMessage m
@@ -2037,6 +2042,7 @@ instance Diagnostic TcRnMessage where
       -> noHints
     TcRnIllegalTupleSection{}
       -> [suggestExtension LangExt.TupleSections]
+    TcRnReifyModuleMissingInfo {} -> noHints
 
 
   diagnosticCode = constructorCode
diff --git a/compiler/GHC/Tc/Errors/Types.hs b/compiler/GHC/Tc/Errors/Types.hs
index 335e7c4965..b7b1a32b87 100644
--- a/compiler/GHC/Tc/Errors/Types.hs
+++ b/compiler/GHC/Tc/Errors/Types.hs
@@ -2762,6 +2762,12 @@ data TcRnMessage where
   -}
   TcRnSectionWithoutParentheses :: HsExpr GhcPs -> TcRnMessage
 
+  {-| TcRnReifyModuleMissingInfo is a warning triggered by attempting to
+      call reifyModule on a module whose interface file lacks the necessary information
+      to satisfy the query. This normally occurs when the module is compiled with `-fno-write-self-recomp-info`.
+  -}
+  TcRnReifyModuleMissingInfo:: Module -> TcRnMessage
+
   deriving Generic
 
 -- | Things forbidden in @type data@ declarations.
diff --git a/compiler/GHC/Tc/Gen/Splice.hs b/compiler/GHC/Tc/Gen/Splice.hs
index 6ba304be16..3bbe9a9af7 100644
--- a/compiler/GHC/Tc/Gen/Splice.hs
+++ b/compiler/GHC/Tc/Gen/Splice.hs
@@ -2808,6 +2808,10 @@ modToTHMod :: Module -> TH.Module
 modToTHMod m = TH.Module (TH.PkgName $ unitString  $ moduleUnit m)
                          (TH.ModName $ moduleNameString $ moduleName m)
 
+-- | Note that reifyModule will not work if the module is compiled with `-fno-write-self-recomp-info`
+-- because the implementation works by consulting the `mi_usages` field which is intended to be only
+-- used for recompilation checking. See #8489 for a ticket which tracks improvement
+-- of this function.
 reifyModule :: TH.Module -> TcM TH.ModuleInfo
 reifyModule (TH.Module (TH.PkgName pkgString) (TH.ModName mString)) = do
   this_mod <- getModule
@@ -2820,9 +2824,20 @@ reifyModule (TH.Module (TH.PkgName pkgString) (TH.ModName mString)) = do
 
       reifyFromIface reifMod = do
         iface <- loadInterfaceForModule (text "reifying module from TH for" <+> ppr reifMod) reifMod
-        let usages = [modToTHMod m | usage <- mi_usages iface,
-                                     Just m <- [usageToModule (moduleUnit reifMod) usage] ]
-        return $ TH.ModuleInfo usages
+        case mi_self_recomp_info iface of
+          NoSelfRecomp -> do
+            -- Arguably this should fail here but GHC.Prim always has NoSelfRecomp, so
+            -- any existing traversals would just stop working. Now they will start warning
+            -- and a user is expected to add a special case to avoid GHC.Prim in their traversal.
+
+            -- An alternative would be to add that special case for GHC.Prim here and make it a hard
+            -- error if reifyModule was attempted to be used with these partial interface files.
+            addDiagnosticTc (TcRnReifyModuleMissingInfo reifMod)
+            return (TH.ModuleInfo [])
+          ModIfaceSelfRecomp{ mi_sr_usages } -> do
+            let usages = [modToTHMod m | usage <- mi_sr_usages
+                                       , Just m <- [usageToModule (moduleUnit reifMod) usage] ]
+            return $ TH.ModuleInfo usages
 
       usageToModule :: Unit -> Usage -> Maybe Module
       usageToModule _ (UsageFile {}) = Nothing
diff --git a/compiler/GHC/Types/Error/Codes.hs b/compiler/GHC/Types/Error/Codes.hs
index ad8028ef38..e157c14a2b 100644
--- a/compiler/GHC/Types/Error/Codes.hs
+++ b/compiler/GHC/Types/Error/Codes.hs
@@ -500,6 +500,8 @@ type family GhcDiagnosticCode c = n | n -> c where
   GhcDiagnosticCode "TcRnMissingClassAssoc"                         = 06205
   GhcDiagnosticCode "TcRnBadFamInstDecl"                            = 06206
   GhcDiagnosticCode "TcRnNotOpenFamily"                             = 06207
+  GhcDiagnosticCode "TcRnReifyModuleMissingInfo"                    = 89264
+
 
