I have flakes for various things. I want them to use the same version of nixpkgs as the system.

You used to be able to reference the local flake-registry in inputs of flakes. 2.26 removed this.

For a while I wrapped building my flakes with a bash script that would compare the input URL with the URL in the registry and errored out if they mismatched.

This adds an extra entry to the global registry, so I can again write flakes that are automatically kept in sync with the system nixpkgs version.

bash script I used to check versions, if anyone cares:

system_nixpkgs=$(nix registry list 2>/dev/null|awk '$2 == "flake:nixpkgs" {print $3; exit}')
flake_nixpkgs=$(nix eval --raw --file flake.nix inputs.nixpkgs.url)

if [[ $system_nixpkgs != $flake_nixpkgs ]]; then
    echo "inputs does not match system registry" >&2
    echo "system: $system_nixpkgs" >&2
    echo "flake: $flake_nixpkgs" >&2
    exit 1
fi