top-level: Only splice as needed for performance

pkgs/stdenv/booter.nix

@@ -63,12 +63,11 @@ stageFuns: let
         __bootPackages = finalSoFar;
       };
     };
-    self =
-      if args.__raw or false
-      then args'
-      else allPackages ((builtins.removeAttrs args' ["selfBuild"]) // {
-        buildPackages = if args.selfBuild or true then self else finalSoFar;
-      });
-  in self;
+  in
+    if args.__raw or false
+    then args'
+    else allPackages ((builtins.removeAttrs args' ["selfBuild"]) // {
+      buildPackages = if args.selfBuild or true then null else finalSoFar;
+    });
 
 in lib.lists.fold folder {} withAllowCustomOverrides

pkgs/top-level/splice.nix

@@ -17,7 +17,11 @@
 # `mkDerivation` knows how to pull out the right ones for `buildDepends` and
 # friends, but a few packages use them directly, so it seemed efficient (to
 # @Ericson2314) to reuse those names, at least initially, to minimize breakage.
-lib: pkgs:
+#
+# For performance reasons, rather than uniformally splice in all cases, we only
+# do so when `pkgs` and `buildPackages` are distinct. The `actuallySplice`
+# parameter there the boolean value of that equality check.
+lib: pkgs: actuallySplice:
 
 let
   defaultBuildScope = pkgs.buildPackages // pkgs.buildPackages.xorg;
@@ -58,7 +62,10 @@ let
     };
   in lib.listToAttrs (map merge (lib.attrNames mash));
 
-  splicedPackages = splicer defaultBuildScope defaultRunScope;
+  splicedPackages =
+    if actuallySplice
+    then splicer defaultBuildScope defaultRunScope
+    else pkgs // pkgs.xorg;
 
 in
 

pkgs/top-level/stage.nix

@@ -45,7 +45,8 @@
   ## Other parameters
   ##
 
-, # The package set used at build-time
+, # The package set used at build-time. If null, `buildPackages` will
+  # be defined internally as the produced package set as itself.
   buildPackages
 
 , # The standard environment to use for building packages.
@@ -84,7 +85,8 @@ let
     };
 
   stdenvBootstappingAndPlatforms = self: super: {
-    buildPackages = buildPackages // { recurseForDerivations = false; };
+    buildPackages = (if buildPackages == null then self else buildPackages)
+      // { recurseForDerivations = false; };
     inherit stdenv
       buildPlatform hostPlatform targetPlatform;
   };
@@ -103,7 +105,7 @@ let
     inherit (buildPlatform) system platform;
   };
 
-  splice = self: super: import ./splice.nix lib self;
+  splice = self: super: import ./splice.nix lib self (buildPackages != null);
 
   allPackages = self: super:
     let res = import ./all-packages.nix