Swift 6 Migration

Overview

Swift 6 enforces data race safety at compile time. Migration involves making implicit isolation explicit and ensuring all shared state is thread-safe.

This skill bundles Apple's complete migration guide. You MUST search it for EVERY error before implementing a fix.

Mandatory Workflow

digraph migration_workflow {
    rankdir=TB;
    node [shape=box];

    "Compiler error/warning" [shape=ellipse];
    "Search migration-guide.md" [label="REQUIRED: Search migration-guide.md\nfor error pattern"];
    "Read matched section" [label="Read the matched FILE: section"];
    "Apply documented fix" [label="Apply Apple's documented fix"];
    "Verify fix" [label="Build and verify"];

    "Compiler error/warning" -> "Search migration-guide.md";
    "Search migration-guide.md" -> "Read matched section";
    "Read matched section" -> "Apply documented fix";
    "Apply documented fix" -> "Verify fix";
    "Verify fix" -> "Compiler error/warning" [label="More errors"];
}

For EACH compiler error/warning:

1. FIRST search migration-guide.md for the error pattern
2. THEN read the matched FILE: section
3. THEN apply the documented fix
4. FINALLY verify the fix

Checklist

Use TodoWrite to track each item:

• Enable -strict-concurrency=complete warnings before Swift 6 mode
• For EACH error: grep migration-guide.md BEFORE implementing fix
• Read the relevant FILE: section from search results
• Verify fix matches Apple's documented approach
• Test runtime behavior after fixes (execution order may change)

Required Searches by Error Type

You MUST run these searches. Do not skip to Quick Reference.

Quick Reference (Starting Points Only)

These are shortcuts for orientation. You MUST verify against migration-guide.md before applying.

Commands

# Build with complete concurrency checking (warnings)
swift build -Xswiftc -strict-concurrency=complete

# Build in Swift 6 mode (errors)
swift build -Xswiftc -swift-version -Xswiftc 6

Package.swift settings:

// Enable strict concurrency per target
.target(
    name: "MyTarget",
    swiftSettings: [.enableExperimentalFeature("StrictConcurrency")]
)

// Or enable Swift 6 mode
swiftLanguageVersions: [.v6]

Common Mistakes

Red Flags - STOP and Search First

These thoughts mean you're about to skip required verification:

• "I know how to fix this"
• "The Quick Reference has the answer"
• "This is a common pattern"
• "I'll check the guide if this doesn't work"
• "My knowledge is sufficient"

All of these mean: Search migration-guide.md FIRST.

Reference Documentation

The migration-guide.md file contains Apple's complete migration documentation (25 bundled files, 3700+ lines):

To find specific content: grep -n "pattern" migration-guide.md