swiftlint

# SwiftLint

SwiftLint enforces Swift style and conventions by linting source files against a configurable rule set. It is the most widely adopted Swift linter. This skill covers setup, configuration, rule selection, suppression, CI integration, and rollout strategy.

SwiftLint is a **style enforcement tool**, not a style guide. For underlying Swift naming and design conventions, see `swift-api-design-guidelines`. For architecture patterns, see `swift-architecture`.

## Contents

- [Recommended Setup](#recommended-setup)
- [Configuration](#configuration)
- [Rule Selection Strategy](#rule-selection-strategy)
- [Suppressions](#suppressions)
- [Baselines](#baselines)
- [Autocorrect](#autocorrect)
- [CI Integration](#ci-integration)
- [Integration Decision Tree](#integration-decision-tree)
- [Multiple Configurations](#multiple-configurations)
- [Common Mistakes](#common-mistakes)
- [Review Checklist](#review-checklist)
- [References](#references)

---

## Recommended Setup

**Default: build tool plugin via `SimplyDanny/SwiftLintPlugins`.**

Add the plugin package to `Package.swift` or via Xcode's package dependencies:

```swift
// Package.swift
dependencies: [
  .package(url: "https://github.com/SimplyDanny/SwiftLintPlugins", from: "<reviewed-version>")
]
```

For SwiftPM targets, apply the plugin:

```swift
.target(
    name: "MyApp",
    plugins: [.plugin(name: "SwiftLintBuildToolPlugin", package: "SwiftLintPlugins")]
)
```

For Xcode projects without a `Package.swift`, add the package dependency in the project settings, then enable the plugin under the target's Build Phases or the package's plugin trust dialog.

The build tool plugin runs SwiftLint automatically on every build. No run script required.

> **First build**: Xcode prompts to trust the plugin. Select "Trust & Enable All" for the SwiftLintPlugins package.

For alternatives (run scripts, command plugin, Homebrew CLI), see [references/plugins-run-scripts-and-integrations.md](references/plugins-run-scripts-and-integrations.md).

## Configuration

Create `.swiftlint.yml` at the project root. SwiftLint discovers this file by walking up from each source file's directory.

```yaml
# .swiftlint.yml — conservative starter config
disabled_rules:
  - trailing_whitespace
  - todo

opt_in_rules:
  - empty_count
  - closure_spacing
  - force_unwrapping
  - sorted_imports
  - vertical_whitespace_opening_braces
  - private_swiftui_state
  - unhandled_throwing_task
  - accessibility_label_for_image

included:
  - Sources
  - Tests

excluded:
  - .build
  - DerivedData
  - "**/.build"
  - "**/Generated"

line_length:
  warning: 140
  error: 200

type_body_length:
  warning: 300
  error: 500

file_length:
  warning: 500
  error: 1000
```

Key configuration options:

| Key | Purpose |
|-----|---------|
| `disabled_rules` | Turn off default-enabled rules |
| `opt_in_rules` | Turn on rules not enabled by default |
| `only_rules` | Use _only_ the listed rules (mutually exclusive with `disabled_rules`/`opt_in_rules`) |
| `analyzer_rules` | Rules requiring compiler logs (run via `swiftlint analyze`) |
| `baseline` | Path to an existing baseline file used to suppress known violations |
| `write_baseline` | Path where SwiftLint should write a new baseline file |
| `included` | Paths to lint (default: current directory) |
| `excluded` | Paths to skip |
| `strict` | Elevate all warnings to errors |
| `lenient` | Downgrade all errors to warnings |
| `allow_zero_lintable_files` | Suppress the error when no Swift files are found |
| `reporter` | Output format: `xcode` (default), `json`, `checkstyle`, `sarif`, `csv`, `emoji`, etc. |

For full configuration details including severity tuning, environment-variable interpolation, and nested/remote configs, see [references/adoption-and-configuration.md](references/adoption-and-configuration.md).

## Rule Selection Strategy

SwiftLint ships with three rule categories:

1. **Default rules** — enabled automatically, cover widely accepted conventions
2. **Opt-in rules** — disabled by default, enable selectively via `opt_in_rules`
3. **Analyzer rules** — require compiler logs, enabled via `analyzer_rules`

Browse the full categorized list at <https://realm.github.io/SwiftLint/rule-directory.html>.

**Recommended approach for new projects:**

1. Start with defaults. Run `swiftlint rules` to see which rules are enabled.
2. Disable rules that conflict with your team's established conventions.
3. Add opt-in rules one at a time. Review violations before committing each addition.
4. Do not use `only_rules` unless you have a specific reason to start from zero.

**Recommended approach for existing codebases:**

1. Start with the default rule set.
2. Create a baseline (see [Baselines](#baselines)) to suppress all existing violations.
3. Enforce zero new violations in CI.
4. Burn down baseline violations incrementally.

Do not transcribe or memorize the rule directory. Look up rule identifiers and configuration options at the official rule directory when needed.

## Suppressions

Suppress SwiftLint for specific lines when a rule produces a false positive or when the violation is intentional and reviewed.

```swift
// swiftlint:disable:next force_cast
let view = object as! UIView

let legacy = try! JSONDecoder().decode(T.self, from: data) // swiftlint:disable:this force_try

// swiftlint:disable:previous large_tuple
```

Disable for a region:

```swift
// swiftlint:disable cyclomatic_complexity
func complexRouter(...) { ... }
// swiftlint:enable cyclomatic_complexity
```

Disable all rules (use sparingly):

```swift
// swiftlint:disable all
// ... generated or legacy code ...
// swiftlint:enable all
```

**Policy:**
- Prefer targeted single-rule suppressions over `all`.
- Always re-enable after the region ends.
- For generated code, prefer `excluded` paths in `.swiftlint.yml` over inline suppressions.
- For test targets with different tolerance, use a child configuration (see [Multiple Configurations](#multiple-configurations)).