fix(nix fmt): remove the default "." argument

When `nix fmt` is called without an argument, Nix appends the "." argument before calling the formatter. The comment in the code is:
> Format the current flake out of the box

This also happens when formatting sub-folders.

This means that the formatter is now unable to distinguish, as an interface, whether the "." argument is coming from the flake or the user's intent to format the current folder. This decision should be up to the formatter.

Treefmt, for example, will automatically look up the project's root and format all the files. This is the desired behaviour. But because the "." argument is passed, it cannot function as expected.

Upstream-PR: https://github.com/nixos/nix/pull/11438

Change-Id: I60fb6b3ed4ec1b24f81b5f0d76c0be98470817ce

1 files changed, 38 insertions, 0 deletions

diff --git a/doc/manual/rl-next/nix-fmt-default-argument.md b/doc/manual/rl-next/nix-fmt-default-argument.md
new file mode 100644
index 000000000..41b8f85bd
--- /dev/null
+++ b/doc/manual/rl-next/nix-fmt-default-argument.md
@@ -0,0 +1,38 @@
+---
+synopsis: Removing the `.` default argument passed to the `nix fmt` formatter
+issues: []
+prs: [11438]
+cls: [1902]
+category: Breaking Changes
+credits: zimbatm
+---
+
+The underlying formatter no longer receives the ". " default argument when `nix fmt` is called with no arguments.
+
+This change was necessary as the formatter wasn't able to distinguish between
+a user wanting to format the current folder with `nix fmt .` or the generic
+`nix fmt`.
+
+The default behaviour is now the responsibility of the formatter itself, and
+allows tools such as treefmt to format the whole tree instead of only the
+current directory and below.
+
+This may cause issues with some formatters: nixfmt, nixpkgs-fmt and alejandra currently format stdin when no arguments are passed.
+
+Here is a small wrapper example that will restore the previous behaviour for such a formatter:
+
+```nix
+{
+  outputs = { self, nixpkgs, systems }:
+    let
+      eachSystem = nixpkgs.lib.genAttrs (import systems) (system: nixpkgs.legacyPackages.${system});
+    in
+    {
+      formatter = eachSystem (pkgs:
+        pkgs.writeShellScriptBin "formatter" ''
+          if [[ $# = 0 ]]; set -- .; fi
+          exec "${pkgs.nixfmt-rfc-style}/bin/nixfmt "$@"
+        '');
+    };
+}
+```