   -- IllegalNewtypeReason
   GhcDiagnosticCode "DoesNotHaveSingleField"                        = 23517
diff --git a/compiler/GHC/Unit/Module/ModGuts.hs b/compiler/GHC/Unit/Module/ModGuts.hs
index d54e836d71..9f14ba9165 100644
--- a/compiler/GHC/Unit/Module/ModGuts.hs
+++ b/compiler/GHC/Unit/Module/ModGuts.hs
@@ -52,7 +52,7 @@ data ModGuts
         mg_exports   :: ![AvailInfo],    -- ^ What it exports
         mg_deps      :: !Dependencies,   -- ^ What it depends on, directly or
                                          -- otherwise
-        mg_usages    :: ![Usage],        -- ^ What was used?  Used for interfaces.
+        mg_usages    :: !(Maybe [Usage]), -- ^ What was used?  Used for interfaces.
 
         mg_used_th   :: !Bool,           -- ^ Did we run a TH splice?
         mg_rdr_env   :: !GlobalRdrEnv,   -- ^ Top-level lexical environment
diff --git a/compiler/GHC/Unit/Module/ModIface.hs b/compiler/GHC/Unit/Module/ModIface.hs
index 1d5280f4fa..9a748c9ea0 100644
--- a/compiler/GHC/Unit/Module/ModIface.hs
+++ b/compiler/GHC/Unit/Module/ModIface.hs
@@ -3,12 +3,16 @@
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE NamedFieldPuns #-}
 
 module GHC.Unit.Module.ModIface
    ( ModIface
    , ModIface_ (..)
    , PartialModIface
    , ModIfaceBackend (..)
+   , ModIfaceSelfRecompBackend (..)
+   , ModIfaceSelfRecomp (..)
+   , isSelfRecompilationInterface
    , IfaceDeclExts
    , IfaceBackendExts
    , IfaceExport
@@ -19,6 +23,13 @@ module GHC.Unit.Module.ModIface
    , mi_semantic_module
    , mi_free_holes
    , mi_mnwib
+   , mi_flag_hash
+   , mi_iface_hash
+   , mi_opt_hash
+   , mi_hpc_hash
+   , mi_plugin_hash
+   , mi_usages
+   , mi_src_hash
    , renameFreeHoles
    , emptyPartialModIface
    , emptyFullModIface
@@ -57,6 +68,9 @@ import GHC.Utils.Binary
 
 import Control.DeepSeq
 import Control.Exception
+import GHC.Utils.Panic
+import GHC.Utils.Outputable
+import GHC.Utils.Misc
 
 {- Note [Interface file stages]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -82,19 +96,11 @@ type ModIface = ModIface_ 'ModIfaceFinal
 -- * Or computed just before writing the iface to disk. (Hashes)
 -- In order to fully instantiate it.
 data ModIfaceBackend = ModIfaceBackend
-  { mi_iface_hash :: !Fingerprint
-    -- ^ Hash of the whole interface
-  , mi_mod_hash :: !Fingerprint
+  { mi_mod_hash :: !Fingerprint
     -- ^ Hash of the ABI only
-  , mi_flag_hash :: !Fingerprint
-    -- ^ Hash of the important flags used when compiling the module, excluding
-    -- optimisation flags
-  , mi_opt_hash :: !Fingerprint
-    -- ^ Hash of optimisation flags
-  , mi_hpc_hash :: !Fingerprint
-    -- ^ Hash of hpc flags
-  , mi_plugin_hash :: !Fingerprint
-    -- ^ Hash of plugins
+  , mi_self_recomp_backend_info :: !ModIfaceSelfRecompBackend
+    -- ^ Information needed for checking self-recompilation.
+    -- See Note [Self recompilation information in interface files]
   , mi_orphan :: !WhetherHasOrphans
     -- ^ Whether this module has orphans
   , mi_finsts :: !WhetherHasFamInst
@@ -136,6 +142,81 @@ type family IfaceBackendExts (phase :: ModIfacePhase) = bk | bk -> phase where
   IfaceBackendExts 'ModIfaceCore = ()
   IfaceBackendExts 'ModIfaceFinal = ModIfaceBackend
 
