Feature Request: Add autocorrection support for `indentation_width` rule

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Feature Request

Description

The indentation_width rule currently does not support autocorrection (--fix). This makes it tedious to fix indentation violations manually, especially in larger codebases or when refactoring code.

According to the official documentation, the rule explicitly states:

• Supports autocorrection: No

Motivation

Indentation issues are purely formatting-related and should be safe to autocorrect. Other style rules like trailing_whitespace already support autocorrection. Adding autocorrection for indentation_width would:

1. Save developers significant time fixing indentation manually
2. Make the rule more practical for teams to enable
3. Align with the behavior of other formatting-focused rules

Steps to Reproduce

1. Create a file with incorrect indentation:

// IndentationTest.swift
import SwiftUI

struct TestView: View {
    var body: some View {
        HStack {
            Text("Hello")
        }
            .frame(maxWidth: .infinity)
            .onTapGesture {
                if true {
                    Task { @MainActor in
            try await doSomething()  // Wrong indentation
                    }
            }
            }
    }

    func doSomething() async throws {}
}

2. Run SwiftLint lint:

$ swiftlint lint IndentationTest.swift

Output:

IndentationTest.swift:13:1: error: Indentation Width Violation: Code should be indented using one tab or 4 spaces (indentation_width)

3. Attempt to fix with --fix:

$ swiftlint --fix IndentationTest.swift

Output:

Correcting Swift files at paths IndentationTest.swift
Correcting 'IndentationTest.swift' (1/1)
Done correcting 1 file!

4. Run lint again - the indentation_width violation still exists:

$ swiftlint lint IndentationTest.swift

Output:

IndentationTest.swift:13:1: error: Indentation Width Violation: Code should be indented using one tab or 4 spaces (indentation_width)

Expected Behavior

Running swiftlint --fix should automatically correct indentation violations by adjusting the whitespace at the beginning of lines to match the expected indentation level.

Environment

• SwiftLint version: 0.63.2
• Installation method: Pre-built binary
• Xcode version: 16.x
• macOS version: macOS 15.x (Sequoia)

Start by locating the implementation and tests for the indentation_width rule, then reproduce the reported case with SwiftLint 0.63.2 and --fix. Done means --fix adjusts leading whitespace for indentation_width violations and a subsequent lint run reports no remaining violation, with regression coverage for the example.