Possible to create overlay to add all packages added to nixpkgs master on top of nixpkgs-unstable?

So I’m currently using an overlay to add an attr master-pkgs to my nixpkgs to access things that have been merged to master but haven’t made it to unstable yet

that looks like this:

# flake.nix
  inputs = {
    nixpkgs.url = "nixpkgs/nixos-unstable";
    nixpkgs-master.url = "github:NixOS/nixpkgs/master";

  outputs = { nixpkgs, nixpkgs-master, ... }: let
    pkgs-master-overlay = self: super: {
      master-pkgs = nixpkgs-master.legacyPackages.x86_64-linux;
    in { ... }

which I can then use like pkgs.master-pkgs.vimPlugins.thing-that-just-got-merged.

This is fine and dandy and good enough, but what I want is to create an overlay that will recursively add all attrs that exist in nixpkgs-master but don’t exist in nixpkgs, so that when I’m specifying a package, I can simply type pkgs.vimPlugins.thing-that-just-got-merged rather than the pkgs.master-pkgs version.

I’ve been playing around with this for the last couple of hours but just cant get it right.

If anyone is already doing this or similar, would you mind sharing your config?

If not, is there any obvious way you can think to accomplish it? Perhaps specific library functions I’ve missed that could be helpful. I’ve played around with what feels like most of the functions in lib.attrsets with no clear path forward.