+-- | The information for a module which is only used when deciding whether to recompile
+-- itself. In particular the external interface of a module is recorded by the ABI
+-- hash
+data ModIfaceSelfRecompBackend = NoSelfRecompBackend | ModIfaceSelfRecompBackend {
+    mi_sr_flag_hash :: !Fingerprint
+    -- ^ Hash of the important flags used when compiling the module, excluding
+    -- optimisation flags
+  , mi_sr_iface_hash :: !Fingerprint
+    -- ^ Hash of the whole interface
+  , mi_sr_opt_hash :: !Fingerprint
+    -- ^ Hash of optimisation flags
+  , mi_sr_hpc_hash :: !Fingerprint
+    -- ^ Hash of hpc flags
+  , mi_sr_plugin_hash :: !Fingerprint
+    -- ^ Hash of plugins
+}
+withSelfRecompBackend :: HasCallStack => (ModIfaceSelfRecompBackend-> t) -> ModIfaceBackend-> t
+
+withSelfRecompBackend f mi =
+  case mi_self_recomp_backend_info mi of
+    NoSelfRecompBackend -> panic "Trying to use self-recomp info"
+    x -> f x
+
+mi_flag_hash :: HasCallStack => ModIfaceBackend -> Fingerprint
+mi_flag_hash = withSelfRecompBackend mi_sr_flag_hash
+mi_iface_hash :: HasCallStack => ModIfaceBackend -> Fingerprint
+mi_iface_hash = withSelfRecompBackend mi_sr_iface_hash
+mi_opt_hash :: HasCallStack => ModIfaceBackend -> Fingerprint
+mi_opt_hash   = withSelfRecompBackend mi_sr_opt_hash
+mi_hpc_hash :: HasCallStack => ModIfaceBackend -> Fingerprint
+mi_hpc_hash   = withSelfRecompBackend mi_sr_hpc_hash
+mi_plugin_hash :: HasCallStack => ModIfaceBackend -> Fingerprint
+mi_plugin_hash = withSelfRecompBackend mi_sr_plugin_hash
+
+isSelfRecompilationInterface :: ModIface -> Bool
+isSelfRecompilationInterface iface =
+  case mi_self_recomp_info iface of
+    NoSelfRecomp -> False
+    ModIfaceSelfRecomp {} -> True
+
+{-
+Note [Self recompilation information in interface files]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The flag -fwrite-self-recomp-info controls whether
+interface files contain the information necessary to answer the
+question:
+
+  Do I need to recompile myself or is this current interface file
+  suitable?
+
+Why? Most packages are only built once either by a distribution or cabal
+and then placed into an immutable store, after which we will never ask
+this question. Therefore we can derive two benefits from omitting this
+information.
+
+* Primary motivation: It vastly reduces the surface area for creating
+  non-deterministic interface files. See issue #10424 which motivated a
+  proper fix to that issue. Distributions have long contained versions
+  of GHC which just have broken self-recompilation checking (in order to
+  get deterministic interface files).
+
+* Secondary motivation: This reduces the size of interface files
+  slightly.. the `mi_usages` field can be quite big but probably this
+  isn't such a great benefit.
+
+* Third motivation: Conceptually clarity about which parts of an
+  interface file are used in order to **communicate** with subsequent
+  packages about the **interface** for a module. And which parts are
+  used to self-communicate during recompilation checking.
+
+The main tracking issue is #22188 but fixes issues such as #10424 in a
+proper way.
+
+-}
 
 
 -- | A 'ModIface' plus a 'ModDetails' summarises everything we know
@@ -162,12 +243,6 @@ data ModIface_ (phase :: ModIfacePhase)
                 -- consulted for directly-imported modules, but not
                 -- for anything else (hence lazy)
 
-        mi_usages   :: [Usage],
-                -- ^ Usages; kept sorted so that it's easy to decide
-                -- whether to write a new iface file (changing usages
-                -- doesn't affect the hash of this module)
-                -- NOT STRICT!  we read this field lazily from the interface file
-                -- It is *only* consulted by the recompilation checker
 
         mi_exports  :: ![IfaceExport],
                 -- ^ Exports
@@ -259,10 +334,40 @@ data ModIface_ (phase :: ModIfacePhase)
                 -- chosen over `ByteString`s.
                 --
 
-        mi_src_hash :: !Fingerprint
-                -- ^ Hash of the .hs source, used for recompilation checking.
+        mi_self_recomp_info :: !ModIfaceSelfRecomp
+                -- ^ Information needed for checking self-recompilation.
+                -- See Note [Self recompilation information in interface files]
      }
 
