# Pastebin TUwQgwTA
    Add primop ‘scopedImport’                                                                           

    ‘scopedImport’ works like ‘import’, except that it takes a set of
    attributes to be added to the lexical scope of the expression,
    essentially extending or overriding the builtin variables.  For
    instance, the expression

      scopedImport { x = 1; } ./foo.nix

    where foo.nix contains ‘x’, will evaluate to 1.

    This has a few applications:

    * It allows getting rid of function argument specifications in package
      expressions. For instance, a package expression like:

        { stdenv, fetchurl, libfoo }:

        stdenv.mkDerivation { ... buildInputs = [ libfoo ]; }

      can now we written as just

        stdenv.mkDerivation { ... buildInputs = [ libfoo ]; }

      and imported in all-packages.nix as:

        bar = scopedImport pkgs ./bar.nix;

      So whereas we once had dependencies listed in three places
      (buildInputs, the function, and the call site), they now only need
      to appear in one place.

    * It allows overriding builtin functions. For instance, to trace all
      calls to ‘map’:

      let
        overrides = {
          map = f: xs: builtins.trace "map called!" (map f xs);

          # Ensure that our override gets propagated by calls to
          # import/scopedImport.
          import = fn: scopedImport overrides fn;

          scopedImport = attrs: fn: scopedImport (overrides // attrs) fn;

          # Also update ‘builtins’.
          builtins = builtins // overrides;
        };
      in scopedImport overrides ./bla.nix

    * Similarly, it allows extending the set of builtin functions. For
      instance, during Nixpkgs/NixOS evaluation, the Nixpkgs library
      functions could be added to the default scope.

    There is a downside: calls to scopedImport are not memoized, unlike
    import. So importing a file multiple times leads to multiple parsings
    / evaluations. It would be possible to construct the AST only once,
    but that would require careful handling of variables/environments.