nixpkgs/nixos/modules/config/nix-flakes.nix

/*
  Manages the flake registry.

  See also
   - ./nix.nix
   - ./nix-channel.nix
 */
{ config, lib, ... }:
let
  inherit (lib)
    filterAttrs
    literalExpression
    mapAttrsToList
    mkDefault
    mkIf
    mkOption
    types
    ;

  cfg = config.nix;

in
{
  options = {
    nix = {
      registry = mkOption {
        type = types.attrsOf (types.submodule (
          let
            referenceAttrs = with types; attrsOf (oneOf [
              str
              int
              bool
              path
              package
            ]);
          in
          { config, name, ... }:
          {
            options = {
              from = mkOption {
                type = referenceAttrs;
                example = { type = "indirect"; id = "nixpkgs"; };
                description = lib.mdDoc "The flake reference to be rewritten.";
              };
              to = mkOption {
                type = referenceAttrs;
                example = { type = "github"; owner = "my-org"; repo = "my-nixpkgs"; };
                description = lib.mdDoc "The flake reference {option}`from` is rewritten to.";
              };
              flake = mkOption {
                type = types.nullOr types.attrs;
                default = null;
                example = literalExpression "nixpkgs";
                description = lib.mdDoc ''
                  The flake input {option}`from` is rewritten to.
                '';
              };
              exact = mkOption {
                type = types.bool;
                default = true;
                description = lib.mdDoc ''
                  Whether the {option}`from` reference needs to match exactly. If set,
                  a {option}`from` reference like `nixpkgs` does not
                  match with a reference like `nixpkgs/nixos-20.03`.
                '';
              };
            };
            config = {
              from = mkDefault { type = "indirect"; id = name; };
              to = mkIf (config.flake != null) (mkDefault (
                {
                  type = "path";
                  path = config.flake.outPath;
                } // filterAttrs
                  (n: _: n == "lastModified" || n == "rev" || n == "revCount" || n == "narHash")
                  config.flake
              ));
            };
          }
        ));
        default = { };
        description = lib.mdDoc ''
          A system-wide flake registry.
        '';
      };
    };
  };

  config = mkIf cfg.enable {
    environment.etc."nix/registry.json".text = builtins.toJSON {
      version = 2;
      flakes = mapAttrsToList (n: v: { inherit (v) from to exact; }) cfg.registry;
    };
  };
}