+data ModIfaceSelfRecomp = NoSelfRecomp
+  | ModIfaceSelfRecomp { mi_sr_src_hash :: !Fingerprint
+                       -- ^ Hash of the .hs source, used for recompilation checking.
+                       , mi_sr_usages   :: [Usage]
+                       -- ^ Usages; kept sorted so that it's easy to decide
+                       -- whether to write a new iface file (changing usages
+                       -- doesn't affect the hash of this module)
+                       -- NOT STRICT!  we read this field lazily from the interface file
+                       -- It is *only* consulted by the recompilation checker
+                       }
+
+instance Outputable ModIfaceSelfRecomp where
+  ppr NoSelfRecomp = text "NoSelfRecomp"
+  ppr (ModIfaceSelfRecomp{mi_sr_src_hash, mi_sr_usages}) = vcat [text "Self-Recomp"
+                                                                , nest 2 (vcat [text "src hash:" <+> ppr mi_sr_src_hash
+                                                                , text "usages:" <+> ppr (length mi_sr_usages)])]
+
+withSelfRecomp :: HasCallStack => (ModIfaceSelfRecomp-> t) -> ModIface_ phase -> t
+withSelfRecomp f mi =
+  case mi_self_recomp_info mi of
+    NoSelfRecomp -> panic "Trying to use self-recomp info"
+    x -> f x
+
+mi_usages :: HasCallStack => ModIface_ phase -> [Usage]
+mi_usages = withSelfRecomp mi_sr_usages
+mi_src_hash :: HasCallStack => ModIface_ phase -> Fingerprint
+mi_src_hash = withSelfRecomp mi_sr_src_hash
+
+
 {-
 Note [Strictness in ModIface]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -337,17 +442,53 @@ renameFreeHoles fhs insts =
         -- It wasn't actually a hole
         | otherwise                           = emptyUniqDSet
 
