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.
I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests
- Concepts is harder to understand, the documentation makes a good
unit vs functional vs integration distinction, but when the
integration tests are just two subdirs within `tests/` this is not
clear.
- Source filtering in the `flake.nix` is more complex. We need to
filter out some of the dirs from `tests/`, rather than simply pick
the dirs we want and take all of them. This is a good sign the
structure of what we are trying to do is not matching the structure
of the files.
With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests
functional/
installer/
nixos/
```
