top-level: Introduce buildPackages for resolving build-time deps

[N.B., this package also applies to the commits that follow it in the same
PR.]

In most cases, buildPackages = pkgs so things work just as before. For
cross compiling, however, buildPackages is resolved as the previous
bootstrapping stage. This allows us to avoid the mkDerivation hacks cross
compiling currently uses today.

To avoid a massive refactor, callPackage will splice together both package
sets. Again to avoid churn, it uses the old `nativeDrv` vs `crossDrv` to do
so. So now, whether cross compiling or not, packages with get a `nativeDrv`
and `crossDrv`---in the non-cross-compiling case they are simply the same
derivation. This is good because it reduces the divergence between the
cross and non-cross dataflow. See `pkgs/top-level/splice.nix` for a comment
along the lines of the preceding paragraph, and the code that does this
splicing.

Also, `forceNativeDrv` is replaced with `forceNativePackages`. The latter
resolves `pkgs` unless the host platform is different from the build
platform, in which case it resolves to `buildPackages`. Note that the
target platform is not important here---it will not prevent
`forcedNativePackages` from resolving to `pkgs`.

--------

Temporarily, we make preserve some dubious decisions in the name of preserving
hashes:

Most importantly, we don't distinguish between "host" and "target" in the
autoconf sense. This leads to the proliferation of *Cross derivations
currently used. What we ought to is resolve native deps of the cross "build
packages" (build = host != target) package set against the "vanilla
packages" (build = host = target) package set. Instead, "build packages"
uses itself, with (informally) target != build in all cases.

This is wrong because it violates the "sliding window" principle of
bootstrapping stages that shifting the platform triple of one stage to the
left coincides with the next stage's platform triple. Only because we don't
explicitly distinguish between "host" and "target" does it appear that the
"sliding window" principle is preserved--indeed it is over the reductionary
"platform double" of just "build" and "host/target".

Additionally, we build libc, libgcc, etc in the same stage as the compilers
themselves, which is wrong because they are used at runtime, not build
time. Fixing this is somewhat subtle, and the solution and problem will be
better explained in the commit that does fix it.

Commits after this will solve both these issues, at the expense of breaking
cross hashes. Native hashes won't be broken, thankfully.

--------

Did the temporary ugliness pan out? Of the packages that currently build in
`release-cross.nix`, the only ones that have their hash changed are
`*.gcc.crossDrv` and `bootstrapTools.*.coreutilsMinimal`. In both cases I
think it doesn't matter.

 1. GCC when doing a `build = host = target = foreign` build (maximally
    cross), still defines environment variables like `CPATH`[1] with
    packages.  This seems assuredly wrong because whether gcc dynamically
    links those, or the programs built by gcc dynamically link those---I
    have no idea which case is reality---they should be foreign. Therefore,
    in all likelihood, I just made the gcc less broken.

 2. Coreutils (ab)used the old cross-compiling infrastructure to depend on
    a native version of itself. When coreutils was overwritten to be built
    with fewer features, the native version it used would also be
    overwritten because the binding was tight. Now it uses the much looser
    `BuildPackages.coreutils` which is just fine as a richer build dep
    doesn't cause any problems and avoids a rebuild.

So, in conclusion I'd say the conservatism payed off. Onward to actually
raking the muck in the next PR!

[1]: https://gcc.gnu.org/onlinedocs/gcc/Environment-Variables.html

pkgs/os-specific/gnu/default.nix

@@ -3,7 +3,7 @@
 args@{ fetchgit, stdenv, autoconf, automake, automake111x, libtool
 , texinfo, glibcCross, hurdPartedCross, libuuid, samba
 , gccCrossStageStatic, gccCrossStageFinal
-, forceNativeDrv, forceSystem, newScope, platform, config, crossSystem
+, forcedNativePackages, forceSystem, newScope, platform, config, crossSystem
 , overrides ? {} }:
 
 with args;
@@ -12,7 +12,7 @@ let
   callPackage = newScope gnu;
 
   gnu = {
-    hurdCross = forceNativeDrv (callPackage ./hurd {
+    hurdCross = forcedNativePackages.callPackage ./hurd {
       inherit fetchgit stdenv autoconf libtool texinfo
         glibcCross hurdPartedCross;
       inherit (gnu) machHeaders mig;
@@ -21,9 +21,9 @@ let
       headersOnly = false;
       cross = assert crossSystem != null; crossSystem;
       gccCross = gccCrossStageFinal;
-    });
+    };
 
