stdenv: refactor (no change in semantics)

This just moves some expressions around in preparation to further changes.

pkgs/stdenv/generic/default.nix

@@ -290,15 +290,15 @@ let
         in [ nativeBuildInputs buildInputs ];
 
       propagatedDependencies' = map lib.chooseDevOutputs propagatedDependencies;
-    in
 
       # Throw an error if trying to evaluate an non-valid derivation
-      assert let v = checkValidity attrs;
+      validityCondition =
+			 let v = checkValidity attrs;
              in if !v.valid
                then throwEvalHelp (removeAttrs v ["valid"])
                else true;
 
-      lib.addPassthru (derivation (
+      derivationArg =
         (removeAttrs attrs
           ["meta" "passthru" "crossAttrs" "pos"
            "__impureHostDeps" "__propagatedImpureHostDeps"
@@ -343,9 +343,8 @@ let
           __propagatedImpureHostDeps = computedPropagatedImpureHostDeps ++ __propagatedImpureHostDeps;
         } // (if outputs' != [ "out" ] then {
           outputs = outputs';
-        } else { })))) (
-      {
-        overrideAttrs = f: mkDerivation (attrs // (f attrs));
+        } else { }));
+
       # The meta attribute is passed in the resulting attribute set,
       # but it's not part of the actual derivation, i.e., it's not
       # passed to the builder and is not a dependency.  But since we
@@ -363,12 +362,20 @@ let
             hasOutput = out: builtins.elem out outs;
           in [( lib.findFirst hasOutput null (["bin" "out"] ++ outs) )];
         }
-          // meta
+        // attrs.meta or {}
           # Fill `meta.position` to identify the source location of the package.
         // lib.optionalAttrs (pos' != null)
           { position = pos'.file + ":" + toString pos'.line; }
         ;
-        inherit passthru;
+
+    in
+
+      assert validityCondition;
+
+      lib.addPassthru (derivation derivationArg) (
+        {
+          overrideAttrs = f: mkDerivation (attrs // (f attrs));
+          inherit meta passthru;
         } //
         # Pass through extra attributes that are not inputs, but
         # should be made available to Nix expressions using the