fix: unset PRJ_ROOT to prevent impure behavior

[GaetanLepage]

When the PRJ_ROOT environment variable is set, treefmt will use it to populate the --tree-root option.
Also, both --tree-root and --tree-root-file options must not be set at the same time, otherwise leading to:

treefmt: error: --tree-root and --tree-root-file can't be used together

As the tree-fmt module is explicitly setting --tree-root-file, I don't see a justification to let PRJ_ROOT leak in this script.

[GaetanLepage]

To give a bit more context, I faced this issue while using devshell which automatically sets the $PRJ_ROOT environment variable.

The combination of devshell and treefmt-nix thus leads to the aforementioned error message.

[emilazy]

cc #219

I think the flake-parts module may still be broken after this, cf.

treefmt-nix/flake-module.nix

Lines 43 to 50 in c9f9703

	options.projectRoot = lib.mkOption {
	type = types.path;
	default = self;
	defaultText = lib.literalExpression "self";
	description = ''
	Path to the root of the project on which treefmt operates
	'';
	};

.

[emilazy]

Or maybe not? projectRoot is only used for the check.

Anyway I was thinking that making projectRootFile optional might be a good idea too.

[GaetanLepage]

Well, I don't know where this should be fix.
But the formatting script, as it is generated right now, should definitely ignore the PRJ_ROOT variable. At best, the latter is not set and thus ignored, but at worst, it generates the error.

Of course, there might be a cleaner solution.

[emilazy]

Right. The potential alternative, I guess, is to make projectRootFile optional and omit --tree-root-file when PRJ_ROOT is set. Not sure what would be best.

(This needs rebasing, by the way, I think.)

[MattSturgeon]

The potential alternative, I guess, is to make projectRootFile optional and omit --tree-root-file when PRJ_ROOT is set. Not sure what would be best.

I don't see why we would want impure environment variables leaking through to treefmt? Isn't it possible that PRJ_ROOT could end up being set to some arbitrary value that is unrelated to what was configured in the treefmt-nix flake module? The environment shouldn't be able to override something explicitly configured in a module and defined in the wrapper script.

[emilazy]

Well, I think searching for the project root is kind of inherently impure to begin with. But sure, that’s fair enough. (projectRoot being an option that’s only used for the flake check, rather than setting --tree-root similarly to projectRootFile, is pretty confusing in itself, but that’s another matter.)

[zimbatm]

This is a bug in treefmt. Env vars should not take precedence over CLI args.

[zimbatm]

alecthomas/kong#446

[zimbatm]

I'll merge this for now (given upstream doesn't think this is an issue)