question: why are @args not forwardable?

[gabyx]

Hello,

Great project!

I have simple question I have a shells/custom/default.nix where:

{
  lib,
  pkgs,
  namespace, # if I remove this because unused, the `args` do not contain it anymore?
  ...
}@args:
let
  toolchains = import ../toolchain.nix args; # args is not the full attrset?
in 
{
...
}

I am unsure why I need to name all keys in args in the function arguments, when I want to forward to my function toolchain.nix.
If I remove namespace in {...}@args the below import does not work anymore? Why is that?