+instance Binary ModIfaceSelfRecompBackend where
+  put_ bh NoSelfRecompBackend = put_ bh (0 :: Int)
+  put_ bh (ModIfaceSelfRecompBackend {mi_sr_flag_hash, mi_sr_iface_hash, mi_sr_plugin_hash, mi_sr_opt_hash, mi_sr_hpc_hash}) = do
+    put_ bh (1 :: Int)
+    put_ bh mi_sr_flag_hash
+    put_ bh mi_sr_iface_hash
+    put_ bh mi_sr_plugin_hash
+    put_ bh mi_sr_opt_hash
+    put_ bh mi_sr_hpc_hash
+
+  get bh = do
+    (tag :: Int) <- get bh
+    case tag of
+      0 -> return NoSelfRecompBackend
+      1 -> do
+        mi_sr_flag_hash <- get bh
+        mi_sr_iface_hash <- get bh
+        mi_sr_plugin_hash <- get bh
+        mi_sr_opt_hash <- get bh
+        mi_sr_hpc_hash <- get bh
+        return (ModIfaceSelfRecompBackend {mi_sr_flag_hash, mi_sr_iface_hash, mi_sr_plugin_hash, mi_sr_opt_hash, mi_sr_hpc_hash})
+      x -> pprPanic "get_ModIfaceSelfRecomp" (ppr x)
+
+instance Binary ModIfaceSelfRecomp where
+  put_ bh NoSelfRecomp = put_ bh (0 :: Int)
+  put_ bh (ModIfaceSelfRecomp{mi_sr_src_hash, mi_sr_usages}) = do
+    put_ bh (1 :: Int)
+    put_ bh mi_sr_src_hash
+    lazyPut bh mi_sr_usages
+
+  get bh = do
+    (tag :: Int) <- get bh
+    case tag of
+      0 -> return NoSelfRecomp
+      1 -> do
+        src_hash    <- get bh
+        usages      <- {-# SCC "bin_usages" #-} lazyGet bh
+        return $ ModIfaceSelfRecomp { mi_sr_src_hash = src_hash, mi_sr_usages = usages }
+      x -> pprPanic "get_ModIfaceSelfRecomp" (ppr x)
+
 -- See Note [Strictness in ModIface] about where we use lazyPut vs put
 instance Binary ModIface where
    put_ bh (ModIface {
                  mi_module    = mod,
                  mi_sig_of    = sig_of,
                  mi_hsc_src   = hsc_src,
-                 mi_src_hash = _src_hash, -- Don't `put_` this in the instance
-                                          -- because we are going to write it
-                                          -- out separately in the actual file
                  mi_deps      = deps,
-                 mi_usages    = usages,
                  mi_exports   = exports,
                  mi_used_th   = used_th,
                  mi_fixities  = fixities,
@@ -366,13 +507,10 @@ instance Binary ModIface where
                  mi_ext_fields = _ext_fields, -- Don't `put_` this in the instance so we
                                               -- can deal with it's pointer in the header
                                               -- when we write the actual file
+                 mi_self_recomp_info = self_recomp,
                  mi_final_exts = ModIfaceBackend {
-                   mi_iface_hash = iface_hash,
+                   mi_self_recomp_backend_info = self_recomp_backend,
                    mi_mod_hash = mod_hash,
-                   mi_flag_hash = flag_hash,
-                   mi_opt_hash = opt_hash,
-                   mi_hpc_hash = hpc_hash,
-                   mi_plugin_hash = plugin_hash,
                    mi_orphan = orphan,
                    mi_finsts = hasFamInsts,
                    mi_exp_hash = exp_hash,
@@ -381,16 +519,12 @@ instance Binary ModIface where
         put_ bh mod
         put_ bh sig_of
         put_ bh hsc_src
-        put_ bh iface_hash
+        put_ bh self_recomp
+        put_ bh self_recomp_backend
         put_ bh mod_hash
-        put_ bh flag_hash
-        put_ bh opt_hash
-        put_ bh hpc_hash
-        put_ bh plugin_hash
         put_ bh orphan
         put_ bh hasFamInsts
         lazyPut bh deps
-        lazyPut bh usages
         put_ bh exports
         put_ bh exp_hash
         put_ bh used_th
@@ -413,16 +547,12 @@ instance Binary ModIface where
         mod         <- get bh
         sig_of      <- get bh
         hsc_src     <- get bh
-        iface_hash  <- get bh
+        self_recomp_info <- get bh
+        self_recomp_backend_info <- get bh
         mod_hash    <- get bh
-        flag_hash   <- get bh
-        opt_hash    <- get bh
-        hpc_hash    <- get bh
-        plugin_hash <- get bh
         orphan      <- get bh
         hasFamInsts <- get bh
         deps        <- lazyGet bh
-        usages      <- {-# SCC "bin_usages" #-} lazyGet bh
         exports     <- {-# SCC "bin_exports" #-} get bh
         exp_hash    <- get bh
         used_th     <- get bh
@@ -444,10 +574,7 @@ instance Binary ModIface where
                  mi_module      = mod,
                  mi_sig_of      = sig_of,
                  mi_hsc_src     = hsc_src,
-                 mi_src_hash = fingerprint0, -- placeholder because this is dealt
-                                             -- with specially when the file is read
                  mi_deps        = deps,
-                 mi_usages      = usages,
                  mi_exports     = exports,
                  mi_used_th     = used_th,
                  mi_anns        = anns,
@@ -467,13 +594,10 @@ instance Binary ModIface where
                  mi_docs        = docs,
                  mi_ext_fields  = emptyExtensibleFields, -- placeholder because this is dealt
                                                          -- with specially when the file is read
+                 mi_self_recomp_info = self_recomp_info,
                  mi_final_exts = ModIfaceBackend {
-                   mi_iface_hash = iface_hash,
+                   mi_self_recomp_backend_info = self_recomp_backend_info,
                    mi_mod_hash = mod_hash,
-                   mi_flag_hash = flag_hash,
-                   mi_opt_hash = opt_hash,
-                   mi_hpc_hash = hpc_hash,
-                   mi_plugin_hash = plugin_hash,
                    mi_orphan = orphan,
                    mi_finsts = hasFamInsts,
                    mi_exp_hash = exp_hash,
@@ -491,9 +615,7 @@ emptyPartialModIface mod
   = ModIface { mi_module      = mod,
                mi_sig_of      = Nothing,
                mi_hsc_src     = HsSrcFile,
-               mi_src_hash    = fingerprint0,
                mi_deps        = noDependencies,
-               mi_usages      = [],
                mi_exports     = [],
                mi_used_th     = False,
                mi_fixities    = [],
@@ -511,6 +633,7 @@ emptyPartialModIface mod
                mi_complete_matches = [],
                mi_docs        = Nothing,
                mi_final_exts  = (),
+               mi_self_recomp_info = NoSelfRecomp,
                mi_ext_fields  = emptyExtensibleFields
              }
 
