13 releases (4 stable)

1.2.1 Oct 31, 2023
1.2.0 Apr 29, 2023
1.0.0 Oct 12, 2022
0.1.8 Sep 13, 2022
0.1.1 Dec 11, 2021

#218 in Unix APIs


1.5K SLoC


Scan .nix files for dead code (unused variable bindings).

Usage with Github Actions

See deadnix-action

Usage with Nix Flakes


$ nix run github:astro/deadnix -- --help
Find dead code in .nix files

Usage: deadnix [OPTIONS] [FILE_PATHS]...

  [FILE_PATHS]...  .nix files, or directories with .nix files inside [default: .]

  -l, --no-lambda-arg                  Don't check lambda parameter arguments
  -L, --no-lambda-pattern-names        Don't check lambda attrset pattern names (don't break nixpkgs callPackage)
  -_, --no-underscore                  Don't check any bindings that start with a _
  -q, --quiet                          Don't print dead code report
  -e, --edit                           Remove unused code and write to source file
  -h, --hidden                         Recurse into hidden subdirectories and process hidden .*.nix files
  -f, --fail                           Exit with 1 if unused code has been found
  -o, --output-format <OUTPUT_FORMAT>  Output format to use [default: human-readable] [possible values: human-readable, json]
      --exclude <EXCLUDES>...          Files to exclude from analysis
  -V, --version                        Print version

Reports contain ANSI color escape codes unless the $NO_COLOR environment variable is set.

The --exclude parameter accepts multiple paths. Separate them with -- to pass [FILE_PATHS]....

Scan for unused code

$ nix run github:astro/deadnix example.nix
Warning: Unused declarations were found.
  1 │unusedArgs@{ unusedArg, usedArg, ... }:
    ·     │           ╰───── Unused lambda pattern: unusedArg
    ·     ╰───────────────── Unused lambda pattern: unusedArgs
  3 │  inherit (builtins) unused_inherit;
    ·                            ╰─────── Unused let binding: unused_inherit
  5 │  unused = "fnord";
    ·     ╰─── Unused let binding: unused
 10 │  shadowed = 42;
    ·      ╰──── Unused let binding: shadowed
 11 │  _unused = unused: false;
    ·     │         ╰─── Unused lambda argument: unused
    ·     ╰───────────── Unused let binding: _unused
 13 │  x = { unusedArg2, x ? args.y, ... }@args: used1 + x;
    ·             ╰───── Unused lambda pattern: unusedArg2

Remove unused code automatically

Do commit your changes into version control before!

$ nix run github:astro/deadnix -- -eq test.nix

Usage with pre-commit

Add the following to your project's .pre-commit-config.yaml:

  - repo: https://github.com/astro/deadnix
    rev: ID # frozen: VERSION
      - id: deadnix
        #args: [--edit] # Uncomment to automatically modify files
        stages: [commit]

Replace ID and VERSION above with the relevant version tag and commit ID for reference, for example:

rev: da39a3ee5e6b4b0d3255bfef95601890afd80709  # frozen: v1.2.3


Renaming of all unused to lambda args to start with _

If you disfavor marking them as unused, use option -l/--no-lambda-arg.

nixpkgs callPackages with multiple imports

callPackages guesses the packages to inject by the names of a packages' lambda attrset pattern names. Some packages alias these with @args to pass them to another import ...nix args.

As the used args are only named in the imported file they will be recognized as dead in the package source file that is imported by callPackage, rendering it unable to guess the dependencies to call the packages with.

Use option -L/--no-lambda-pattern-names in this case.

Skipping reports for certain declarations

As of 1.1.0 deadnix supports the following pragma to skip check of bindings in the following line:

# deadnix: skip

What if the produced reports are wrong?

Please open an issue. Do not forget to include the .nix code that produces incorrect results.

Commercial Support

The author can be hired to implement the features that you wish, or to integrate this tool into your toolchain.


~43K SLoC