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

doc/manual/change-authors.yml

@@ -147,3 +147,6 @@ winter:
 
 yshui:
   github: yshui
+
+zimbatm:
+  github: zimbatm

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 "$@"
+        '');
+    };
+}
+```

src/nix/fmt.cc

@@ -39,14 +39,8 @@ struct CmdFmt : SourceExprCommand {
         Strings programArgs{app.program};
 
         // Propagate arguments from the CLI
-        if (args.empty()) {
-            // Format the current flake out of the box
-            programArgs.push_back(".");
-        } else {
-            // User wants more power, let them decide which paths to include/exclude
-            for (auto &i : args) {
-                programArgs.push_back(i);
-            }
+        for (auto &i : args) {
+            programArgs.push_back(i);
         }
 
         runProgramInStore(store, UseSearchPath::DontUse, app.program, programArgs);

tests/functional/fmt.sh

@@ -26,7 +26,10 @@ cat << EOF > flake.nix
   };
 }
 EOF
-nix fmt ./file ./folder | grep 'Formatting: ./file ./folder'
+# No arguments check
+[[ "$(nix fmt)" = "Formatting(0):" ]]
+# Argument forwarding check
+nix fmt ./file ./folder | grep 'Formatting(2): ./file ./folder'
 nix flake check
 nix flake show | grep -P "package 'formatter'"
 

tests/functional/fmt.simple.sh

@@ -1,3 +1,3 @@
 #!/usr/bin/env bash
 
-echo Formatting: "${@}"
+echo "Formatting(${#}):" "${@}"