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.

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

grep -n -A 30 "PreconcurrencyImport" migration-guide.md

Task

grep -n -A 30 "Boundaries.swift" migration-guide.md

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.

let

@MainActor

grep -n "Globals.swift" migration-guide.md

Sendable

@unchecked Sendable

grep -n "Sendable" migration-guide.md

await

nonisolated

@MainActor

grep -n "actor-isolated" migration-guide.md

@Sendable

grep -n "closure" migration-guide.md

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

@unchecked Sendable

grep -n "unchecked" migration-guide.md

nonisolated(unsafe)

grep -n "nonisolated" migration-guide.md

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):

FILE: Guide.docc/CommonProblems.md

FILE: Guide.docc/DataRaceSafety.md

FILE: Guide.docc/IncrementalAdoption.md

FILE: Guide.docc/Swift6Mode.md

FILE: Guide.docc/CompleteChecking.md

FILE: Sources/Examples/ConformanceMismatches.swift

FILE: Sources/Examples/Globals.swift

To find specific content:

grep -n "pattern" migration-guide.md