libkookie: update base.nix shell to allow for empty userPackages

infra/libkookie/shells/README.md

@@ -1,4 +1,4 @@
-# nix builders
+# Development shells
 
 To make development on NixOS slightly less excruciating, libkookie
 ships with a set of development shells.  These are environments that
@@ -12,18 +12,26 @@ other `shell.nix` files.
 
 ## How to use
 
-Create a `shell.nix` for a project that you want to include.  The tool
-`msh` (Make SHell) is available to generate these based on common
-defaults.  Afterwards both `lorri` and `nix-shell` will be able to
-pick up the required environment for development.
+Create a `shell.nix` for a project that you want to include (and
+optionally `.envrc` for lorri).  The tool `msh` (Make SHell) is
+available to generate these based on common defaults.  Afterwards both
+`lorri` and `nix-shell` will be able to pick up the required
+environment for development.
 
 ```nix
-{...}: import <shells/rust> {}
+{...}: import <shells/rust/stable> {}
 ```
 
 The above snippet includes the basic Rust development toolchain.  You
 can also provide a set of additional packages to install
 
 ```nix
-{ pkgs, ... }: import <shells/rust/stable> { packages = [ pkgs.hello ]; }
+import <shells/rust/stable> { 
+    userPackages = (pkgs: [ pkgs.hello ]);
+}
 ```
+
+You can also include `<shells/rust/nightly>` which will get the latest
+nightly compiler with a complete toolchain.  This feature is
+implemented via the
+[oxalica/rust-overlay](https://github.com/oxalica/rust-overlay/).

infra/libkookie/shells/base.nix

@@ -1,4 +1,4 @@
-name: mkBuildInputs: { userPackages }:
+name: mkBuildInputs: { userPackages ? _: [] }:
 with import <nixpkgs> {};
 stdenv.mkDerivation {
   inherit name;