-    hurdCrossIntermediate = forceNativeDrv (callPackage ./hurd {
+    hurdCrossIntermediate = forcedNativePackages.callPackage ./hurd {
       inherit fetchgit stdenv autoconf libtool texinfo glibcCross;
       inherit (gnu) machHeaders mig;
       hurdPartedCross = null;
@@ -42,7 +42,7 @@ let
       # libshouldbeinlibc.
       buildTarget = "libihash libstore libshouldbeinlibc";
       installTarget = "libihash-install libstore-install libshouldbeinlibc-install";
-    });
+    };
 
     hurdHeaders = callPackage ./hurd {
       automake = automake111x;
@@ -58,13 +58,13 @@ let
       hurd = null;
     };
 
-    libpthreadCross = forceNativeDrv (callPackage ./libpthread {
+    libpthreadCross = forcedNativePackages.callPackage ./libpthread {
       inherit fetchgit stdenv autoconf automake libtool glibcCross;
       inherit (gnu) machHeaders hurdHeaders;
       hurd = gnu.hurdCrossIntermediate;
       gccCross = gccCrossStageStatic;
       cross = assert crossSystem != null; crossSystem;
-    });
+    };
 
     # In theory GNU Mach doesn't have to be cross-compiled.  However, since it
     # has to be built for i586 (it doesn't work on x86_64), one needs a cross

pkgs/stdenv/adapters.nix

@@ -66,12 +66,10 @@ rec {
 
             # In nixpkgs, sometimes 'null' gets in as a buildInputs element,
             # and we handle that through isAttrs.
-            getNativeDrv = drv: drv.nativeDrv or drv;
+            nativeBuildInputsDrvs = nativeBuildInputs;
-            getCrossDrv = drv: drv.crossDrv or drv;
+            buildInputsDrvs = buildInputs;
-            nativeBuildInputsDrvs = map getNativeDrv nativeBuildInputs;
+            propagatedBuildInputsDrvs = propagatedBuildInputs;
-            buildInputsDrvs = map getCrossDrv buildInputs;
+            propagatedNativeBuildInputsDrvs = propagatedNativeBuildInputs;
-            propagatedBuildInputsDrvs = map getCrossDrv propagatedBuildInputs;
-            propagatedNativeBuildInputsDrvs = map getNativeDrv propagatedNativeBuildInputs;
 
             # The base stdenv already knows that nativeBuildInputs and
             # buildInputs should be built with the usual gcc-wrapper
@@ -88,10 +86,7 @@ rec {
                 (drv: builtins.isAttrs drv && drv ? nativeDrv) buildInputs;
             nativeInputsFromBuildInputs = stdenv.lib.filter hostAsNativeDrv buildInputsNotNull;
 
-            # We should overwrite the input attributes in crossDrv, to overwrite
+        in      stdenv.mkDerivation (args // {
-            # the defaults for only-native builds in the base stdenv
-            crossDrv = if cross == null then nativeDrv else
-                stdenv.mkDerivation (args // {
                     name = name + "-" + cross.config;
                     nativeBuildInputs = nativeBuildInputsDrvs
                       ++ nativeInputsFromBuildInputs
@@ -112,9 +107,6 @@ rec {
 
                     crossConfig = cross.config;
                 } // args.crossAttrs or {});
-        in nativeDrv // {
-          inherit crossDrv nativeDrv;
-        };
     } // {
       inherit cross gccCross binutilsCross;
       ccCross = gccCross;

