top-level: Lay the groundwork for {build,host,target}Platform

The long term goal is a big replace:
  { inherit system platform; } => buildPlatform
  crossSystem => hostPlatform
  stdenv.cross => targetPlatform
And additionally making sure each is defined even when not cross compiling.

This commit refactors the bootstrapping code along that vision, but leaves
the old identifiers with their null semantics in place so packages can be
modernized incrementally.

pkgs/stdenv/adapters.nix

@@ -108,7 +108,7 @@ rec {
                     crossConfig = cross.config;
                 } // args.crossAttrs or {});
     } // {
-      inherit cross gccCross binutilsCross;
+      inherit gccCross binutilsCross;
       ccCross = gccCross;
     };
 

pkgs/stdenv/cross/default.nix

@@ -1,10 +1,10 @@
 { lib
-, system, platform, crossSystem, config, overlays
+, localSystem, crossSystem, config, overlays
 }:
 
 let
   bootStages = import ../. {
-    inherit lib system platform overlays;
+    inherit lib localSystem overlays;
     crossSystem = null;
     # Ignore custom stdenvs when cross compiling for compatability
     config = builtins.removeAttrs config [ "replaceStdenv" ];
@@ -14,21 +14,25 @@ in bootStages ++ [
 
   # Build Packages
   (vanillaPackages: {
-    inherit system platform crossSystem config overlays;
+    buildPlatform = localSystem;
+    hostPlatform = localSystem;
+    targetPlatform = crossSystem;
+    inherit 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 // {
-      # Needed elsewhere as a hacky way to pass the target
-      cross = crossSystem;
       overrides = _: _: {};
     };
   })
 
   # Run Packages
   (buildPackages: {
-    inherit system platform crossSystem config overlays;
+    buildPlatform = localSystem;
+    hostPlatform = crossSystem;
+    targetPlatform = crossSystem;
+    inherit config overlays;
     selfBuild = false;
     stdenv = if crossSystem.useiOSCross or false
       then let

pkgs/stdenv/custom/default.nix

@@ -1,12 +1,12 @@
 { lib
-, system, platform, crossSystem, config, overlays
+, localSystem, crossSystem, config, overlays
 }:
 
 assert crossSystem == null;
 
 let
   bootStages = import ../. {
-    inherit lib system platform crossSystem overlays;
+    inherit lib localSystem crossSystem overlays;
     # Remove config.replaceStdenv to ensure termination.
     config = builtins.removeAttrs config [ "replaceStdenv" ];
   };
@@ -15,7 +15,10 @@ in bootStages ++ [
 
   # Additional stage, built using custom stdenv
   (vanillaPackages: {
-    inherit system platform crossSystem config overlays;
+    buildPlatform = localSystem;
+    hostPlatform = localSystem;
+    targetPlatform = localSystem;
+    inherit config overlays;
     stdenv = config.replaceStdenv { pkgs = vanillaPackages; };
   })
 

pkgs/stdenv/darwin/default.nix

@@ -1,11 +1,12 @@
 { lib
-, system, platform, crossSystem, config, overlays
+, localSystem, crossSystem, config, overlays
 
 # Allow passing in bootstrap files directly so we can test the stdenv bootstrap process when changing the bootstrap tools
 , bootstrapFiles ? let
   fetch = { file, sha256, executable ? true }: import <nix/fetchurl.nix> {
     url = "http://tarballs.nixos.org/stdenv-darwin/x86_64/33f59c9d11b8d5014dfd18cc11a425f6393c884a/${file}";
-    inherit sha256 system executable;
+    inherit (localSystem) system;
+    inherit sha256 executable;
   }; in {
     sh      = fetch { file = "sh";    sha256 = "1rx4kg6358xdj05z0m139a0zn4f4zfmq4n4vimlmnwyfiyn4x7wk"; };
     bzip2   = fetch { file = "bzip2"; sha256 = "104qnhzk79vkbp2yi0kci6lszgfppvrwk3rgxhry842ly1xz2r7l"; };
@@ -18,6 +19,8 @@
 assert crossSystem == null;
 
 let
+  inherit (localSystem) system platform;
+
   libSystemProfile = ''
     (import "${./standard-sandbox.sb}")
   '';
@@ -98,7 +101,10 @@ in rec {
       };
 
     in {
-      inherit system platform crossSystem config overlays;
+      buildPlatform = localSystem;
+      hostPlatform = localSystem;
+      targetPlatform = localSystem;
+      inherit config overlays;
       stdenv = thisStdenv;
     };
 
