lib/systems: elaborate properly with non-matching system / config / parsed args

lib/systems/default.nix

@@ -6,9 +6,9 @@ let
     filterAttrs
     foldl
     hasInfix
+    isAttrs
     isFunction
     isList
-    isString
     mapAttrs
     optional
     optionalAttrs
@@ -55,24 +55,34 @@ let
   */
   flakeExposed = import ./flake-systems.nix { };
 
+  # Turn localSystem or crossSystem, which could be system-string or attrset, into
+  # attrset.
+  systemToAttrs = systemOrArgs:
+    if isAttrs systemOrArgs then systemOrArgs else { system = systemOrArgs; };
+
   # Elaborate a `localSystem` or `crossSystem` so that it contains everything
   # necessary.
   #
   # `parsed` is inferred from args, both because there are two options with one
   # clearly preferred, and to prevent cycles. A simpler fixed point where the RHS
   # always just used `final.*` would fail on both counts.
-  elaborate = args': let
-    args = if isString args' then { system = args'; }
-           else args';
+  elaborate = systemOrArgs: let
+    allArgs = systemToAttrs systemOrArgs;
+
+    # Those two will always be derived from "config", if given, so they should NOT
+    # be overridden further down with "// args".
+    args = builtins.removeAttrs allArgs [ "parsed" "system" ];
 
     # TODO: deprecate args.rustc in favour of args.rust after 23.05 is EOL.
     rust = args.rust or args.rustc or {};
 
     final = {
       # Prefer to parse `config` as it is strictly more informative.
-      parsed = parse.mkSystemFromString (if args ? config then args.config else args.system);
-      # Either of these can be losslessly-extracted from `parsed` iff parsing succeeds.
+      parsed = parse.mkSystemFromString (args.config or allArgs.system);
+      # This can be losslessly-extracted from `parsed` iff parsing succeeds.
       system = parse.doubleFromSystem final.parsed;
+      # TODO: This currently can't be losslessly-extracted from `parsed`, for example
+      # because of -mingw32.
       config = parse.tripleFromSystem final.parsed;
       # Determine whether we can execute binaries built for the provided platform.
       canExecute = platform:
@@ -435,5 +445,6 @@ in
     inspect
     parse
     platforms
+    systemToAttrs
     ;
 }

lib/tests/systems.nix

@@ -78,6 +78,18 @@ lib.runTests (
     expr = toLosslessStringMaybe (lib.systems.elaborate "x86_64-linux" // { something = "extra"; });
     expected = null;
   };
+  test_elaborate_config_over_system = {
+    expr = (lib.systems.elaborate { config = "i686-unknown-linux-gnu"; system = "x86_64-linux"; }).system;
+    expected = "i686-linux";
+  };
+  test_elaborate_config_over_parsed = {
+    expr = (lib.systems.elaborate { config = "i686-unknown-linux-gnu"; parsed = (lib.systems.elaborate "x86_64-linux").parsed; }).parsed.cpu.arch;
+    expected = "i686";
+  };
+  test_elaborate_system_over_parsed = {
+    expr = (lib.systems.elaborate { system = "i686-linux"; parsed = (lib.systems.elaborate "x86_64-linux").parsed; }).parsed.cpu.arch;
+    expected = "i686";
+  };
 }
 
 # Generate test cases to assert that a change in any non-function attribute makes a platform unequal

pkgs/top-level/stage.nix

@@ -246,7 +246,7 @@ let
       })] ++ overlays;
       ${if stdenv.hostPlatform == stdenv.buildPlatform
         then "localSystem" else "crossSystem"} = {
-        parsed = makeMuslParsedPlatform stdenv.hostPlatform.parsed;
+        config = lib.systems.parse.tripleFromSystem (makeMuslParsedPlatform stdenv.hostPlatform.parsed);
       };
     } else throw "Musl libc only supports 64-bit Linux systems.";
 
@@ -258,9 +258,9 @@ let
       })] ++ overlays;
       ${if stdenv.hostPlatform == stdenv.buildPlatform
         then "localSystem" else "crossSystem"} = {
-        parsed = stdenv.hostPlatform.parsed // {
+        config = lib.systems.parse.tripleFromSystem (stdenv.hostPlatform.parsed // {
           cpu = lib.systems.parse.cpuTypes.i686;
-        };
+        });
       };
     } else throw "i686 Linux package set can only be used with the x86 family.";
 
@@ -270,9 +270,9 @@ let
         pkgsx86_64Darwin = super';
       })] ++ overlays;
       localSystem = {
-        parsed = stdenv.hostPlatform.parsed // {
+        config = lib.systems.parse.tripleFromSystem (stdenv.hostPlatform.parsed // {
           cpu = lib.systems.parse.cpuTypes.x86_64;
-        };
+        });
       };
     } else throw "x86_64 Darwin package set can only be used on Darwin systems.";
 
@@ -311,10 +311,11 @@ let
       })] ++ overlays;
       crossSystem = {
         isStatic = true;
-        parsed =
+        config = lib.systems.parse.tripleFromSystem (
           if stdenv.hostPlatform.isLinux
           then makeMuslParsedPlatform stdenv.hostPlatform.parsed
-          else stdenv.hostPlatform.parsed;
+          else stdenv.hostPlatform.parsed
+        );
         gcc = lib.optionalAttrs (stdenv.hostPlatform.system == "powerpc64-linux") { abi = "elfv2"; } //
           stdenv.hostPlatform.gcc or {};
       };