pkgs/stdenv/booter.nix

@@ -57,12 +57,18 @@ stageFuns: let
   # debugging purposes.
   folder = stageFun: finalSoFar: let
     args = stageFun finalSoFar;
-    stdenv = args.stdenv // {
+    args' = args // {
-      # For debugging
+      stdenv = args.stdenv // {
-      __bootPackages = finalSoFar;
+        # For debugging
+        __bootPackages = finalSoFar;
+      };
     };
-    args' = args // { inherit stdenv; };
+    self =
-  in
+      if args.__raw or false
-    (if args.__raw or false then lib.id else allPackages) args';
+      then args'
+      else allPackages ((builtins.removeAttrs args' ["selfBuild"]) // {
+        buildPackages = if args.selfBuild or true then self else finalSoFar;
+      });
+  in self;
 
 in lib.lists.fold folder {} withAllowCustomOverrides

pkgs/stdenv/cross/default.nix

@@ -12,13 +12,11 @@ let
 
 in bootStages ++ [
 
-  # Build Packages.
+  # Build Packages
-  #
-  # For now, this is just used to build the native stdenv. Eventually, it
-  # should be used to build compilers and other such tools targeting the cross
-  # platform. Then, `forceNativeDrv` can be removed.
   (vanillaPackages: {
     inherit system platform crossSystem config overlays;
+    # Should be false, but we're trying to preserve hashes for now
+    selfBuild = true;
     # It's OK to change the built-time dependencies
     allowCustomOverrides = true;
     stdenv = vanillaPackages.stdenv // {
@@ -28,9 +26,10 @@ in bootStages ++ [
     };
   })
 
-  # Run packages
+  # Run Packages
   (buildPackages: {
     inherit system platform crossSystem config overlays;
+    selfBuild = false;
     stdenv = if crossSystem.useiOSCross or false
       then let
           inherit (buildPackages.darwin.ios-cross {

pkgs/stdenv/generic/default.nix

@@ -115,7 +115,19 @@ let
     , sandboxProfile ? ""
     , propagatedSandboxProfile ? ""
     , ... } @ attrs:
-    let
+    let # Rename argumemnts to avoid cycles
+      buildInputs__ = buildInputs;
+      nativeBuildInputs__ = nativeBuildInputs;
+      propagatedBuildInputs__ = propagatedBuildInputs;
+      propagatedNativeBuildInputs__ = propagatedNativeBuildInputs;
+    in let
+      getNativeDrv = drv: drv.nativeDrv or drv;
+      getCrossDrv = drv: drv.crossDrv or drv;
+      nativeBuildInputs = map getNativeDrv nativeBuildInputs__;
+      buildInputs = map getCrossDrv buildInputs__;
+      propagatedBuildInputs = map getCrossDrv propagatedBuildInputs__;
+      propagatedNativeBuildInputs = map getNativeDrv propagatedNativeBuildInputs__;
+    in let
       pos' =
         if pos != null then
           pos

pkgs/stdenv/linux/make-bootstrap-tools-cross.nix

@@ -55,11 +55,12 @@ let
     if toolsArch == "armv6l" then raspberrypiCrossSystem else
     if toolsArch == "armv7l" then armv7l-hf-multiplatform-crossSystem else null;
 
-  pkgs = pkgsFun ({inherit system;} // selectedCrossSystem);
+  pkgsUnspliced = pkgsFun ({inherit system;} // selectedCrossSystem);
+  pkgs = pkgsUnspliced.splicedPackages;
 
-  inherit (pkgs) stdenv nukeReferences cpio binutilsCross;
+  inherit (pkgsUnspliced.buildPackages) stdenv nukeReferences cpio binutilsCross;
 
