Change `Backend` type and remove direct dependencieswip/backend-as-record

With this change, `Backend` becomes an abstract type
(there are no more exposed value constructors).
Decisions that were formerly made by asking "is the
current back end equal to (or different from) this named value
constructor?" are now made by interrogating the back end about
its properties, which are functions exported by `GHC.Driver.Backend`.

There is a description of how to migrate code using `Backend` in the
user guide.

Clients using the GHC API can find a backdoor to access the Backend
datatype in GHC.Driver.Backend.Internal.

Bumps haddock submodule.

Fixes #20927

1 files changed, 11 insertions, 14 deletions

diff --git a/compiler/GHC/Driver/Main.hs b/compiler/GHC/Driver/Main.hs
index ddc86ac3e3..36dcf24237 100644
--- a/compiler/GHC/Driver/Main.hs
+++ b/compiler/GHC/Driver/Main.hs
@@ -824,24 +824,24 @@ hscRecompStatus
         return $ HscRecompNeeded $ fmap (mi_iface_hash . mi_final_exts) mb_checked_iface
       UpToDateItem checked_iface -> do
         let lcl_dflags = ms_hspp_opts mod_summary
-        case backend lcl_dflags of
-          -- No need for a linkable, we're good to go
-          NoBackend -> do
-            msg $ UpToDate
-            return $ HscUpToDate checked_iface Nothing
+        if not (backendGeneratesCode (backend lcl_dflags)) then
+            -- No need for a linkable, we're good to go
+          do msg $ UpToDate
+             return $ HscUpToDate checked_iface Nothing
+        else
           -- Do need linkable
-          _ -> do
+          do
             -- Check to see whether the expected build products already exist.
             -- If they don't exists then we trigger recompilation.
             recomp_linkable_result <- case () of
                -- Interpreter can use either already loaded bytecode or loaded object code
-               _ | Interpreter <- backend lcl_dflags -> do
+               _ | backendCanReuseLoadedCode (backend lcl_dflags) -> do
                      let res = checkByteCode old_linkable
                      case res of
                        UpToDateItem _ -> pure res
                        _ -> liftIO $ checkObjects lcl_dflags old_linkable mod_summary
                  -- Need object files for making object files
-                 | backendProducesObject (backend lcl_dflags) -> liftIO $ checkObjects lcl_dflags old_linkable mod_summary
+                 | backendWritesFiles (backend lcl_dflags) -> liftIO $ checkObjects lcl_dflags old_linkable mod_summary
                  | otherwise -> pprPanic "hscRecompStatus" (text $ show $ backend lcl_dflags)
             case recomp_linkable_result of
               UpToDateItem linkable -> do
@@ -1001,7 +1001,7 @@ hscDesugarAndSimplify summary (FrontendTypecheck tc_result) tc_warnings mb_old_h
   -- interface file.
   case mb_desugar of
       -- Just cause we desugared doesn't mean we are generating code, see above.
-      Just desugared_guts | bcknd /= NoBackend -> do
+      Just desugared_guts | backendGeneratesCode bcknd -> do
           plugins <- liftIO $ readIORef (tcg_th_coreplugins tc_result)
           simplified_guts <- hscSimplify' plugins desugared_guts
 
@@ -1088,10 +1088,7 @@ hscMaybeWriteIface
   -> IO ()
 hscMaybeWriteIface logger dflags is_simple iface old_iface mod_location = do
     let force_write_interface = gopt Opt_WriteInterface dflags
-        write_interface = case backend dflags of
-                            NoBackend    -> False
-                            Interpreter  -> False
-                            _            -> True
+        write_interface = backendWritesFiles (backend dflags)
 
         write_iface dflags' iface =
           let !iface_name = if dynamicNow dflags' then ml_dyn_hi_file mod_location else ml_hi_file mod_location
@@ -2397,4 +2394,4 @@ showModuleIndex (i,n) = text "[" <> pad <> int i <> text " of " <> int n <> text
 writeInterfaceOnlyMode :: DynFlags -> Bool
 writeInterfaceOnlyMode dflags =
  gopt Opt_WriteInterface dflags &&
- NoBackend == backend dflags
+ not (backendGeneratesCode (backend dflags))