top-level: Simplify impure and pure fallback

This is now possible, since the `platform` attribute has been removed in
PR #107214. I've been waiting to do a cleanup like this for a long time!

2 files changed, 24 insertions, 34 deletions

diff --git a/pkgs/top-level/default.nix b/pkgs/top-level/default.nix
index dfa68ba31b84..10cf36d4d13e 100644
--- a/pkgs/top-level/default.nix
+++ b/pkgs/top-level/default.nix
@@ -1,9 +1,8 @@
 /* This function composes the Nix Packages collection. It:
 
-     1. Applies the final stage to the given `config` if it is a function
+     1. Elaborates `localSystem` and `crossSystem` with defaults as needed.
 
-     2. Infers an appropriate `platform` based on the `system` if none is
-        provided
+     2. Applies the final stage to the given `config` if it is a function
 
      3. Defaults to no non-standard config and no cross-compilation target
 
@@ -50,6 +49,14 @@ let # Rename the function arguments
 in let
   lib = import ../../lib;
 
+  localSystem = lib.systems.elaborate args.localSystem;
+
+  # Condition preserves sharing which in turn affects equality.
+  crossSystem =
+    if crossSystem0 == null || crossSystem0 == args.localSystem
+    then localSystem
+    else lib.systems.elaborate crossSystem0;
+
   # Allow both:
   # { /* the config */ } and
   # { pkgs, ... } : { /* the config */ }
@@ -58,17 +65,6 @@ in let
     then config0 { inherit pkgs; }
     else config0;
 
-  # From a minimum of `system` or `config` (actually a target triple, *not*
-  # nixpkgs configuration), infer the other one and platform as needed.
-  localSystem = lib.systems.elaborate (if builtins.isAttrs args.localSystem then (
-    # Allow setting the platform in the config file. This take precedence over
-    # the inferred platform, but not over an explicitly passed-in one.
-    builtins.intersectAttrs { platform = null; } config1
-    // args.localSystem) else args.localSystem);
-
-  crossSystem = if crossSystem0 == null then localSystem
-                else lib.systems.elaborate crossSystem0;
-
   configEval = lib.evalModules {
     modules = [
       ./config.nix
diff --git a/pkgs/top-level/impure.nix b/pkgs/top-level/impure.nix
index 3ba6c08a140b..6f7383c8e7a0 100644
--- a/pkgs/top-level/impure.nix
+++ b/pkgs/top-level/impure.nix
@@ -12,17 +12,15 @@ let
 
 in
 
-{ # We combine legacy `system` and `platform` into `localSystem`, if
-  # `localSystem` was not passed. Strictly speaking, this is pure desugar, but
-  # it is most convient to do so before the impure `localSystem.system` default,
-  # so we do it now.
-  localSystem ? builtins.intersectAttrs { system = null; platform = null; } args
+{ # We put legacy `system` into `localSystem`, if `localSystem` was not passed.
+  # If neither is passed, assume we are building packages on the current
+  # (build, in GNU Autotools parlance) platform.
+  localSystem ? { system = args.system or builtins.currentSystem; }
 
-, # These are needed only because nix's `--arg` command-line logic doesn't work
-  # with unnamed parameters allowed by ...
-  system ? localSystem.system
-, platform ? localSystem.platform
-, crossSystem ? null
+# These are needed only because nix's `--arg` command-line logic doesn't work
+# with unnamed parameters allowed by ...
+, system ? localSystem.system
+, crossSystem ? localSystem
 
 , # Fallback: The contents of the configuration file found at $NIXPKGS_CONFIG or
   # $HOME/.config/nixpkgs/config.nix.
@@ -77,15 +75,11 @@ in
 , ...
 } @ args:
 
-# If `localSystem` was explicitly passed, legacy `system` and `platform` should
-# not be passed.
-assert args ? localSystem -> !(args ? system || args ? platform);
+# If `localSystem` was explicitly passed, legacy `system` should
+# not be passed, and vice-versa.
+assert args ? localSystem -> !(args ? system);
+assert args ? system -> !(args ? localSystem);
 
-import ./. (builtins.removeAttrs args [ "system" "platform" ] // {
-  inherit config overlays crossSystem crossOverlays;
-  # Fallback: Assume we are building packages on the current (build, in GNU
-  # Autotools parlance) system.
-  localSystem = if builtins.isString localSystem then localSystem
-                else (if args ? localSystem then {}
-                      else { system = builtins.currentSystem; }) // localSystem;
+import ./. (builtins.removeAttrs args [ "system" ] // {
+  inherit config overlays localSystem;
 })