@@ -518,13 +641,9 @@ emptyFullModIface :: Module -> ModIface
 emptyFullModIface mod =
     (emptyPartialModIface mod)
       { mi_decls = []
-      , mi_final_exts = ModIfaceBackend
-        { mi_iface_hash = fingerprint0,
+      , mi_final_exts = ModIfaceBackend {
           mi_mod_hash = fingerprint0,
-          mi_flag_hash = fingerprint0,
-          mi_opt_hash = fingerprint0,
-          mi_hpc_hash = fingerprint0,
-          mi_plugin_hash = fingerprint0,
+          mi_self_recomp_backend_info = NoSelfRecompBackend, -- TODO
           mi_orphan = False,
           mi_finsts = False,
           mi_exp_hash = fingerprint0,
@@ -550,19 +669,31 @@ emptyIfaceHashCache _occ = Nothing
 -- Take care, this instance only forces to the degree necessary to
 -- avoid major space leaks.
 instance (NFData (IfaceBackendExts (phase :: ModIfacePhase)), NFData (IfaceDeclExts (phase :: ModIfacePhase))) => NFData (ModIface_ phase) where
-  rnf (ModIface f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 f12
+  rnf (ModIface f1 f2 f3 f4 f6 f7 f8 f9 f10 f11 f12
                 f13 f14 f15 f16 f17 f18 f19 f20 f21 f22 f23 f24) =
-    rnf f1 `seq` rnf f2 `seq` f3 `seq` f4 `seq` f5 `seq` f6 `seq` rnf f7 `seq` f8 `seq`
+    rnf f1 `seq` rnf f2 `seq` f3 `seq` f4 `seq` f6 `seq` rnf f7 `seq` f8 `seq`
     f9 `seq` rnf f10 `seq` rnf f11 `seq` rnf f12 `seq` f13 `seq` rnf f14 `seq` rnf f15 `seq` rnf f16 `seq`
     rnf f17 `seq` f18 `seq` rnf f19 `seq` rnf f20 `seq` f21 `seq` f22 `seq` f23 `seq` rnf f24
     `seq` ()
 
 
 instance NFData (ModIfaceBackend) where
-  rnf (ModIfaceBackend f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 f12 f13)
+  rnf (ModIfaceBackend f1 f2 f3 f4 f5 f6 f7 f8 f9)
     = rnf f1 `seq` rnf f2 `seq` rnf f3 `seq` rnf f4 `seq`
       rnf f5 `seq` rnf f6 `seq` rnf f7 `seq` rnf f8 `seq`
-      rnf f9 `seq` rnf f10 `seq` rnf f11 `seq` rnf f12 `seq` rnf f13
+      rnf f9
+
+instance NFData ModIfaceSelfRecompBackend where
+  -- Sufficient as all fields are strict (and simple)
+  rnf NoSelfRecompBackend = ()
+  -- Written like this so if you add another field you have to think about it
+  rnf !(ModIfaceSelfRecompBackend _ _ _ _ _) = ()
+instance NFData ModIfaceSelfRecomp where
+  -- Sufficient as all fields are strict (and simple)
+  rnf NoSelfRecomp = ()
+  -- MP: Note does not deeply force Usages but the old ModIface logic didn't either, so
+  -- I left it as a shallow force.
+  rnf (ModIfaceSelfRecomp src_hash usages) = src_hash `seq` usages `seq` ()
 
 
 forceModIface :: ModIface -> IO ()
diff --git a/docs/users_guide/phases.rst b/docs/users_guide/phases.rst
index aa171c2055..b955719398 100644
--- a/docs/users_guide/phases.rst
+++ b/docs/users_guide/phases.rst
@@ -653,6 +653,24 @@ Options affecting code generation
     depend on the optimisation level. Any definitions which are already included in
     an interface file (via an unfolding for an exported identifier) are reused.
 
