recent nix package definitions lack a meta.mainProgram attribute

[mcsimw]

Describe the bug

When using the latest versions of Nix from git master branch through the nixpkgs overlay, a warning is displayed during nixos-rebuild evaluations and nix flake check.

This is what my last rebuild looked like

building the system configuration...
evaluation warning: getExe: Package "nix-2.26.0pre20250108_5230d3e" does not have the meta.mainProgram attribute. We'll assume that the main program has the same name for now, but this behavior is deprecated, because it leads to surprising errors when the assumption does not hold. If the package has a main program, please set `meta.mainProgram` in its definition to make this warning go away. Otherwise, if the package does not have a main program, or if you don't control its definition, use getExe' to specify the name to the program, such as lib.getExe' foo "bar".
stopping the following units: systemd-modules-load.service
activating the configuration...
setting up /etc...
reloading user units for mcsimw...
restarting sysinit-reactivation.target
reloading the following units: dbus-broker.service
restarting the following units: systemd-networkd.service, systemd-resolved.service, systemd-udevd.service
starting the following units: systemd-modules-load.service

Steps To Reproduce

I add nix to my flake inputs

inputs = {
  nix.url = "github:nixos/nix";
  nixpkgs.url = "github:nixos/nixpkgs";
  flake-parts = {
     url = "github:hercules-ci/flake-parts";
     inputs.nixpkgs-lib.follows = "nixpkgs";
  };
};

I ensured inputs was passed into my system configuration and added the overlay

outputs =
  inputs:
  inputs.flake-parts.lib.mkFlake { inherit inputs; } {
    flake.nixosConfigurations.compoooter = inputs.nixpkgs.lib.nixosSystem {
      specialArgs = { inherit inputs; };
      modules = [
        ./configuration.nix
        {
          nixpkgs.overlays = [ inputs.nix.overlays.default ];
        }
      ];
    };
    systems = [
      "x86_64-linux"
    ];
  };

Expected behavior

It's unclear to me why the meta.mainProgram attribute was removed recently, as this evaluation warning has existed for some time. There may be a valid reason for its removal, but I wanted to report it to ensure it is addressed if unintended.

Metadata

nix-env (Nix) 2.26.0pre

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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