pkgs/overlays/unstable: factor out adding unstable package set to different file

pkgs/overlays/unstable-from-source.nix

@@ -0,0 +1,30 @@
+{
+  unstableSource,
+}:
+
+self: super:
+
+let
+  useUnstable = self.config.useUnstable or true;
+
+  unstablePkgsForNixpkgs = nixpkgs: import unstableSource {
+    # localSystem -> pkgs.stdenv.buildPlatform
+    localSystem = nixpkgs.stdenv.buildPlatform;
+    # crossSystem -> pkgs.stdenv.hostPlatform or pkgs.stdenv.targetPlatform ??
+    # passing below
+    # config -> pkgs.config
+    config = nixpkgs.config;
+    # overlays -> partial of pkgs.overlays
+    overlays = nixpkgs.overlays;
+    # crossOverlays -> partial of pkgs.overlays
+    # crossOverlays are merged to overlays, not sure what issues that might raise.
+    # ignoring.
+  } // (if nixpkgs.stdenv.buildPlatform == nixpkgs.stdenv.hostPlatform then {} else {
+    # workaround for some odd structured packages that changes behaviour
+    # when crossSystem is passed.
+    crossSystem = nixpkgs.stdenv.hostPlatform;
+  });
+in
+{
+  unstable = if useUnstable then unstablePkgsForNixpkgs self else self;
+}

pkgs/overlays/unstable.nix

@@ -24,26 +24,10 @@ let
   ];
 
   nixpkgsRevision = (builtins.readFile "${unstablePkgsExprs}/.git-revision");
-  unstablePkgsForNixpkgs = nixpkgs: import unstablePkgsExprs {
-    # localSystem -> pkgs.stdenv.buildPlatform
-    localSystem = nixpkgs.stdenv.buildPlatform;
-    # crossSystem -> pkgs.stdenv.hostPlatform or pkgs.stdenv.targetPlatform ??
-    # passing below
-    # config -> pkgs.config
-    config = nixpkgs.config;
-    # overlays -> partial of pkgs.overlays
-    overlays = nixpkgs.overlays;
-    # crossOverlays -> partial of pkgs.overlays
-    # crossOverlays are merged to overlays, not sure what issues that might raise.
-    # ignoring.
-  } // (if nixpkgs.stdenv.buildPlatform == nixpkgs.stdenv.hostPlatform then {} else {
-    # workaround for some odd structured packages that changes behaviour
-    # when crossSystem is passed.
-    crossSystem = nixpkgs.stdenv.hostPlatform;
-  });
 in
-{
+import ./unstable-from-source.nix {
-  unstable = if useUnstable then unstablePkgsForNixpkgs self else self;
+  unstableSource = unstablePkgsExprs;
+} self super // {
   unstableVersion = self.lib.optionalString useUnstable nixpkgsVersion;
   unstableRevision = self.lib.optionalString useUnstable nixpkgsRevision;
 }