Add support for configuring legacyPackages

API_GUIDE.md

@@ -711,6 +711,44 @@ For example:
 }
 ```
 
+### legacyPackages
+
+```
+Type: Pkgs -> Pkgs
+```
+
+The `legacyPackages` option allows you to configure the flake's `legacyPackges`
+output. It can be set to a function that takes the package set and returns the
+package set to be used as the corresponding system's legacyPackages output.
+
+For example:
+
+```nix
+{
+  inputs = {
+    flakelight.url = "github:nix-community/flakelight";
+    nixpkgs.url = "nixpkgs/nixpkgs-unstable";
+  };
+  outputs = { flakelight, nixpkgs, ... }:
+    flakelight ./. {
+      legacyPackages = pkgs: nixpkgs.legacyPackages.${pkgs.system};
+    };
+}
+```
+
+To export the package set used for calling package definitions and other options
+that take functions passed the package set, you can do the following:
+
+```nix
+{
+  inputs.flakelight.url = "github:nix-community/flakelight";
+  outputs = { flakelight, ... }:
+    flakelight ./. {
+      legacyPackages = pkgs: pkgs;
+    };
+}
+```
+
 ### formatter
 
 ```

builtinModules/legacyPackages.nix

@@ -0,0 +1,19 @@
+# flakelight -- Framework for simplifying flake setup
+# Copyright (C) 2023 Archit Gupta <archit@accelbread.com>
+# SPDX-License-Identifier: MIT
+
+{ config, lib, genSystems, ... }:
+let
+  inherit (lib) mkIf mkOption;
+  inherit (lib.types) functionTo nullOr pkgs;
+in
+{
+  options.legacyPackages = mkOption {
+    type = nullOr (functionTo pkgs);
+    default = null;
+  };
+
+  config.outputs = mkIf (config.legacyPackages != null) {
+    legacyPackages = genSystems config.legacyPackages;
+  };
+}

tests/default.nix

@@ -287,6 +287,22 @@ in
     })
     (f: (import f.packages.x86_64-linux.pkg2));
 
+  legacyPackages-set-pkgs = test
+    (flakelight ./empty {
+      inputs = { inherit nixpkgs; };
+      legacyPackages = pkgs: pkgs;
+    })
+    (f: f.legacyPackages.x86_64-linux.hello
+      == nixpkgs.legacyPackages.x86_64-linux.hello);
+
+  legacyPackages-set-nixpkgs = test
+    (flakelight ./empty {
+      inputs = { inherit nixpkgs; };
+      legacyPackages = pkgs: nixpkgs.legacyPackages.${pkgs.system};
+    })
+    (f: f.legacyPackages.x86_64-linux.hello
+      == nixpkgs.legacyPackages.x86_64-linux.hello);
+
   devShell = test
     (flakelight ./empty {
       devShell = {