nixredo/builder.nix

buildArgs@{ lib ? import ./lib.nix {}, nixpkgs ? <nixpkgs>, pkgs ? import nixpkgs {} }:
let
  inherit (lib) searchPath whichdo d1 d2;

  resolve = path: let
    builder = (whichdo path);
    in if builder == "" then throw "No rule to build ${path}"
       else runBuilder (d1 path builder) (d2 path builder) builder;

  runBuilder = d1: d2: builder:
    with pkgs.lib;
    with builtins;
    let
      # Funktionalise pkgs; our main entry point to resolve default*nix build scripts.
      funcPkgs = { __functor = self: arg: resolve arg; } // pkgs;

      _builder = if (tryEval (let
        # Needs to be done as nix chokes on empty files rn.
        content = readFile builder;
      in assert content != "" && (! (hasPrefix "#!" content)); import builder)).success
                  then import builder
                  else ''
                    #!/bin/sh
                    ${/. + builder} $d1 $d2 $out >$out
                  '';
      __builder = if isFunction _builder then _builder funcPkgs
                  else _builder;

      args = { stdenv = pkgs.stdenvNoCC; } // (if isString __builder
                                                then { builder = __builder; }
                                                else  __builder);

      drv = (args.deriver or args.stdenv.mkDerivation) ({
        name = pkgs.lib.strings.sanitizeDerivationName d1;
        buildCommand = args.builder;
        passAsFile = [ "buildCommand" ];
        preferLocalBuild = true;
        allowSubstitutes = false;
        inherit d1 d2;
      } // (removeAttrs args [ "builder" "system" "deriver" "stdenv" ]));
    in drv; # myself
in resolve