+.. ghc-flag:: -fwrite-self-recomp-info
+    :shortdesc: Write information for self-recompilation checking in an interface file
+    :type: dynamic
+    :category: codegen
+
+    :default: on
+
+    Include information in an interface file which can be used in future to determine
+    whether we need to recompile a module or can reuse the existing interface.
+
+    This is intended to be turned off in situations where you know you will never try
+    to recompile a module, such as when compiling a package for distribution.
+    The advantage is that by omitting unecessary information to do with dependencies
+    there is less chance of build paths leaking into the interface file and affecting
+    determinism.
+
+
+
 
 .. ghc-flag:: -fobject-code
     :shortdesc: Generate object code
diff --git a/docs/users_guide/using-warnings.rst b/docs/users_guide/using-warnings.rst
index 977bb69941..d61fc46319 100644
--- a/docs/users_guide/using-warnings.rst
+++ b/docs/users_guide/using-warnings.rst
@@ -2355,6 +2355,21 @@ of ``-W(no-)*``.
     triggered whenever this happens, and can be addressed by enabling the
     extension.
 
+.. ghc-flag:: -Wreify-module-missing-info
+    :shortdesc: warn when `reifyModule`
+    :type: dynamic
+    :reverse: -Wno-reify-module-missing-info
+
+    :since: 9.6.1
+
+    The `reifyModule` function in the Template Haskell API can fail to find the necessary
+    information when an interface file is generated with `-fno-write-self-recomp-info`. This
+    is due to a shortcoming in `reifyModule` tracked by :ghc-ticket:`8489`.
+
+    This flag warns the user when they try to call `reifyModule` on a module where this
+    situation occurs so that they know the traversal has ended prematurely.
+
+
 If you're feeling really paranoid, the :ghc-flag:`-dcore-lint` option is a good choice.
 It turns on heavyweight intra-pass sanity-checking within GHC. (It checks GHC's
 sanity, not yours.)
diff --git a/testsuite/tests/annotations/should_compile/th/annth_compunits.stderr b/testsuite/tests/annotations/should_compile/th/annth_compunits.stderr
new file mode 100644
index 0000000000..417b2a5a56
--- /dev/null
+++ b/testsuite/tests/annotations/should_compile/th/annth_compunits.stderr
@@ -0,0 +1,11 @@
+
+annth.hs:12:4: warning: [GHC-89264] [-Wreify-module-missing-info (in -Wdefault)]
+    • GHC.Prim can't be reified due to missing information in its interface file.
+      Possible cause: GHC.Prim was compiled with -fno-write-self-recomp-info
+    • In the untyped splice:
+        $(do anns <- traverseModuleAnnotations
+             runIO $ print (anns :: [String])
+             anns <- reifyAnnotations (AnnLookupName 'testValue)
+             runIO $ print (anns :: [String])
+             anns <- reifyAnnotations (AnnLookupName 'testValueTH)
+             ....)
diff --git a/testsuite/tests/annotations/should_compile/th/annth_make.stderr b/testsuite/tests/annotations/should_compile/th/annth_make.stderr
new file mode 100644
index 0000000000..417b2a5a56
--- /dev/null
+++ b/testsuite/tests/annotations/should_compile/th/annth_make.stderr
@@ -0,0 +1,11 @@
+
+annth.hs:12:4: warning: [GHC-89264] [-Wreify-module-missing-info (in -Wdefault)]
+    • GHC.Prim can't be reified due to missing information in its interface file.
+      Possible cause: GHC.Prim was compiled with -fno-write-self-recomp-info
+    • In the untyped splice:
+        $(do anns <- traverseModuleAnnotations
+             runIO $ print (anns :: [String])
+             anns <- reifyAnnotations (AnnLookupName 'testValue)
+             runIO $ print (anns :: [String])
+             anns <- reifyAnnotations (AnnLookupName 'testValueTH)
+             ....)
diff --git a/testsuite/tests/driver/self-recomp/Makefile b/testsuite/tests/driver/self-recomp/Makefile
new file mode 100644
index 0000000000..c6ac89fba1