-  glibc = pkgs.libcCross;
+  glibc = pkgs.libcCross.nativeDrv;
   bash = pkgs.bash.crossDrv;
   findutils = pkgs.findutils.crossDrv;
   diffutils = pkgs.diffutils.crossDrv;
@@ -71,7 +72,7 @@ let
   gnumake = pkgs.gnumake.crossDrv;
   patch = pkgs.patch.crossDrv;
   patchelf = pkgs.patchelf.crossDrv;
-  gcc = pkgs.gcc.cc.crossDrv;
+  gcc = pkgs.gcc.crossDrv.cc;
   gmpxx = pkgs.gmpxx.crossDrv;
   mpfr = pkgs.mpfr.crossDrv;
   zlib = pkgs.zlib.crossDrv;
@@ -86,17 +87,17 @@ in
 rec {
 
 
-  coreutilsMinimal = (pkgs.coreutils.override (args: {
+  coreutilsMinimal = pkgs.coreutils.override (args: {
     # We want coreutils without ACL/attr support.
     aclSupport = false;
     attrSupport = false;
     # Our tooling currently can't handle scripts in bin/, only ELFs and symlinks.
     singleBinary = "symlinks";
-  })).crossDrv;
+  });
 
-  tarMinimal = (pkgs.gnutar.override { acl = null; }).crossDrv;
+  tarMinimal = pkgs.gnutar.override { acl = null; };
 
-  busyboxMinimal = (pkgs.busybox.override {
+  busyboxMinimal = pkgs.busybox.override {
     useMusl = true;
     enableStatic = true;
     enableMinimal = true;
@@ -109,13 +110,13 @@ rec {
       CONFIG_TAR y
       CONFIG_UNXZ y
     '';
-  }).crossDrv;
+  };
 
   build =
 
     stdenv.mkDerivation {
       name = "stdenv-bootstrap-tools-cross";
-      crossConfig = stdenv.cross.config;
+      crossConfig = pkgsUnspliced.crossSystem.config;
 
       buildInputs = [nukeReferences cpio binutilsCross];
 

pkgs/top-level/all-packages.nix

@@ -10,26 +10,12 @@ self: pkgs:
 
 with pkgs;
 
-let
-  defaultScope = pkgs // pkgs.xorg;
-in
-
 {
 
   # Allow callPackage to fill in the pkgs argument
   inherit pkgs;
 
 
-  # We use `callPackage' to be able to omit function arguments that
-  # can be obtained from `pkgs' or `pkgs.xorg' (i.e. `defaultScope').
-  # Use `newScope' for sets of packages in `pkgs' (see e.g. `gnome'
-  # below).
-  callPackage = newScope {};
-
-  callPackages = lib.callPackagesWith defaultScope;
-
-  newScope = extra: lib.callPackageWith (defaultScope // extra);
-
   # Override system. This is useful to build i686 packages on x86_64-linux.
   forceSystem = system: kernel: nixpkgsFun {
     inherit system;
@@ -39,15 +25,9 @@ in
   # Used by wine, firefox with debugging version of Flash, ...
   pkgsi686Linux = forceSystem "i686-linux" "i386";
 
-  callPackage_i686 = lib.callPackageWith (pkgsi686Linux // pkgsi686Linux.xorg);
+  callPackage_i686 = pkgsi686Linux.callPackage;
 
-  forceNativeDrv = drv:
+  forcedNativePackages = if crossSystem == null then pkgs else buildPackages;
-    # Even when cross compiling, some packages come from the stdenv's
-    # bootstrapping package set. Those packages are only built for the native
-    # platform.
-    if crossSystem != null && drv ? crossDrv
-    then drv // { crossDrv = drv.nativeDrv; }
-    else drv;
 
   # A stdenv capable of building 32-bit binaries.  On x86_64-linux,
   # it uses GCC compiled with multilib support; on i686-linux, it's
@@ -4771,41 +4751,45 @@ in
 
   gccApple = throw "gccApple is no longer supported";
 
