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, 6 insertions, 12 deletions

diff --git a/compiler/GHC/Driver/CodeOutput.hs b/compiler/GHC/Driver/CodeOutput.hs
index c073c40323..5b3f614d8e 100644
--- a/compiler/GHC/Driver/CodeOutput.hs
+++ b/compiler/GHC/Driver/CodeOutput.hs
@@ -43,7 +43,6 @@ import GHC.Utils.TmpFs
 
 import GHC.Utils.Error
 import GHC.Utils.Outputable
-import GHC.Utils.Panic
 import GHC.Utils.Logger
 import GHC.Utils.Exception (bracket)
 import GHC.Utils.Ppr (Mode(..))
@@ -120,13 +119,11 @@ codeOutput logger tmpfs llvm_config dflags unit_state this_mod filenm location g
                   ; emitInitializerDecls this_mod stubs
                   ; return (stubs, a) }
 
-        ; (stubs, a) <- case backend dflags of
-                 NCG         -> outputAsm logger dflags this_mod location filenm
-                                          final_stream
-                 ViaC        -> outputC logger dflags filenm final_stream pkg_deps
-                 LLVM        -> outputLlvm logger llvm_config dflags filenm final_stream
-                 Interpreter -> panic "codeOutput: Interpreter"
-                 NoBackend   -> panic "codeOutput: NoBackend"
+        ; (stubs, a) <- case backendCodeOutput (backend dflags) of
+                 NcgCodeOutput  -> outputAsm logger dflags this_mod location filenm
+                                             final_stream
+                 ViaCCodeOutput -> outputC logger dflags filenm final_stream pkg_deps
+                 LlvmCodeOutput -> outputLlvm logger llvm_config dflags filenm final_stream
         ; stubs_exist <- outputForeignStubs logger tmpfs dflags unit_state this_mod location stubs
         ; return (filenm, stubs_exist, foreign_fps, a)
         }
@@ -313,8 +310,7 @@ outputForeignStubs logger tmpfs dflags unit_state mod location stubs
    cplusplus_ftr = "#if defined(__cplusplus)\n}\n#endif\n"
 
 
--- Don't use doOutput for dumping the f. export stubs
--- since it is more than likely that the stubs file will
+-- It is more than likely that the stubs file will
 -- turn out to be empty, in which case no file should be created.
 outputForeignStubs_help :: FilePath -> String -> String -> String -> IO Bool
 outputForeignStubs_help _fname ""      _header _footer = return False
@@ -392,5 +388,3 @@ ipInitCode do_info_table platform this_mod ents
                          | ipe <- ipes
                          ] ++ [text "NULL"])
       <> semi
-
-