`nix fmt` broken

[makuru-org]

nix fmt doesnt do anything and freezes, when used as a flake input, tested with multiple flakes.

Example flake:

{
  inputs = {
    naersk.url = "github:nix-community/naersk/master";
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
    utils.url = "github:numtide/flake-utils";
  };

  outputs =
    {
      self,
      nixpkgs,
      utils,
      naersk,
    }:
    utils.lib.eachDefaultSystem (
      system:
      let
        pkgs = import nixpkgs { inherit system; };
        naersk-lib = pkgs.callPackage naersk { };
      in
      {
        formatter = pkgs.nixfmt-tree;
        defaultPackage = naersk-lib.buildPackage {
          src = ./.;
          verston = "0.1.0";
          nativeBuildInputs = with pkgs; [
            which
            cmake
            kdePackages.wrapQtAppsHook
            kdePackages.qttools
          ];
          buildInputs = with pkgs; [ kdePackages.qtbase ];
        };
        devShell =
          with pkgs;
          mkShell {
            buildInputs = [
              cargo
              rustc
              rustfmt
              pre-commit
              rustPackages.clippy
              cmake
              kdePackages.full
            ];
            RUST_SRC_PATH = rustPlatform.rustLibSrc;
          };
      }
    );
}

[jfly]

@makuru-org, I am not able to reproduce the issue. Perhaps it's related to your locked deps? Try nix flake update, and if that doesn't fix things, please share a full git repo where you can repro the issue.

You could also try running nix build .#formatter.<system> to see if that gives you any useful output.

[makuru-org]

@makuru-org, I am not able to reproduce the issue. Perhaps it's related to your locked deps? Try nix flake update, and if that doesn't fix things, please share a full git repo where you can repro the issue.

You could also try running nix build .#formatter to see if that gives you any useful output.

The problem, why its not reproducible is, is because I accidentally sent the one with nixfmt-tree,
It should have been: formatter = pkgs.nixfmt-rfc-style;.

[jfly]

Ah, this is happening because if you pass nixfmt no arguments, it tries to read from stdin, which feels like a hang.

Things didn't always work this way. nix fmt used to always pass the formatter an argument, but that changed in NixOS/nix#11438.

@makuru-org, nixfmt is not (no longer) meant to be used as a flake formatter. As you've already discovered, the supported way to do this is with nixfmt-tree. We recently updated nix fmt --help to explain this, but it has not landed in a release of nix yet. This is also documented on nix.dev here: https://nix.dev/guides/recipes/autoformatting. We're also working on adding this to our README.

@infinisil @piegamesde, is it too late to deprecate nixfmt's current stdin support in favor of requiring users to explicitly pass a --stdin flag to activate it? That would allow us to give people a clear error message here instead.

[piegamesde]

I do like being able to just pipe files into nixfmt, but I get the issue. Here's what other programs also do:

• Many programs recognize "-" as a special file that means stdin (instead of having a dedicated flag, which parses differently etc.)
• Some like nom display a warning when X time has passed without any input

I wonder, can we maybe detect a piped stdin from a terminal stdin? The current code always assumes a pipe and will never work from an interactive terminal (as there is no way to "close" the stdin interactively which is required for nixfmt to start processing). So we could detect that and then print a warning/error accordingly

[jfly]

Many programs recognize "-" as a special file that means stdin (instead of having a dedicated flag, which parses differently etc.)

This is a common idiom, but in my experience, less common with autoformatters. Some formatters behave differently based on the name of the file they're formatting. To allow this to work with stdin, these formatters need some mechanism for getting the filename as a command line and the file contents via stdin.

Some like nom display a warning when X time has passed without any input

That's a neat idea!

The current code always assumes a pipe and will never work from an interactive terminal (as there is no way to "close" the stdin interactively which is required for nixfmt to start processing)

That's not true. I can use ctrl-d to close stdin interactively:

---

Whatever we decide here, I sure would like us to work with treefmt to ensure it's compatible with their potential future stdin specification.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/formatting-team-meeting-2025-05-13/64230/1

[jfly]

I'm closing this in favor of #305