nixos: add nixpkgs.scopedOverlays option (#254)

so we can add overlays that apply within configurations without
effecting the base nixpkgs

Author: bacchanalia

nixos/modules/misc/nixpkgs.nix

@@ -101,8 +101,36 @@ let
           ;
       };
 
-  finalPkgs = if opt.pkgs.isDefined then cfg.pkgs.appendOverlays cfg.overlays else defaultPkgs;
+  overlayedPkgs = if opt.pkgs.isDefined then cfg.pkgs.appendOverlays cfg.overlays else defaultPkgs;
+  splicedPackages = overlayedPkgs.__splicedPackages;
 
+  finalPkgs = if (cfg.scopedOverlays or [ ]) == [ ] then splicedPackages else
+    let
+      # cribbed of lib.makeScope
+      withScope = base: extension:
+        let
+          self = packages self // scope;
+          packages = self: extension self base;
+          scope = {
+            inherit extension packages;
+
+            newScope = scope: base.newScope (self // scope);
+
+            callPackage = self.newScope {};
+            callPackages = base.callPackagesWith self;
+            callPackageWith = autoArgs: base.callPackageWith (self // autoArgs);
+            callPackagesWith = autoArgs: base.callPackagesWith (self // autoArgs);
+
+            appendOverlays = fs: withScope (base.appendOverlays fs) extension;
+            extend = f: self.appendOverlays [f];
+
+            overrideScope = f: lib.makeScope base.newScope (lib.extends f packages);
+            appendOverlaysToScope = fs: withScope base (lib.composeManyExtensions ([extension] ++ fs));
+            extendScope = f: self.appendOverlaysToScope [f];
+          };
+        in base // self;
+    in
+    withScope splicedPackages (lib.composeManyExtensions cfg.scopedOverlays);
 in
 
 {
@@ -194,6 +222,25 @@ in
       '';
     };
 
+    scopedOverlays = lib.mkOption {
+      default = [];
+      example = lib.literalExpression
+        ''
+          [
+            (self: super: {
+              # Log commands run in builds of configuration files
+              runCommand = name: args: text: super.runCommand name args '''
+                set +x
+                ''${text}
+              ''';
+          ]
+        '';
+      type = lib.types.listOf overlayType;
+      description = ''
+        List of overlays to apply in a new scope after `nixpkgs.overlays` is applied accessed trough the `pgks` module argument.
+      '';
+    };
+
     hostPlatform = lib.mkOption {
       type = lib.types.either lib.types.str lib.types.attrs; # TODO utilize lib.systems.parsedPlatform
       example = {
@@ -356,7 +403,7 @@ in
         # which is somewhat costly for Nixpkgs. With an explicit priority, we only
         # evaluate the wrapper to find out that the priority is lower, and then we
         # don't need to evaluate `finalPkgs`.
-        lib.mkOverride lib.modules.defaultOverridePriority finalPkgs.__splicedPackages;
+        lib.mkOverride lib.modules.defaultOverridePriority finalPkgs;
     };
 
     assertions =