Add diagnostic codes

This MR adds diagnostic codes, assigning unique numeric codes to
error and warnings, e.g.

  error: [GHC-53633]
  Pattern match is redundant

This is achieved as follows:

  - a type family GhcDiagnosticCode that gives the diagnostic code
    for each diagnostic constructor,
  - a type family ConRecursInto that specifies whether to recur into
    an argument of the constructor to obtain a more fine-grained code
    (e.g. different error codes for different 'deriving' errors),
  - generics machinery to generate the value-level function assigning
    each diagnostic its error code; see Note [Diagnostic codes using generics]
    in GHC.Types.Error.Codes.

The upshot is that, to add a new diagnostic code, contributors only need
to modify the two type families mentioned above. All logic relating to
diagnostic codes is thus contained to the GHC.Types.Error.Codes module,
with no code duplication.

This MR also refactors error message datatypes a bit, ensuring we can
derive Generic for them, and cleans up the logic around constraint
solver reports by splitting up 'TcSolverReportInfo' into separate
datatypes (see #20772).

Fixes #21684

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/GHC/Linker/Loader.hs b/compiler/GHC/Linker/Loader.hs
index 6c2829c432..286c50416c 100644
--- a/compiler/GHC/Linker/Loader.hs
+++ b/compiler/GHC/Linker/Loader.hs
@@ -1505,7 +1505,7 @@ load_dyn interp hsc_env crash_early dll = do
         else
           when (diag_wopt Opt_WarnMissedExtraSharedLib diag_opts)
             $ logMsg logger
-                (mkMCDiagnostic diag_opts $ WarningWithFlag Opt_WarnMissedExtraSharedLib)
+                (mkMCDiagnostic diag_opts (WarningWithFlag Opt_WarnMissedExtraSharedLib) Nothing)
                   noSrcSpan $ withPprStyle defaultUserStyle (note err)
   where
     diag_opts = initDiagOpts (hsc_dflags hsc_env)
@@ -1693,7 +1693,7 @@ locateLib interp hsc_env is_hs lib_dirs gcc_dirs lib0
       , not loading_dynamic_hs_libs
       , interpreterProfiled interp
       = do
-          let diag = mkMCDiagnostic diag_opts WarningWithoutFlag
+          let diag = mkMCDiagnostic diag_opts WarningWithoutFlag Nothing
           logMsg logger diag noSrcSpan $ withPprStyle defaultErrStyle $
             text "Interpreter failed to load profiled static library" <+> text lib <> char '.' $$
               text " \tTrying dynamic library instead. If this fails try to rebuild" <+>