Use flake compat to provide the nix flakes dev shell to non flake users (#2926)

* Use flake compat to provide the nix flakes dev shell to non flake users

* Move shell.nix to .nix sub folder

.nix/flake.lock

@@ -1,5 +1,19 @@
 {
   "nodes": {
+    "flake-compat": {
+      "locked": {
+        "lastModified": 1733328505,
+        "narHash": "sha256-NeCCThCEP3eCl2l/+27kNNK7QrwZB1IJCrXfrbv5oqU=",
+        "rev": "ff81ac966bb2cae68946d5ed5fc4994f96d0ffec",
+        "revCount": 69,
+        "type": "tarball",
+        "url": "https://api.flakehub.com/f/pinned/edolstra/flake-compat/1.1.0/01948eb7-9cba-704f-bbf3-3fa956735b52/source.tar.gz"
+      },
+      "original": {
+        "type": "tarball",
+        "url": "https://flakehub.com/f/edolstra/flake-compat/1.tar.gz"
+      }
+    },
     "flake-utils": {
       "inputs": {
         "systems": "systems"
@@ -52,6 +66,7 @@
     },
     "root": {
       "inputs": {
+        "flake-compat": "flake-compat",
         "flake-utils": "flake-utils",
         "nixpkgs": "nixpkgs",
         "nixpkgs-unstable": "nixpkgs-unstable",

.nix/flake.nix

@@ -24,6 +24,9 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
     flake-utils.url = "github:numtide/flake-utils";
+
+    # This is used to provide a identical development shell at `shell.nix` for users that do not use flakes
+    flake-compat.url = "https://flakehub.com/f/edolstra/flake-compat/1.tar.gz";
   };
 
   outputs = { nixpkgs, nixpkgs-unstable, rust-overlay, flake-utils, ... }:

.nix/shell.nix

@@ -0,0 +1,31 @@
+# This is a helper file for people using NixOS as their operating system.
+# If you don't know what this file does, you can safely ignore it.
+
+# If you are using Nix as your package manager, you can run 'nix-shell .nix'
+# in the root directory of the project and Nix will open a bash shell
+# with all the packages needed to build and run Graphite installed.
+# A shell.nix file is used in the Nix ecosystem to define a development
+# environment with specific dependencies. When you enter a Nix shell using
+# this file, it ensures that all the specified tools and libraries are
+# available regardless of the host system's configuration. This provides
+# a reproducible development environment across different machines and developers.
+
+# You can enter the Nix shell and run Graphite like normal with:
+# > npm start
+# Or you can run it like this without needing to first enter the Nix shell:
+# > nix-shell .nix --command "npm start"
+
+# Uses flake compat to provide a development shell that is identical to the one defined in the flake
+(import
+  (
+    let
+      lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+      nodeName = lock.nodes.root.inputs.flake-compat;
+    in
+    fetchTarball {
+      url = lock.nodes.${nodeName}.locked.url;
+      sha256 = lock.nodes.${nodeName}.locked.narHash;
+    }
+  )
+  { src = ./.; }
+).shellNix