version: "1.0.1" name: swift description: 'Expert guidance on Swift best practices, patterns, and implementation. Use when developers mention: (1) Swift configuration or environment variables, (2) swift-log or logging patterns, (3) OpenTelemetry or swift-otel, (4) Swift Testing framework or @Test macro, (5) Foundation avoidance or cross-platform Swift, (6) platform-specific code organization, (7) Span or memory safety patterns, (8) non-copyable types (~Copyable), (9) API design patterns or access modifiers.'

Swift

Swift is a modern general-purpose programming language.

Reference Files

Load these files as needed for specific topics:

• `references/swift-configuration.md` - Swift Configuration: reading config from environment variables, files, CLI arguments; provider hierarchy, namespacing, hot reloading, secret handling
• `references/swift-log.md` - Swift Log logging API: log levels, structured logging, best practices for libraries, metadata, custom handlers
• `references/swift-otel.md` - Swift OTel: OpenTelemetry backend for server apps (preferred for Linux); OTLP export for logs, metrics, tracing; framework integration
• `references/swift-testing.md` - Swift Testing framework: @Test macro, #expect/#require assertions, traits, parameterized tests, test suites, parallel execution, XCTest migration
• `references/debugging.md` - Debugging tips: Terminal UI on Linux (alternate screen buffer), GitHub Actions log analysis

Swift Testing Test Names

Prefer backtick-escaped, sentence-style test function names so the test description lives in the function name:

@Test
func `does something very special in a certain edge case`() async throws {
    // ...
}

Use @Test("...") display names only when a separate display name is specifically needed.

Access Modifiers

Keep types and functions internal unless they need to be public for external use. This prevents accidental exposure of implementation details and makes access level errors easier to fix.

Member Organization

Sort members by visibility in this order:

1. Public
2. Internal
3. Private

Public members should come first and be grouped by topic using // MARK: - ... sections that read like Apple API reference groups. Put an empty line both above and below every // MARK: - ... heading. Prefer present-progressive gerund phrases that describe the capability, such as:

// MARK: - Working with Child Items
 
public func addChild(_ child: Child) { ... }
 
// MARK: - Managing Life-Cycle
 
public func start() async throws { ... }

Use visibility markers for non-public sections:

// MARK: - Internal
 
internal func prepareStorage() { ... }
 
// MARK: - Private
 
private func rebuildIndex() { ... }

Do not mix internal or private helpers into public topic sections. If public API has only one obvious topic, there is no need for a // MARK: - <Topic> before the internal/private sections.

Protocol Conformance Organization

Choose conformance placement based on what the protocol means:

• If the protocol represents an "is-a" relationship, declare the conformance inline with the type declaration.
• Example: public enum BlahError: Error { ... }
• If the protocol describes an ability of a type, typically an -able protocol, put the conformance in an extension at the bottom of the same file as the protocol.
• Use a // MARK: - <ProtocolName> heading immediately before the extension.

public struct Blah { ... }
 
// MARK: - Codable
 
extension Blah: Codable {
    // Codable-specific implementation
}

Foundation Avoidance Policy

Avoid Foundation in core library code when possible:

• Foundation types (Data, Date, UUID, etc.) should be avoided in public APIs for libraries targeting:
• Embedded Swift
• Cross-platform consistency
• Binary size reduction (FoundationEssentials is 15-40MB)
• Use Swift standard library types instead:
• [UInt8] instead of Data for byte buffers
• ContinuousClock.Instant or custom types instead of Date
• Byte-based initializers instead of UUID strings
• Always use internal import Foundation or internal import FoundationEssentials, never public import

#if canImport(FoundationEssentials)
    internal import FoundationEssentials
#else
    internal import Foundation
#endif

InternalImportsByDefault Feature

When using InternalImportsByDefault in Package.swift, all imports are internal by default unless explicitly marked with public import.

When to use `public import`:

• When types from the imported module are exposed in public API (return types, parameters, protocol conformances)
• Example: public import ServiceLifecycle when conforming to ServiceLifecycle.Service in a public type
• Never use `public import Foundation` - keep Foundation internal

Platform-Specific File Organization

Use a +platform suffix convention for platform-specific implementations:

• PlatformDeviceDiscovery+macos.swift - macOS implementation
• PlatformDeviceDiscovery+linux.swift - Linux implementation
• PlatformDeviceDiscovery+default.swift - Fallback for other platforms

When adding methods to a protocol, all platform files must be updated to maintain conformance.