@@ -316,7 +322,10 @@ in rec {
     stage3
     stage4
     (prevStage: {
-      inherit system crossSystem platform config overlays;
+      buildPlatform = localSystem;
+      hostPlatform = localSystem;
+      targetPlatform = localSystem;
+      inherit config overlays;
       stdenv = stdenvDarwin prevStage;
     })
   ];

pkgs/stdenv/default.nix

@@ -7,7 +7,7 @@
 { # Args just for stdenvs' usage
   lib
   # Args to pass on to the pkgset builder, too
-, system, platform, crossSystem, config, overlays
+, localSystem, crossSystem, config, overlays
 } @ args:
 
 let
@@ -51,4 +51,4 @@ in
     "i686-cygwin" = stagesNative;
     "x86_64-cygwin" = stagesNative;
     "x86_64-freebsd" = stagesFreeBSD;
-  }.${system} or stagesNative
+  }.${localSystem.system} or stagesNative

pkgs/stdenv/freebsd/default.nix

@@ -1,8 +1,9 @@
 { lib
-, system, platform, crossSystem, config, overlays
+, localSystem, crossSystem, config, overlays
 }:
 
 assert crossSystem == null;
+let inherit (localSystem) system; in
 
 
 [
@@ -58,7 +59,10 @@ assert crossSystem == null;
   })
 
   (prevStage: {
-    inherit system crossSystem platform config overlays;
+    buildPlatform = localSystem;
+    hostPlatform = localSystem;
+    targetPlatform = localSystem;
+    inherit config overlays;
     stdenv = import ../generic {
       name = "stdenv-freebsd-boot-3";
       inherit system config;

pkgs/stdenv/linux/default.nix

@@ -4,21 +4,23 @@
 # compiler and linker that do not search in default locations,
 # ensuring purity of components produced by it.
 { lib
-, system, platform, crossSystem, config, overlays
-
-, bootstrapFiles ?
-    if system == "i686-linux" then import ./bootstrap-files/i686.nix
-    else if system == "x86_64-linux" then import ./bootstrap-files/x86_64.nix
-    else if system == "armv5tel-linux" then import ./bootstrap-files/armv5tel.nix
-    else if system == "armv6l-linux" then import ./bootstrap-files/armv6l.nix
-    else if system == "armv7l-linux" then import ./bootstrap-files/armv7l.nix
-    else if system == "mips64el-linux" then import ./bootstrap-files/loongson2f.nix
-    else abort "unsupported platform for the pure Linux stdenv"
+, localSystem, crossSystem, config, overlays
+
+, bootstrapFiles ? { # switch
+    "i686-linux" = import ./bootstrap-files/i686.nix;
+    "x86_64-linux" = import ./bootstrap-files/x86_64.nix;
+    "armv5tel-linux" = import ./bootstrap-files/armv5tel.nix;
+    "armv6l-linux" = import ./bootstrap-files/armv6l.nix;
+    "armv7l-linux" = import ./bootstrap-files/armv7l.nix;
+    "mips64el-linux" = import ./bootstrap-files/loongson2f.nix;
+  }.${localSystem.system}
+    or (abort "unsupported platform for the pure Linux stdenv")
 }:
 
 assert crossSystem == null;
 
 let
+  inherit (localSystem) system platform;
 
   commonPreHook =
     ''
@@ -91,7 +93,10 @@ let
       };
 
     in {
-      inherit system platform crossSystem config overlays;
+      buildPlatform = localSystem;
+      hostPlatform = localSystem;
+      targetPlatform = localSystem;
+      inherit config overlays;
       stdenv = thisStdenv;
     };
 
@@ -246,7 +251,10 @@ in
   # dependency (`nix-store -qR') on bootstrapTools or the first
   # binutils built.
   (prevStage: {
-    inherit system crossSystem platform config overlays;
+    buildPlatform = localSystem;
+    hostPlatform = localSystem;
+    targetPlatform = localSystem;
+    inherit config overlays;
     stdenv = import ../generic rec {
       inherit system config;
 

pkgs/stdenv/native/default.nix

@@ -1,10 +1,11 @@
 { lib
-, system, platform, crossSystem, config, overlays
+, localSystem, crossSystem, config, overlays
 }:
 
 assert crossSystem == null;
 
 let
+  inherit (localSystem) system platform;
 
   shell =
     if system == "i686-freebsd" || system == "x86_64-freebsd" then "/usr/local/bin/bash"
@@ -134,7 +135,10 @@ in
 
   # First build a stdenv based only on tools outside the store.
   (prevStage: {
-    inherit system crossSystem platform config overlays;
+    buildPlatform = localSystem;
+    hostPlatform = localSystem;
+    targetPlatform = localSystem;
+    inherit config overlays;
     stdenv = makeStdenv {
       inherit (prevStage) cc fetchurl;
     } // { inherit (prevStage) fetchurl; };
@@ -143,7 +147,10 @@ in
   # Using that, build a stdenv that adds the ‘xz’ command (which most systems
   # don't have, so we mustn't rely on the native environment providing it).
   (prevStage: {
-    inherit system crossSystem platform config overlays;
+    buildPlatform = localSystem;
+    hostPlatform = localSystem;
+    targetPlatform = localSystem;
+    inherit config overlays;
     stdenv = makeStdenv {
       inherit (prevStage.stdenv) cc fetchurl;
       extraPath = [ prevStage.xz ];