-  gccCrossStageStatic = let
+  gccCrossStageStatic = assert crossSystem != null; let
     libcCross1 =
       if stdenv.cross.libc == "msvcrt" then windows.mingw_w64_headers
       else if stdenv.cross.libc == "libSystem" then darwin.xcode
       else null;
     in wrapGCCCross {
-      gcc = forceNativeDrv (gcc.cc.override {
+      gcc = forcedNativePackages.gcc.cc.override {
         cross = crossSystem;
         crossStageStatic = true;
         langCC = false;
         libcCross = libcCross1;
         enableShared = false;
-      });
+        # Why is this needed?
+        inherit (forcedNativePackages) binutilsCross;
+      };
       libc = libcCross1;
       binutils = binutilsCross;
       cross = crossSystem;
   };
 
   # Only needed for mingw builds
-  gccCrossMingw2 = wrapGCCCross {
+  gccCrossMingw2 = assert crossSystem != null; wrapGCCCross {
     gcc = gccCrossStageStatic.gcc;
     libc = windows.mingw_headers2;
     binutils = binutilsCross;
     cross = assert crossSystem != null; crossSystem;
   };
 
-  gccCrossStageFinal = wrapGCCCross {
+  gccCrossStageFinal = assert crossSystem != null; wrapGCCCross {
-    gcc = forceNativeDrv (gcc.cc.override {
+    gcc = forcedNativePackages.gcc.cc.override {
       cross = crossSystem;
       crossStageStatic = false;
 
       # XXX: We have troubles cross-compiling libstdc++ on MinGW (see
       # <http://hydra.nixos.org/build/4268232>), so don't even try.
       langCC = crossSystem.config != "i686-pc-mingw32";
-    });
+      # Why is this needed?
+      inherit (forcedNativePackages) binutilsCross;
+    };
     libc = libcCross;
     binutils = binutilsCross;
     cross = crossSystem;
@@ -5499,12 +5483,12 @@ in
   wrapGCCCross =
     {gcc, libc, binutils, cross, shell ? "", name ? "gcc-cross-wrapper"}:
 
-    forceNativeDrv (callPackage ../build-support/gcc-cross-wrapper {
+    forcedNativePackages.callPackage ../build-support/gcc-cross-wrapper {
       nativeTools = false;
       nativeLibc = false;
       noLibc = (libc == null);
       inherit gcc binutils libc shell name cross;
-    });
+    };
 
   # prolog
   yap = callPackage ../development/compilers/yap { };
@@ -6084,12 +6068,12 @@ in
     gold = false;
   });
 
-  binutilsCross = assert crossSystem != null; lowPrio (forceNativeDrv (
+  binutilsCross = assert crossSystem != null; lowPrio (
     if crossSystem.libc == "libSystem" then darwin.cctools_cross
-    else binutils.override {
+    else forcedNativePackages.binutils.override {
       noSysDirs = true;
       cross = crossSystem;
-    }));
+    });
 
   bison2 = callPackage ../development/tools/parsing/bison/2.x.nix { };
   bison3 = callPackage ../development/tools/parsing/bison/3.x.nix { };
@@ -6558,9 +6542,9 @@ in
      cross_renaming: we should make all programs use pkgconfig as
      nativeBuildInput after the renaming.
      */
-  pkgconfig = forceNativeDrv (callPackage ../development/tools/misc/pkgconfig {
+  pkgconfig = forcedNativePackages.callPackage ../development/tools/misc/pkgconfig {
     fetchurl = fetchurlBoot;
-  });
+  };
   pkgconfigUpstream = lowPrio (pkgconfig.override { vanilla = true; });
 
   postiats-utilities = callPackage ../development/tools/postiats-utilities {};
@@ -7336,10 +7320,10 @@ in
     withGd = true;
   };
 
-  glibcCross = forceNativeDrv (glibc.override {
+  glibcCross = forcedNativePackages.glibc.override {
     gccCross = gccCrossStageStatic;
     linuxHeaders = linuxHeadersCross;
-  });
+  };
 
