Use mocked builders to get default pkg name faster

Since evaluating pkgs is expensive, this speeds up evaluation of flakes
with `packages.default` by attempting to get the name using a pkgs set
containing only mocked builders that just return the name.

builtinModules/packages.nix

@@ -5,8 +5,8 @@
 { config, lib, inputs, flakelight, genSystems, ... }:
 let
   inherit (builtins) parseDrvName tryEval;
-  inherit (lib) filterAttrs findFirst mapAttrs mapAttrs' mkIf mkMerge
-    mkOption nameValuePair optionalAttrs;
+  inherit (lib) filterAttrs findFirst mapAttrs mapAttrs' mkIf mkMerge mkOption
+    nameValuePair optionalAttrs;
   inherit (lib.types) lazyAttrsOf nullOr str uniq;
   inherit (flakelight) supportedSystem;
   inherit (flakelight.types) overlay packageDef;
@@ -49,12 +49,14 @@ in
           getName = pkg: pkg.pname or (parseDrvName pkg.name).name;
           inherit (prev.stdenv.hostPlatform) system;
           baseNixpkgs = inputs.nixpkgs.legacyPackages.${system};
+          mockPkgs = import ../misc/nameMockedPkgs.nix prev;
 
           defaultPkgName = findFirst (x: (tryEval x).success)
             (throw ("Could not determine the name of the default package; " +
               "please set the `pname` flakelight option to the intended name."))
             [
               (assert config.pname != null; config.pname)
+              (getName (mockPkgs.callPackage config.packages.default { }))
               (getName (baseNixpkgs.callPackage config.packages.default { }))
               (getName (import inputs.nixpkgs {
                 inherit (prev.stdenv.hostPlatform) system;

misc/nameMockedPkgs.nix

@@ -0,0 +1,49 @@
+# flakelight -- Framework for simplifying flake setup
+# Copyright (C) 2023 Archit Gupta <archit@accelbread.com>
+# SPDX-License-Identifier: MIT
+
+# This is a fake pkgs set to enable efficiently extracting a derivation's name
+
+real:
+let
+  inherit (real) lib;
+
+  callPackageWith = autoArgs: fn: args:
+    let
+      f = if lib.isFunction fn then fn else import fn;
+      fargs = lib.functionArgs f;
+      mock = lib.mapAttrs (_: _: { }) (lib.filterAttrs (_: v: !v) fargs);
+    in
+    f (mock // builtins.intersectAttrs fargs autoArgs // args);
+in
+lib.fix (self: {
+  pkgs = self;
+  lib = lib // { inherit callPackageWith; };
+
+  callPackage = callPackageWith self;
+
+  stdenv = real.stdenv // {
+    mkDerivation = args:
+      if lib.isFunction args then lib.fix args else args;
+  };
+
+  runCommandWith = { name, ... }: _: { inherit name; };
+  runCommand = name: _: _: { inherit name; };
+  runCommandLocal = name: _: _: { inherit name; };
+  runCommandCC = name: _: _: { inherit name; };
+  writeTextFile = { name, ... }: { inherit name; };
+  writeText = name: _: { inherit name; };
+  writeTextDir = path: _: { name = builtins.baseNameOf path; };
+  writeScript = name: _: { inherit name; };
+  writeScriptBin = name: _: { inherit name; };
+  writeShellScript = name: _: { inherit name; };
+  writeShellScriptBin = name: _: { inherit name; };
+  writeShellApplication = { name, ... }: { inherit name; };
+  writeCBin = pname: _: { inherit pname; };
+  concatTextFile = { name, ... }: { inherit name; };
+  concatText = name: _: { inherit name; };
+  concatScript = name: _: { inherit name; };
+  symlinkJoin = { name, ... }: { inherit name; };
+  linkFarm = name: _: { inherit name; };
+  linkFarmFromDrvs = name: _: { inherit name; };
+})