nixfmt converts symbolic links to a copy

[badcold]

I've tried to reformat my machine's configuration.nix, which was a symbolic link and it made a copy of the file at the link's location.

[badcold@i7-3770k:~/demo]$ nixfmt --version
nixfmt nixpkgs-unstable-2025-04-04

[badcold@i7-3770k:~/demo]$ ls -la
total 16
drwxr-xr-x  4 badcold users 4096 Jun  9 11:11 .
drwx------ 72 badcold users 4096 Jun  9 11:10 ..
drwxr-xr-x  7 badcold users 4096 Jun  9 11:11 .git
drwxr-xr-x  2 badcold users 4096 Jun  9 11:10 elsewhere
lrwxrwxrwx  1 badcold users   18 Jun  9 11:11 test.nix -> elsewhere/test.nix

[badcold@i7-3770k:~/demo]$ nixfmt test.nix

[badcold@i7-3770k:~/demo]$ ls -la
total 20
drwxr-xr-x  4 badcold users 4096 Jun  9 11:12 .
drwx------ 72 badcold users 4096 Jun  9 11:10 ..
drwxr-xr-x  7 badcold users 4096 Jun  9 11:11 .git
drwxr-xr-x  2 badcold users 4096 Jun  9 11:10 elsewhere
-rw-r--r--  1 badcold users  120 Jun  9 11:12 test.nix

[MattSturgeon]

I wonder if the intended behaviour should be to complain (warn or error) when a file is a symlink, and skip it? 🤔

To me it seems equally surprising to format some (potentially out-of-tree) file that a symlink points to. Keep in mind, that unlike something like treefmt, nixfmt doesn't know where the root of your project is.

Maybe we could have a "follow symlinks" CLI flag, which would disable the error and actually format the pointed-to file.

Although IDK if anyone will be keen to spend time working on a CLI flag as we are trying to reduce complexity in nixfmt by recommending people use treefmt for formatting their projects.

[badcold]

To me it seems equally surprising to format some (potentially out-of-tree) file that a symlink points to.

Really? I was under the impression that it's a commonly used pattern to have multiple NixOS configuration.nix in the same git repo.
When I read "format the file in place" I did not expect that the file would be recreated. But that could be on me, after a quick search it seems that's also how sed and perl are behaving by default.

I'll take a look at treefmt.

[jfly]

+1 to what @MattSturgeon said about using treefmt. However, we would accept a PR that follows the symlink and formats the file it points at.