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, 0 deletions

diff --git a/docs/users_guide/9.6.1-notes.rst b/docs/users_guide/9.6.1-notes.rst
index 2e8302ad22..d78ec969b9 100644
--- a/docs/users_guide/9.6.1-notes.rst
+++ b/docs/users_guide/9.6.1-notes.rst
@@ -60,6 +60,8 @@ Language
   add an injectivity annotation to the type family in the case that
   the type family is in fact injective.
 
+- Error messages are now assigned unique error codes, of the form ``[GHC-12345]``.
+
 Compiler
 ~~~~~~~~