--- /dev/null
+++ b/testsuite/tests/driver/self-recomp/Makefile
@@ -0,0 +1,38 @@
+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk
+
+TEST_HC_OPTS_NO_RTSOPTS = $(filter-out -rtsopts,$(TEST_HC_OPTS))
+
+# -----------------------------------------------------------------------------
+# One-shot compilations, non-hierarchical modules
+
+# Check that modifying flags doesn't affect interface
+SelfRecomp01:
+	"$(TEST_HC)" $(TEST_HC_OPTS) SelfRecomp01.hs -fno-write-self-recomp-info -v0
+	"$(TEST_HC)" --show-iface SelfRecomp01.hi > iface1
+	rm SelfRecomp01.hi
+	"$(TEST_HC)" $(TEST_HC_OPTS) SelfRecomp01.hs -fno-write-self-recomp-info -Iidir -v0
+	"$(TEST_HC)" --show-iface SelfRecomp01.hi > iface2
+	diff iface1 iface2
+
+# Check that the result of addDependentFile doesn't end up in interface
+SelfRecomp02:
+	"$(TEST_HC)" $(TEST_HC_OPTS) SelfRecomp02.hs -fno-write-self-recomp-info -v0
+	"$(TEST_HC)" --show-iface SelfRecomp02.hi > iface
+	[ -z $(grep iface SelfRecomp02.hs) ]
+
+# Check that modifying source doesn't affect interface
+SelfRecomp03:
+	"$(TEST_HC)" $(TEST_HC_OPTS) SelfRecomp03.hs -fno-write-self-recomp-info -v0
+	"$(TEST_HC)" --show-iface SelfRecomp03.hi > iface1
+	rm SelfRecomp03.hi
+	echo "" >> SelfRecomp03.hs
+	"$(TEST_HC)" $(TEST_HC_OPTS) SelfRecomp03.hs -fno-write-self-recomp-info -v0
+	"$(TEST_HC)" --show-iface SelfRecomp03.hi > iface2
+	diff iface1 iface2
+
+# Check that if you don't have recompilation info then you always recompile.
+SelfRecomp04:
+	"$(TEST_HC)" $(TEST_HC_OPTS) SelfRecomp04.hs -fno-write-self-recomp-info -fhide-source-paths
+	"$(TEST_HC)" $(TEST_HC_OPTS) SelfRecomp04.hs -fno-write-self-recomp-info -fhide-source-paths
diff --git a/testsuite/tests/driver/self-recomp/SelfRecomp01.hs b/testsuite/tests/driver/self-recomp/SelfRecomp01.hs
new file mode 100644
index 0000000000..1d6400e92b
--- /dev/null
+++ b/testsuite/tests/driver/self-recomp/SelfRecomp01.hs
@@ -0,0 +1,2 @@
+module SelfRecomp01 where
+
diff --git a/testsuite/tests/driver/self-recomp/SelfRecomp02.hs b/testsuite/tests/driver/self-recomp/SelfRecomp02.hs
new file mode 100644
index 0000000000..dd38566b39
--- /dev/null
+++ b/testsuite/tests/driver/self-recomp/SelfRecomp02.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE TemplateHaskell #-}
+module SelfRecomp02 where
+
+import Language.Haskell.TH.Syntax
+
+main = $(addDependentFile "SelfRecomp02.hs" >> [| print () |])
diff --git a/testsuite/tests/driver/self-recomp/SelfRecomp03.hs b/testsuite/tests/driver/self-recomp/SelfRecomp03.hs
new file mode 100644
index 0000000000..ca181a1f34
--- /dev/null
+++ b/testsuite/tests/driver/self-recomp/SelfRecomp03.hs
@@ -0,0 +1,2 @@
+module SelfRecomp03 where
+
diff --git a/testsuite/tests/driver/self-recomp/SelfRecomp04.hs b/testsuite/tests/driver/self-recomp/SelfRecomp04.hs
new file mode 100644
index 0000000000..8a956509e9
--- /dev/null
+++ b/testsuite/tests/driver/self-recomp/SelfRecomp04.hs
@@ -0,0 +1 @@
+module SelfRecomp04 where
diff --git a/testsuite/tests/driver/self-recomp/SelfRecomp04.stdout b/testsuite/tests/driver/self-recomp/SelfRecomp04.stdout
new file mode 100644
index 0000000000..5dc637533d
--- /dev/null
+++ b/testsuite/tests/driver/self-recomp/SelfRecomp04.stdout
@@ -0,0 +1,2 @@
+[1 of 1] Compiling SelfRecomp04
+[1 of 1] Compiling SelfRecomp04 [Old interface lacks recompilation info]
diff --git a/testsuite/tests/driver/self-recomp/all.T b/testsuite/tests/driver/self-recomp/all.T
new file mode 100644
index 0000000000..91713d459d
--- /dev/null
+++ b/testsuite/tests/driver/self-recomp/all.T
@@ -0,0 +1,4 @@
+test('SelfRecomp01', normal, makefile_test, ['SelfRecomp01'])
+test('SelfRecomp02', normal, makefile_test, ['SelfRecomp02'])
+test('SelfRecomp03', [copy_files], makefile_test, ['SelfRecomp03'])
+test('SelfRecomp04', normal, makefile_test, ['SelfRecomp04'])