pkgs/stdenv/nix/default.nix

@@ -9,9 +9,9 @@ assert crossSystem == null;
 bootStages ++ [
   (prevStage: let
     inherit (prevStage) stdenv;
-    inherit (stdenv) system platform;
   in {
-    inherit system platform crossSystem config;
+    inherit (prevStage) buildPlatform hostPlatform targetPlatform;
+    inherit config overlays;
 
     stdenv = import ../generic rec {
       inherit config;

pkgs/top-level/default.nix

@@ -83,7 +83,8 @@ in let
   boot = import ../stdenv/booter.nix { inherit lib allPackages; };
 
   stages = stdenvStages {
-    inherit lib system platform crossSystem config overlays;
+    localSystem = { inherit system platform; };
+    inherit lib crossSystem config overlays;
   };
 
   pkgs = boot stages;

pkgs/top-level/stage.nix

@@ -9,10 +9,43 @@
    import `pkgs/default.nix` or `default.nix`. */
 
 
-{ # The system (e.g., `i686-linux') for which to build the packages.
-  system
-
-, # the package set used at build-time
+{ ## Misc parameters kept the same for all stages
+  ##
+
+  # Utility functions, could just import but passing in for efficiency
+  lib
+
+, # Use to reevaluate Nixpkgs; a dirty hack that should be removed
+  nixpkgsFun
+
+  ## Platform parameters
+  ##
+  ## The "build" "host" "target" terminology below comes from GNU Autotools. See
+  ## its documentation for more information on what those words mean. Note that
+  ## each should always be defined, even when not cross compiling.
+  ##
+  ## For purposes of bootstrapping, think of each stage as a "sliding window"
+  ## over a list of platforms. Specifically, the host platform of the previous
+  ## stage becomes the build platform of the current one, and likewise the
+  ## target platform of the previous stage becomes the host platform of the
+  ## current one.
+  ##
+
+, # The platform on which packages are built. Consists of `system`, a
+  # string (e.g.,`i686-linux') identifying the most import attributes of the
+  # build platform, and `platform` a set of other details.
+  buildPlatform
+
+, # The platform on which packages run.
+  hostPlatform
+
+, # The platform which build tools (especially compilers) build for in this stage,
+  targetPlatform
+
+  ## Other parameters
+  ##
+
+, # The package set used at build-time
   buildPackages
 
 , # The standard environment to use for building packages.
@@ -24,21 +57,19 @@
   allowCustomOverrides
 
 , # Non-GNU/Linux OSes are currently "impure" platforms, with their libc
-  # outside of the store.  Thus, GCC, GFortran, & co. must always look for
-  # files in standard system directories (/usr/include, etc.)
-  noSysDirs ? (system != "x86_64-freebsd" && system != "i686-freebsd"
-               && system != "x86_64-solaris"
-               && system != "x86_64-kfreebsd-gnu")
+  # outside of the store.  Thus, GCC, GFortran, & co. must always look for files
+  # in standard system directories (/usr/include, etc.)
+  noSysDirs ? buildPlatform.system != "x86_64-freebsd"
+           && buildPlatform.system != "i686-freebsd"
+           && buildPlatform.system != "x86_64-solaris"
+           && buildPlatform.system != "x86_64-kfreebsd-gnu"
 
 , # The configuration attribute set
   config
 
-, overlays # List of overlays to use in the fix-point.
-
-, crossSystem
-, platform
-, lib
-, nixpkgsFun
+, # A list of overlays (Additional `self: super: { .. }` customization
+  # functions) to be fixed together in the produced package set
+  overlays
 }:
 
 let
@@ -53,10 +84,24 @@ let
     };
 
   stdenvBootstappingAndPlatforms = self: super: {
-    stdenv = stdenv // { inherit platform; };
     buildPackages = buildPackages // { recurseForDerivations = false; };
-    inherit
-      system platform crossSystem;
+    inherit stdenv
+      buildPlatform hostPlatform targetPlatform;
+  };
+
+  # The old identifiers for cross-compiling. These should eventually be removed,
+  # and the packages that rely on them refactored accordingly.
+  platformCompat = self: super: let
+    # TODO(@Ericson2314) this causes infinite recursion
+    #inherit (self) buildPlatform hostPlatform targetPlatform;
+  in {
+    stdenv = super.stdenv // {
+      inherit (buildPlatform) platform;
+    } // lib.optionalAttrs (targetPlatform != buildPlatform) {
+      cross = targetPlatform;
+    };
+    inherit (buildPlatform) system platform;
+    crossSystem = if targetPlatform != buildPlatform then targetPlatform else null;
   };
 
   splice = self: super: import ./splice.nix lib self;
@@ -89,6 +134,7 @@ let
   # The complete chain of package set builders, applied from top to bottom
   toFix = lib.foldl' (lib.flip lib.extends) (self: {}) ([
     stdenvBootstappingAndPlatforms
+    platformCompat
     stdenvAdapters
     trivialBuilders
     splice