   # We can choose:
   libcCrossChooser = name: if name == "glibc" then glibcCross
@@ -10910,7 +10894,7 @@ in
     cmdline = callPackage ../os-specific/darwin/command-line-tools {};
     apple-source-releases = callPackage ../os-specific/darwin/apple-source-releases { };
   in apple-source-releases // rec {
-    cctools_cross = callPackage (forceNativeDrv (callPackage ../os-specific/darwin/cctools/port.nix {}).cross) {
+    cctools_cross = callPackage (forcedNativePackages.callPackage ../os-specific/darwin/cctools/port.nix {}).cross {
       cross = assert crossSystem != null; crossSystem;
       inherit maloader;
       xctoolchain = xcode.toolchain;
@@ -11148,13 +11132,13 @@ in
 
   linuxHeaders = linuxHeaders_4_4;
 
-  linuxHeaders24Cross = forceNativeDrv (callPackage ../os-specific/linux/kernel-headers/2.4.nix {
+  linuxHeaders24Cross = forcedNativePackages.callPackage ../os-specific/linux/kernel-headers/2.4.nix {
     cross = assert crossSystem != null; crossSystem;
-  });
+  };
 
-  linuxHeaders26Cross = forceNativeDrv (callPackage ../os-specific/linux/kernel-headers/4.4.nix {
+  linuxHeaders26Cross = forcedNativePackages.callPackage ../os-specific/linux/kernel-headers/4.4.nix {
     cross = assert crossSystem != null; crossSystem;
-  });
+  };
 
   linuxHeaders_3_18 = callPackage ../os-specific/linux/kernel-headers/3.18.nix { };
 

pkgs/top-level/release-cross.nix

@@ -32,8 +32,10 @@ let
 in
 
 {
-  # These `nativeDrv`s should be identical to their vanilla ones --- cross
+  # These derivations from a cross package set's `buildPackages` should be
-  # compiling should not affect the native derivation.
+  # identical to their vanilla equivalents --- none of these package should
+  # observe the target platform which is the only difference between those
+  # package sets.
   ensureUnaffected = let
     # Absurd values are fine here, as we are not building anything. In fact,
     # there probably a good idea to try to be "more parametric" --- i.e. avoid
@@ -47,8 +49,12 @@ in
     # good idea lest there be some irrelevant pass-through debug attrs that
     # cause false negatives.
     testEqualOne = path: system: let
-      f = attrs: builtins.toString (lib.getAttrFromPath path (allPackages attrs));
+      f = path: attrs: builtins.toString (lib.getAttrFromPath path (allPackages attrs));
-    in assert f { inherit system; } == f { inherit system crossSystem; }; true;
+    in assert
+        f path { inherit system; }
+        ==
+        f (["buildPackages"] ++ path) { inherit system crossSystem; };
+      true;
 
     testEqual = path: systems: forAllSupportedSystems systems (testEqualOne path);
 

pkgs/top-level/release-lib.nix

@@ -76,8 +76,9 @@ rec {
    * parameter for allPackages, defining the target platform for cross builds,
    * and triggering the build of the host derivation (cross built - crossDrv). */
   mapTestOnCross = crossSystem: mapAttrsRecursive
-    (path: systems: testOnCross crossSystem systems
+    (path: systems: testOnCross crossSystem systems (pkgs: addMetaAttrs
-      (pkgs: addMetaAttrs { maintainers = crossMaintainers; } (getAttrFromPath path pkgs)));
+      { maintainers = crossMaintainers; }
+      (getAttrFromPath path pkgs.splicedPackages)));
 
