Add default setup back into roots

Author: hamishmack

ci.nix

@@ -1,6 +1,6 @@
 # 'supportedSystems' restricts the set of systems that we will evaluate for. Useful when you're evaluating
 # on a machine with e.g. no way to build the Darwin IFDs you need!
-{ ifdLevel ? 3
+{ ifdLevel ? 1
 , checkMaterialization ? false
 , system ? builtins.currentSystem
 , evalSystem ? builtins.currentSystem or "x86_64-linux"

overlays/haskell.nix

@@ -1120,6 +1120,10 @@ final: prev: {
           }) // final.lib.optionalAttrs (ifdLevel > 1) {
             # Things that require two levels of IFD to build (inputs should be in level 1)
             nix-tools-unchecked = final.pkgsBuildBuild.haskell-nix.nix-tools-unchecked;
+            # This is the setup using the prefered Cabal library.
+            default-setup = final.buildPackages.haskell-nix.compiler.${compiler-nix-name}.defaultSetupFor "some-package";
+            # This is the one used when that one is not allowed.
+            setup-cabal-from-ghc = final.buildPackages.haskell-nix.compiler.${compiler-nix-name}.defaultSetup.useCabalFromGHC;
           } // final.lib.optionalAttrs (ifdLevel > 1
             && final.haskell-nix.haskellLib.isCrossHost
             # GHCJS builds its own template haskell runner.