Merge pull request #1536 from zupo/patch-1

Document how to get a single package from nixpkgs-unstable

docs/common-patterns.md

@@ -1,11 +1,29 @@
-## Using `nixpkgs-unstable` instead of `devenv-nixpkgs`
+## Getting a recent version of a package from `nixpkgs-unstable`
 
-### In the file `devenv.yaml` edit the `nixpkgs` input to point to nixpkgs-unstable
+By default, devenv [uses a fork of nixpkgs](https://devenv.sh/blog/2024/03/20/devenv-10-rewrite-in-rust/#devenv-nixpkgs) with additional fixes. This fork can be several months behind `nixpkgs-unstable`. You can still get a more recently updated package from `nixpkgs-unstable` into your devenv.
+
+1. Add `nixpkgs-unstable` input to `devenv.yaml`:
 
 ```yaml
 inputs:
   nixpkgs:
-    url: github:NixOS/nixpkgs/nixpkgs-unstable
+    url: github:cachix/devenv-nixpkgs/rolling
+  nixpkgs-unstable:
+    url: github:nixos/nixpkgs/nixpkgs-unstable
+```
+
+2. Use the package in your `devenv.nix`:
+
+```nix
+{ pkgs, inputs, ... }:
+let
+  pkgs-unstable = import inputs.nixpkgs-unstable { system = pkgs.stdenv.system; };
+in
+{
+  packages = [
+    pkgs-unstable.elmPackages.elm-test-rs
+  ];
+}
 ```
 
 ## Nix patterns