config.structuredAttrsByDefault: add option

pkgs/stdenv/generic/make-derivation.nix

@@ -156,7 +156,7 @@ let
 
 # Experimental.  For simple packages mostly just works,
 # but for anything complex, be prepared to debug if enabling.
-, __structuredAttrs ? false
+, __structuredAttrs ? config.structuredAttrsByDefault or false
 
 , env ? { }
 

pkgs/top-level/config.nix

@@ -47,6 +47,10 @@ let
       feature = "set `strictDeps` to true by default";
     };
 
+    structuredAttrsByDefault = mkMassRebuild {
+      feature = "set `__structuredAttrs` to true by default";
+    };
+
     enableParallelBuildingByDefault = mkMassRebuild {
       feature = "set `enableParallelBuilding` to true by default";
     };