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.

Error Pattern	Required Search Command
global variable is not concurrency-safe	grep -n -A 30 "Unsafe Global" migration-guide.md
does not conform to.*Sendable	grep -n -A 30 "Sendable Types" migration-guide.md
cannot pass argument of non-sendable type	grep -n -A 30 "ConformanceMismatches" migration-guide.md
actor-isolated.*cannot be referenced	grep -n -A 30 "actor-isolated" migration-guide.md
@preconcurrency needed	grep -n -A 30 "PreconcurrencyImport" migration-guide.md
Task or async/await migration	grep -n -A 30 "Boundaries.swift" migration-guide.md
GCD/DispatchQueue migration	grep -n -A 30 "DispatchQueue" migration-guide.md

Quick Reference (Starting Points Only)

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

Problem	Likely Direction	Verify With
Mutable global var	let, @MainActor, or actor	grep -n "Globals.swift" migration-guide.md
Non-Sendable class	Sendable, @unchecked Sendable, or actor	grep -n "Sendable" migration-guide.md
Actor isolation error	await, nonisolated, or @MainActor	grep -n "actor-isolated" migration-guide.md
Closure capturing	@Sendable closure or restructure	grep -n "closure" migration-guide.md
Legacy callback API	withCheckedContinuation	grep -n "continuation" migration-guide.md

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

Mistake	Why It's Wrong	Better Approach
Adding @unchecked Sendable everywhere	Hides real data races	Search guide first: grep -n "unchecked" migration-guide.md
Using nonisolated(unsafe)	Compiler trusts you but runtime doesn't	Search: grep -n "nonisolated" migration-guide.md
Skipping the migration guide	Miss Apple's recommended patterns	ALWAYS search before fixing

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):

Topic	Search Pattern
Common errors	FILE: Guide.docc/CommonProblems.md
Data race safety	FILE: Guide.docc/DataRaceSafety.md
Incremental adoption	FILE: Guide.docc/IncrementalAdoption.md
Swift 6 mode	FILE: Guide.docc/Swift6Mode.md
Complete checking	FILE: Guide.docc/CompleteChecking.md
Sendable examples	FILE: Sources/Examples/ConformanceMismatches.swift
Global variable patterns	FILE: Sources/Examples/Globals.swift

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