languages/nix: change nixpkgs-fmt to nixfmt (rfc-style)

docs/release-notes/rl-0.7.md

@@ -72,6 +72,15 @@ one type, you can simply change
 - `vim.autocomplete.*` -> `vim.autocomplete.nvim-cmp.*`
 - `vim.autopairs.enable` -> `vim.autopairs.nvim-autopairs.enable`
 
+### `nixpkgs-fmt` removed in favor of `nixfmt` {#sec-nixpkgs-fmt-deprecation}
+
+`nixpkgs-fmt` has been archived for a while, and it's finally being removed in
+favor of nixfmt (more information can be found
+[here](https://github.com/nix-community/nixpkgs-fmt?tab=readme-ov-file#nixpkgs-fmt---nix-code-formatter-for-nixpkgs).
+
+To migrate to `nixfmt`, simply change `vim.languages.nix.format.type` to
+`nixfmt`.
+
 ## Changelog {#sec-release-0.7-changelog}
 
 [ItsSorae](https://github.com/ItsSorae):
@@ -289,3 +298,4 @@ one type, you can simply change
 [nezia1](https://github.com/nezia1):
 
 - Add [biome](https://github.com/biomejs/biome) support for Typescript, CSS and Svelte. Enable them via [](#opt-vim.languages.ts.format.type), [](#opt-vim.languages.css.format.type) and [](#opt-vim.languages.svelte.format.type) respectively.
+- Replace [nixpkgs-fmt](https://github.com/nix-community/nixpkgs-fmt) with [nixfmt](https://github.com/NixOS/nixfmt) (nixfmt-rfc-style).

modules/plugins/languages/nix.nix

@@ -5,6 +5,7 @@
   ...
 }: let
   inherit (builtins) attrNames;
+  inherit (lib) concatStringsSep;
   inherit (lib.options) mkEnableOption mkOption;
   inherit (lib.modules) mkIf mkMerge;
   inherit (lib.lists) isList;
@@ -62,10 +63,10 @@
                 command = {"${cfg.format.package}/bin/alejandra", "--quiet"},
               },
             ''}
-          ${optionalString (cfg.format.type == "nixpkgs-fmt")
+          ${optionalString (cfg.format.type == "nixfmt")
             ''
               formatting = {
-                command = {"${cfg.format.package}/bin/nixpkgs-fmt"},
+                command = {"${cfg.format.package}/bin/nixfmt"},
               },
             ''}
             },
@@ -90,10 +91,19 @@
       '';
     };
 
-    nixpkgs-fmt = {
-      package = pkgs.nixpkgs-fmt;
-      # Never need to use null-ls for nixpkgs-fmt
+    nixfmt = {
+      package = pkgs.nixfmt-rfc-style;
+      nullConfig = ''
+        table.insert(
+          ls_sources,
+          null_ls.builtins.formatting.nixfmt.with({
+            command = "${cfg.format.package}/bin/nixfmt"
+          })
+        )
+      '';
     };
+
+    nixpkgs-fmt = null; # removed
   };
 
   defaultDiagnosticsProvider = ["statix" "deadnix"];
@@ -175,6 +185,12 @@ in {
 
   config = mkIf cfg.enable (mkMerge [
     {
+      assertions = [
+        {
+          assertion = cfg.format.type != "nixpkgs-fmt";
+          message = "nixpkgs-fmt has been archived upstream. Please use one of the following instead: ${concatStringsSep ", " (attrNames formats)}";
+        }
+      ];
       vim.pluginRC.nix = ''
         vim.api.nvim_create_autocmd("FileType", {
           pattern = "nix",