flake-linter

A tool to detect and report duplicate dependencies in Nix flakes.

The Problem

Nix flakes use a lock file (flake.lock) to pin dependencies to specific versions, ensuring reproducible builds. However, when multiple flakes in your dependency tree depend on the same upstream flake (like nixpkgs or flake-utils), they might reference different versions of it. This creates several issues:

Why Duplicate Dependencies Are Problematic

1. Increased Closure Size: Each version of a dependency must be downloaded and stored separately, significantly increasing disk usage and download times.

2. Evaluation Performance: Nix has to evaluate multiple versions of the same flake, slowing down your build times and nix flake commands.

3. Inconsistent Behavior: Different parts of your project might use different versions of the same packages, leading to subtle bugs and version conflicts.

When This Happens

This commonly occurs when:

• Your project uses multiple third-party flakes
• Those flakes haven't synchronized their flake.lock updates
• Transitive dependencies pin different versions of common utilities (like flake-utils, nixpkgs, etc.)

Solution

flake-linter scans your flake.lock file and identifies all cases where the same dependency appears multiple times with different versions, helping you spot these issues before they impact your project.

Usage

$ cd your-flake-project
$ nix run github:Mic92/flake-linter
github:numtide/flake-utils has multiple versions:
  flake-utils is used by: crane, pre-commit-hooks-nix, root, rust-overlay
  flake-utils_2 is used by: lanzaboote

Options

# Run on a specific flake.lock file
$ flake-linter /path/to/flake.lock

# Show all dependencies (including single versions)
$ flake-linter --verbose

# Exit with error code if duplicates are found (useful for CI)
$ flake-linter --fail-if-multiple-versions

Installation

Run directly with Nix:

nix run github:Mic92/flake-linter

Or add to your flake inputs for regular use:

{
  inputs.flake-linter.url = "github:Mic92/flake-linter";
}

What to Do When Duplicates Are Found

1. Update Inputs: Run nix flake update to try getting your dependencies on the same versions
2. Override Inputs: Use flake input follows to force dependencies to use your version:

   inputs.some-flake.inputs.nixpkgs.follows = "nixpkgs";
   

License

MIT