Add descriptive error in case of pkgs error

Author: infinisil

default.nix

@@ -1,7 +1,6 @@
 { nurpkgs ? import <nixpkgs> {} # For nixpkgs dependencies used by NUR itself
   # Dependencies to call NUR repos with
-, pkgs ? throw "NUR call didn't receive a pkgs argument, but the evaluation requires it."
-}:
+, pkgs ? null }:
 
 let
   inherit (nurpkgs) fetchgit fetchzip lib;
@@ -44,7 +43,7 @@ let
       };
 
    createRepo = name: attr: import ./lib/evalRepo.nix {
-     inherit name pkgs lib;
+     inherit name pkgs lib attr;
      src = repoSource name attr + "/" + (attr.file or "");
    };
 

lib/evalRepo.nix

@@ -1,20 +1,35 @@
 { name
+, attr
 , src
 , pkgs # Do not use this for anything other than passing it along as an argument to the repository
 , lib
 }:
 let
 
+  prettyName = "${name}";
+
   # Arguments passed to each repositories default.nix
-  passedArgs = { inherit pkgs lib; };
+  passedArgs = {
+    inherit lib;
+    pkgs = if pkgs != null then pkgs else throw ''
+      NUR import call didn't receive a pkgs argument, but the evaluation of NUR's ${prettyName} repository requires it.
+
+      This is either because
+        - You're trying to use a package from that repository, but didn't pass a `pkgs` argument to the NUR import.
+          In that case, refer to the installation instructions at https://github.com/nix-community/nur#installation on how to properly import NUR
+
+        - You're trying to use a module/overlay from that repository, but it didn't properly declare their module.
+          In that case, inform the maintainer of the repository: ${attr.url}
+    '';
+  };
 
   expr = import src;
   args = builtins.functionArgs expr;
   # True if not all arguments are either passed by default (e.g. pkgs) or defaulted (e.g. foo ? 10)
   usesCallPackage = ! lib.all (arg: lib.elem arg (lib.attrNames passedArgs) || args.${arg}) (lib.attrNames args);
 
 in if usesCallPackage then lib.warn ''
-    NUR repository ${name} is using the deprecated callPackage syntax which
+    NUR repository ${prettyName} is using the deprecated callPackage syntax which
     might result in infinite recursion when used with NixOS modules.
   '' (pkgs.callPackages src {})
   else expr (builtins.intersectAttrs args passedArgs)