Linux C Library Support

Support both Glibc and Musl for Linux compatibility:

#if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
import Darwin
#elseif canImport(Glibc)
import Glibc
#elseif canImport(Musl)
import Musl
#endif

Avoid Repetitive Code in Selection Logic

When selecting from multiple options with preference ordering, use sorting instead of multiple conditional blocks:

Bad - Repetitive:

if !preferBluetooth {
    for interface in interfaces {
        if case .lan(let device) = interface {
            return .lan(device)
        }
    }
}
for interface in interfaces {
    if case .bluetooth(let device) = interface {
        return .bluetooth(device)
    }
}
if preferBluetooth {
    for interface in interfaces {
        if case .lan(let device) = interface {
            return .lan(device)
        }
    }
}

Good - Sort once, iterate once:

let sorted = interfaces.sorted { a, b in
    if preferBluetooth {
        return a.type == "Bluetooth" && b.type != "Bluetooth"
    } else {
        return a.type == "LAN" && b.type != "LAN"
    }
}
 
for interface in sorted {
    switch interface {
    case .lan(let device): return .lan(device)
    case .bluetooth(let device): return .bluetooth(device)
    default: continue
    }
}

Memory Safety Patterns (Swift 6.2+)

Swift 6.2 introduces opt-in strict memory safety checking via .strictMemorySafety() in Package.swift.

Span Lifetime Constraints:

• Span<T> is lifetime-dependent - it borrows the memory of its backing storage
• Cannot cross async boundaries
• Cannot escape closure scope
• Cannot pass to async callbacks

Solution: Asymmetric API Design Use Span for parsing (read-only, synchronous, borrowed) and [UInt8] for writing (owned, can cross boundaries):

public struct Characteristic<Value: Sendable>: Sendable {
    // Parsing uses Span - borrowed, synchronous access
    internal let parse: @Sendable (borrowing Span<UInt8>) throws -> Value
 
    // Writing uses [UInt8] - owned, can cross closure boundaries
    public typealias WithBytes = ([UInt8]) -> Void
    internal let write: @Sendable (Value) -> (WithBytes) -> Void
}

Safe Integer Loading from Bytes:

// UNSAFE: unsafeLoad
return span.bytes.unsafeLoad(as: UInt64.self)
 
// SAFE: Manual byte-by-byte assembly
var value: UInt64 = 0
for i in 0..<8 {
    value |= UInt64(span[i]) << (i * 8)
}
return value

Span-Based Computed Properties with _read/_modify

With the LifetimeDependence experimental feature, computed properties can return non-escapable types like RawSpan and MutableRawSpan using _read and _modify accessors:

// Enable in Package.swift:
swiftSettings: [
    .enableExperimentalFeature("LifetimeDependence"),
]
 
// Read-only span access
public var bytes: RawSpan {
    _read {
        var mapInfo = GstMapInfo()
        guard mapBuffer(&mapInfo) else { fatalError("Failed to map") }
        defer { unmapBuffer(&mapInfo) }
        yield RawSpan(_unsafeStart: mapInfo.data, byteCount: Int(mapInfo.size))
    }
}
 
// Mutable span access with Copy-on-Write
public var mutableBytes: MutableRawSpan {
    _read {
        fatalError("Cannot read mutableBytes")
    }
    _modify {
        // Ensure unique ownership before write (CoW)
        if !isKnownUniquelyReferenced(&storage) {
            storage = storage.copy()!
        }
        var mapInfo = GstMapInfo()
        guard mapBuffer(&mapInfo) else { fatalError("Failed to map") }
        defer { unmapBuffer(&mapInfo) }
        var span = MutableRawSpan(_unsafeStart: mapInfo.data, byteCount: Int(mapInfo.size))
        yield &span
    }
}

Known Compiler Issue (Swift 6.2.3): The LifetimeDependenceScopeFixup pass can crash when using span.withUnsafeBytes in certain contexts. Workaround: provide separate closure-based methods for C interop that don't go through the span accessor.

Non-Copyable Types

Use ~Copyable for move-only types that should not be duplicated:

public struct ResourceHandle: ~Copyable {
    // Can only be moved, not copied
}
 
public struct ServiceRegistration: @unchecked Sendable, ~Copyable { ... }