feat: cabal2nix integration for IFD-less evalutation

nix/build-haskell-package.nix

@@ -22,6 +22,20 @@ let
       name = "${name}";
       inherit src;
     };
+
+  callPackageKeepDeriver = src: args:
+    pkgs.haskell.lib.compose.overrideCabal
+      (orig: {
+        inherit src; # Override original source path to use new
+        passthru = orig.passthru or { } // {
+          # When using cabal2nix, it is often useful
+          # to debug a failure by inspecting the Nix expression
+          # generated by cabal2nix. This can be accessed via this
+          # cabal2nixDeriver field.
+          cabal2nixDeriver = src;
+        };
+      })
+      (self.callPackage src args);
 in
 
 name: root:
@@ -31,6 +45,19 @@ lib.pipe root
     (mkNewStorePath "source-${name}")
     (x: log.traceDebug "${name}.mkNewStorePath ${x.outPath}" x)
 
-    (root: self.callCabal2nix name root { })
-    (x: log.traceDebug "${name}.cabal2nixDeriver ${x.cabal2nixDeriver.outPath}" x)
+     (root:
+        # Check if cached cabal2nix generated nix expression is present,
+        # if present use it with callPackage
+        # to avoid IFD
+        let pkg = 
+          if lib.pathExists (lib.concatStringsSep "/" [ root "default.nix" ])
+          then 
+            (log.traceDebug "${name}.callPackageKeepDeriver ${root}") 
+              callPackageKeepDeriver root { }
+          else
+            (log.traceDebug "${name}.callCabal2nix ${root}")
+              self.callCabal2nix name root { };
+        in 
+          (log.traceDebug "${name}.cabal2nixDeriver ${pkg.cabal2nixDeriver.outPath}" pkg)
+     )
   ]

nix/modules/project/packages/default.nix

@@ -61,8 +61,7 @@ in
           getOrMkPackage = name: cfg:
             if lib.types.path.check cfg.source
             then
-              log.traceDebug "${name}.callCabal2nix ${cfg.source}"
-                (build-haskell-package name cfg.source)
+              (build-haskell-package name cfg.source)
             else
               log.traceDebug "${name}.callHackage ${cfg.source}"
                 (self.callHackage name cfg.source { });