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;
@@ -48,7 +52,7 @@ 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;
 
@@ -56,6 +60,7 @@ in lib.init bootStages ++ [
         ++ 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: {