Nestix

Nestix is a structural formatter for Nix. It rewrites structure (currently focused on attribute sets) rather than acting as a full pretty-printer.

It is designed to be used alongside a conventional Nix formatter such as alejandra or nixfmt:

1. Run alejandra or nixfmt to standardize whitespace and line breaking.
2. Run Nestix to normalize and (where appropriate) flatten attribute-set structure.

Quickstart

Run Nestix after your usual Nix formatter. Example (single file):

alejandra file.nix # or nixfmt
nestix file.nix

Format a repository (recursively formats *.nix files in place):

nestix .

Check mode (exits with code 2 if changes would be made):

nestix --check .

Read from stdin and write to stdout:

cat file.nix | nestix

What it changes

Nestix focuses on transformations that conventional formatters typically do not perform:

• Flattening small attribute-set subtrees into dotted attribute assignments (when it improves readability).
• Normalizing indentation when Nestix needs to move code.

Example (flattening an attribute set):

# Before
{
  attr1 = {
    attr2 = {
      attr3 = true;
    };
    attr4 = true;
  };
}

# After
{
  attr1.attr2.attr3 = true;
  attr1.attr4 = true;
}

Nestix does not blindly flatten everything. It keeps (or expands) nesting when flattening would reduce readability.

What it does not try to do

• Replace alejandra or nixfmt: Nestix does not aim to make every Nix expression "pretty" on its own.
• Impose a style guide: Nestix is unopinionated and attempts to preserve the existing style (alignment, indentation, etc).

Installation

Nix (flakes)

Run without installing:

nix run github:Noah765/nestix

Install into your profile:

nix profile install github:Noah765/nestix

Cargo (from Git)

cargo install --git https://github.com/Noah765/nestix

NixOS (flake)

Minimal NixOS system flake snippet

{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  inputs.nestix = {
    url = "github:Noah765/nestix";
    inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = {
    nixpkgs,
    nestix,
    ...
  }: let
    system = "x86_64-linux";
  in {
    nixosConfigurations.example = nixpkgs.lib.nixosSystem {
      inherit system;
      modules = [
        {
          environment.systemPackages = [nestix.packages.${system}.default];
        }
      ];
    };
  };
}

Usage

nestix [OPTIONS] [INCLUDE]...

• If no INCLUDE paths are provided, Nestix reads Nix code from stdin.
• If paths are provided, Nestix traverses them recursively and formats *.nix files in place.

Common flags:

• --check: Do not write changes; print files that need formatting; exit with code 2 if any do.
• --exclude <PATH>: Skip a path (may be repeated).

See nestix --help for the full CLI help text.

Performance

Nestix is intended to be fast enough to run over large file trees. As a rough datapoint, nestix /path/to/nixpkgs takes about 8 seconds on an AMD Ryzen 5 3600.

Contributing / Development

• Nix dev shell: nix develop
• Build: cargo build --release
• Test: cargo test

If you encounter a bug or would like a new formatting rule, please open an issue. Small, focused, reproducible snippets are extremely helpful.