Mark treefmt-nix as optional by default

Closes #3

flake-dev.lock

@@ -0,0 +1,48 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1722630782,
+        "narHash": "sha256-hMyG9/WlUi0Ho9VkRrrez7SeNlDzLxalm9FwY7n/Noo=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "d04953086551086b44b6f3c6b7eeb26294f207da",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs",
+        "treefmt-nix": "treefmt-nix"
+      }
+    },
+    "treefmt-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1722330636,
+        "narHash": "sha256-uru7JzOa33YlSRwf9sfXpJG+UAV+bnBEYMjrzKrQZFw=",
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "rev": "768acdb06968e53aa1ee8de207fd955335c754b7",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "treefmt-nix",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake-update.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+
+set -e
+set -x
+
+nix flake update \
+  --reference-lock-file flake.lock \
+  --output-lock-file flake.lock
+
+nix flake update \
+  --reference-lock-file flake-dev.lock \
+  --override-input treefmt-nix 'github:numtide/treefmt-nix' \
+  --output-lock-file flake-dev.lock

flake.nix

@@ -6,15 +6,12 @@
     treefmt-nix = {
       url = "github:numtide/treefmt-nix";
       inputs.nixpkgs.follows = "nixpkgs";
+      follows = "";
     };
   };
 
   outputs =
-    {
-      self,
-      nixpkgs,
-      treefmt-nix,
-    }:
+    { self, nixpkgs, ... }:
     let
       systems = [
         "aarch64-darwin"
@@ -23,7 +20,8 @@
         "x86_64-linux"
       ];
       eachSystem = f: nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});
-      treefmtEval = eachSystem (pkgs: treefmt-nix.lib.evalModule pkgs ./treefmt.nix);
+      treefmt-nix = import ./treefmt-nix.nix;
+      treefmtEval = eachSystem (pkgs: treefmt-nix.evalModule pkgs ./treefmt.nix);
       lib = import ./lib.nix;
     in
     {

treefmt-nix.nix

@@ -0,0 +1,10 @@
+let
+  lock = builtins.fromJSON (builtins.readFile ./flake-dev.lock);
+  node = lock.nodes.treefmt-nix.locked;
+  inherit (node) owner repo rev;
+  path = fetchTarball {
+    url = "https://github.com/${owner}/${repo}/archive/${rev}.tar.gz";
+    sha256 = node.narHash;
+  };
+in
+import path