stdenv: implement crossOverlays

crossOverlays only apply to the packages being built, not the build
packages. It is useful when you don’t care what is used to build your
packages, just what is being built. The idea relies heavily on the
cross compiling infrastructure. Using this implies that we need to
create a cross stdenv.

pkgs/stdenv/cross/default.nix

@@ -1,11 +1,14 @@
 { lib
-, localSystem, crossSystem, config, overlays
+, localSystem, crossSystem, config, overlays, crossOverlays ? []
 }:
 
 let
   bootStages = import ../. {
     inherit lib localSystem overlays;
-    crossSystem = null;
+
+    crossSystem = localSystem;
+    crossOverlays = [];
+
     # Ignore custom stdenvs when cross compiling for compatability
     config = builtins.removeAttrs config [ "replaceStdenv" ];
   };
@@ -33,7 +36,8 @@ in lib.init bootStages ++ [
 
   # Run Packages
   (buildPackages: {
-    inherit config overlays;
+    inherit config;
+    overlays = overlays ++ crossOverlays;
     selfBuild = false;
     stdenv = buildPackages.stdenv.override (old: rec {
       buildPlatform = localSystem;
@@ -48,14 +52,15 @@ in lib.init bootStages ++ [
 
       cc = if crossSystem.useiOSPrebuilt or false
              then buildPackages.darwin.iosSdkPkgs.clang
-           else if crossSystem.useAndroidPrebuilt
+           else if crossSystem.useAndroidPrebuilt or false
              then buildPackages.androidenv."androidndkPkgs_${crossSystem.ndkVer}".gcc
            else buildPackages.gcc;
 
       extraNativeBuildInputs = old.extraNativeBuildInputs
         ++ lib.optionals
              (hostPlatform.isLinux && !buildPlatform.isLinux)
              [ buildPackages.patchelf buildPackages.paxctl ]
+        ++ lib.optional hostPlatform.isDarwin buildPackages.clang
         ++ lib.optional
              (let f = p: !p.isx86 || p.libc == "musl"; in f hostPlatform && !(f buildPlatform))
              buildPackages.updateAutotoolsGnuConfigScriptsHook

pkgs/stdenv/custom/default.nix

@@ -2,7 +2,7 @@
 , localSystem, crossSystem, config, overlays
 }:
 
-assert crossSystem == null;
+assert crossSystem == localSystem;
 
 let
   bootStages = import ../. {

pkgs/stdenv/darwin/default.nix

@@ -1,5 +1,5 @@
 { lib
-, localSystem, crossSystem, config, overlays
+, localSystem, crossSystem, config, overlays, crossOverlays ? []
 
 # Allow passing in bootstrap files directly so we can test the stdenv bootstrap process when changing the bootstrap tools
 , bootstrapFiles ? let
@@ -16,7 +16,7 @@
   }
 }:
 
-assert crossSystem == null;
+assert crossSystem == localSystem;
 
 let
   inherit (localSystem) system platform;

pkgs/stdenv/default.nix

@@ -7,7 +7,7 @@
 { # Args just for stdenvs' usage
   lib
   # Args to pass on to the pkgset builder, too
-, localSystem, crossSystem, config, overlays
+, localSystem, crossSystem, config, overlays, crossOverlays ? []
 } @ args:
 
 let
@@ -36,7 +36,7 @@ let
 
   # Select the appropriate stages for the platform `system'.
 in
-  if crossSystem != null then stagesCross
+  if crossSystem != localSystem || crossOverlays != [] then stagesCross
   else if config ? replaceStdenv then stagesCustom
   else { # switch
     "i686-linux" = stagesLinux;

pkgs/stdenv/freebsd/default.nix

@@ -2,7 +2,7 @@
 , localSystem, crossSystem, config, overlays
 }:
 
-assert crossSystem == null;
+assert crossSystem == localSystem;
 let inherit (localSystem) system; in
 
 

pkgs/stdenv/linux/default.nix

@@ -4,7 +4,7 @@
 # compiler and linker that do not search in default locations,
 # ensuring purity of components produced by it.
 { lib
-, localSystem, crossSystem, config, overlays
+, localSystem, crossSystem, config, overlays, crossOverlays ? []
 
 , bootstrapFiles ?
   let table = {
@@ -32,7 +32,7 @@
   in files
 }:
 
-assert crossSystem == null;
+assert crossSystem == localSystem;
 
 let
   inherit (localSystem) system platform;

pkgs/stdenv/native/default.nix

@@ -2,7 +2,7 @@
 , localSystem, crossSystem, config, overlays
 }:
 
-assert crossSystem == null;
+assert crossSystem == localSystem;
 
 let
   inherit (localSystem) system;

pkgs/stdenv/nix/default.nix

@@ -1,10 +1,10 @@
 { lib
-, crossSystem, config, overlays
+, crossSystem, localSystem, config, overlays
 , bootStages
 , ...
 }:
 
-assert crossSystem == null;
+assert crossSystem == localSystem;
 
 bootStages ++ [
   (prevStage: {

pkgs/top-level/default.nix

@@ -22,16 +22,18 @@
   # `*Platform`s.
   localSystem
 
-, # The system packages will ultimately be run on. Null if the two should be the
-  # same.
-  crossSystem ? null
+, # The system packages will ultimately be run on.
+  crossSystem ? localSystem
 
 , # Allow a configuration attribute set to be passed in as an argument.
   config ? {}
 
 , # List of overlays layers used to extend Nixpkgs.
   overlays ? []
 
+, # List of overlays to apply to target packages only.
+  crossOverlays ? []
+
 , # A function booting the final package set for a specific standard
   # environment. See below for the arguments given to that function, the type of
   # list it returns.
@@ -61,7 +63,8 @@ in let
     builtins.intersectAttrs { platform = null; } config
     // args.localSystem);
 
-  crossSystem = lib.mapNullable lib.systems.elaborate crossSystem0;
+  crossSystem = if crossSystem0 == null then localSystem
+                else lib.systems.elaborate crossSystem0;
 
   # A few packages make a new package set to draw their dependencies from.
   # (Currently to get a cross tool chain, or forced-i686 package.) Rather than
@@ -91,7 +94,7 @@ in let
   boot = import ../stdenv/booter.nix { inherit lib allPackages; };
 
   stages = stdenvStages {
-    inherit lib localSystem crossSystem config overlays;
+    inherit lib localSystem crossSystem config overlays crossOverlays;
   };
 
   pkgs = boot stages;