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, 1 insertions, 1 deletions

diff --git a/ghc/GHCi/UI.hs b/ghc/GHCi/UI.hs
index c1ce849651..3751711b9d 100644
--- a/ghc/GHCi/UI.hs
+++ b/ghc/GHCi/UI.hs
@@ -619,7 +619,7 @@ ghciLogAction lastErrLocations old_log_action
               dflags msg_class srcSpan msg = do
     old_log_action dflags msg_class srcSpan msg
     case msg_class of
-        MCDiagnostic SevError _reason -> case srcSpan of
+        MCDiagnostic SevError _reason _code -> case srcSpan of
             RealSrcSpan rsp _ -> modifyIORef lastErrLocations
                 (++ [(srcLocFile (realSrcSpanStart rsp), srcLocLine (realSrcSpanStart rsp))])
             _ -> return ()