mozlint

name	mozlint
description	You MUST use this skill when working with Firefox's linting infrastructure (mozlint), adding new linters, modifying existing linters, running linters, or dealing with linting issues.

Overview

Firefox's linting infrastructure (mozlint) provides a unified way to run various linters across the codebase. It integrates multiple linters including ESLint, Stylelint, ruff, clippy, clang-format, and many others.

Workflow

• Run ./mach lint without arguments to lint all modified files in your working directory
• Run ./mach lint --fix to automatically fix issues where possible
• Run ./mach lint <path> to lint specific files or directories
• Run ./mach lint --list to see all available linters
• Run ./mach lint --linter <name> to run a specific linter (e.g., ./mach lint --linter eslint)
• Use ./mach lint --outgoing to lint only outgoing changes (useful before pushing)
• Use ./mach lint --rev <rev> to lint changes in a specific revision
• Use ./mach lint --warnings to also show warnings, not just errors

Python linters

Python-based linters are the right choice when the linter primarily calls external programs (e.g., eslint, ruff, clang-format) or needs complex Python library integrations. They use types like external, string, regex, structured_log, or global, with a payload of the form module:function.

mozcheck (Rust linter binary)

Rust-based checks in mozcheck are preferred over new Python linters whenever feasible.

tools/lint/mozcheck/ is a Rust crate hosting fast, in-process checks (currently file-perm, file-whitespace, license, rejected-words, test-manifest-toml, trojan-source). The Python shim at tools/lint/mozcheck/__init__.py finds or builds the binary and invokes it in batch mode.

Wire a YAML to it with type: external, payload: mozcheck:lint, check: <subcommand>, and support-files: ['tools/lint/mozcheck/**']. Extra YAML keys are forwarded as the check's config.

To add a check: add src/<name>.rs, register it in src/main.rs and src/batch.rs, create the YAML, and add tests under tools/lint/test/. Prefer this over a new Python linter for self-contained file checks.

Adding New Linters

See docs/code-quality/lint/create.rst for the full guide on creating a new linter.

• Linter configurations are in tools/lint/ directory
• Each linter has a YAML configuration file (e.g., tools/lint/eslint.yml, tools/lint/ruff.yml). Adding the YAML file is what registers the linter with mozlint.
• Python-based linters are typically in tools/lint/python/
• To add a new linter:
  1. Create a YAML configuration in tools/lint/
  2. If it's a custom linter, implement the Python module in tools/lint/python/ (or add a check to tools/lint/mozcheck/ for fast file checks)
  3. Add tests in tools/lint/test/
• To run the full mozlint test suite (slow): ./mach python-test --subsuite mozlint --run-slow
• To run a single mozlint test: ./mach python-test --subsuite mozlint <test_name>

Common Linters

• eslint: JavaScript and JSX linting
• stylelint: CSS and related files linting
• ruff: Python linting and formatting
• clippy: Rust linting
• clang-format: C/C++ code formatting
• prettier: JavaScript/JSON/YAML formatting
• shellcheck: Shell script linting
• yamllint: YAML file linting
• rstcheck: reStructuredText linting
• license: License header checking

Configuration Files

• ESLint: eslint.config.mjs
• Stylelint: .stylelintrc.js, .stylelintignore
• Ruff: pyproject.toml
• Prettier: .prettierrc.js, .prettierignore
• List of third party code: tools/rewriting/ThirdPartyPaths.txt

Tips

• Use ./mach lint --verbose for more detailed output when debugging linter issues
• Check docs/code-quality/lint/ for documentation on specific linters
• Some linters may require additional setup (e.g., node modules for ESLint)
• Run ./mach bootstrap if linters are missing dependencies
• To exclude files from linting, use the appropriate ignore file or add exclusions in the linter's YAML config