 
   /* Recursively map a (nested) set of derivations to an isomorphic

pkgs/top-level/splice.nix

@@ -0,0 +1,76 @@
+# The `splicedPackages' package set, and its use by `callPackage`
+#
+# The `buildPackages` pkg set is a new concept, and the vast majority package
+# expression (the other *.nix files) are not designed with it in mind. This
+# presents us with a problem with how to get the right version (build-time vs
+# run-time) of a package to a consumer that isn't used to thinking so cleverly.
+#
+# The solution is to splice the package sets together as we do below, so every
+# `callPackage`d expression in fact gets both versions. Each# derivation (and
+# each derivation's outputs) consists of the run-time version, augmented with a
+# `nativeDrv` field for the build-time version, and `crossDrv` field for the
+# run-time version.
+#
+# We could have used any names we want for the disambiguated versions, but
+# `crossDrv` and `nativeDrv` were somewhat similarly used for the old
+# cross-compiling infrastructure. The names are mostly invisible as
+# `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:
+
+let
+  defaultBuildScope = pkgs.buildPackages // pkgs.buildPackages.xorg;
+  # TODO(@Ericson2314): we shouldn't preclude run-time fetching by removing
+  # these attributes. We should have a more general solution for selecting
+  # whether `nativeDrv` or `crossDrv` is the default in `defaultScope`.
+  pkgsWithoutFetchers = lib.filterAttrs (n: _: !lib.hasPrefix "fetch" n) pkgs;
+  defaultRunScope = pkgsWithoutFetchers // pkgs.xorg;
+
+  splicer = buildPkgs: runPkgs: let
+    mash = buildPkgs // runPkgs;
+    merge = name: {
+      inherit name;
+      value = let
+        defaultValue = mash.${name};
+        buildValue = buildPkgs.${name} or {};
+        runValue = runPkgs.${name} or {};
+        augmentedValue = defaultValue
+          // (lib.optionalAttrs (buildPkgs ? ${name}) { nativeDrv = buildValue; })
+          // (lib.optionalAttrs (runPkgs ? ${name}) { crossDrv = runValue; });
+        # Get the set of outputs of a derivation
+        getOutputs = value:
+          lib.genAttrs (value.outputs or []) (output: value.${output});
+      in
+        # Certain *Cross derivations will fail assertions, but we need their
+        # nativeDrv. We are assuming anything that fails to evaluate is an
+        # attrset (including derivation) and thus can be unioned.
+        if !(builtins.tryEval defaultValue).success then augmentedValue
+        # The derivation along with its outputs, which we recur
+        # on to splice them together.
+        else if lib.isDerivation defaultValue then augmentedValue
+          // splicer (getOutputs buildValue) (getOutputs runValue)
+        # Just recur on plain attrsets
+        else if lib.isAttrs defaultValue then splicer buildValue runValue
+        # Don't be fancy about non-derivations. But we could have used used
+        # `__functor__` for functions instead.
+        else defaultValue;
+    };
+  in lib.listToAttrs (map merge (lib.attrNames mash));
+
+  splicedPackages = splicer defaultBuildScope defaultRunScope;
+
+in
+
+{
+  splicedPackages = splicedPackages // { recurseForDerivations = false; };
+
+  # We use `callPackage' to be able to omit function arguments that can be
+  # obtained `pkgs` or `buildPackages` and their `xorg` package sets. Use
+  # `newScope' for sets of packages in `pkgs' (see e.g. `gnome' below).
+  callPackage = pkgs.newScope {};
+
+  callPackages = lib.callPackagesWith splicedPackages;
+
+  newScope = extra: lib.callPackageWith (splicedPackages // extra);
+}

pkgs/top-level/stage.nix

@@ -12,6 +12,9 @@
 { # The system (e.g., `i686-linux') for which to build the packages.
   system
 
+, # the package set used at build-time
+  buildPackages
+
 , # The standard environment to use for building packages.
   stdenv
 
@@ -51,10 +54,13 @@ let
 
   stdenvBootstappingAndPlatforms = self: super: {
     stdenv = stdenv // { inherit platform; };
+    buildPackages = buildPackages // { recurseForDerivations = false; };
     inherit
       system platform crossSystem;
   };
 
+  splice = self: super: import ./splice.nix lib self;
+
   allPackages = self: super:
     let res = import ./all-packages.nix
       { inherit lib nixpkgsFun noSysDirs config; }
@@ -85,6 +91,7 @@ let
     stdenvBootstappingAndPlatforms
     stdenvAdapters
     trivialBuilders
+    splice
     allPackages
     aliases
     stdenvOverrides