Use existing haddock-library and haddock-api on GHC >9.12

Author: hamishmack

lib/call-cabal-project-to-nix.nix

@@ -461,6 +461,8 @@ let
                 cabal_file=${ghcSrc}/compiler/${name}.cabal.in
               elif [ -f ${ghcSrc}/libraries/${name}/${name}.cabal.in ]; then
                 cabal_file=${ghcSrc}/libraries/${name}/${name}.cabal.in
+              elif [ -f ${ghcSrc}/utils/haddock/${name}/${name}.cabal ]; then
+                cabal_file=${ghcSrc}/utils/haddock/${name}/${name}.cabal
               fi
               if [[ "$cabal_file" != "" ]]; then
                 fixed_cabal_file=$(mktemp)

overlays/haskell.nix

@@ -1126,6 +1126,9 @@ final: prev: {
             "xhtml"
           ] ++ final.lib.optionals (builtins.compareVersions ghc.version "9.4" > 0) [
             "system-cxx-std-lib"
+          ] ++ final.lib.optionals (builtins.compareVersions ghc.version "9.12" > 0) [
+            "haddock-api"
+            "haddock-library"
           ] ++ final.lib.optionals (
                   !final.stdenv.targetPlatform.isGhcjs
                && !final.stdenv.targetPlatform.isWindows