borrowing & consuming — Parameter Ownership

Explicit ownership modifiers for performance optimization and noncopyable type support.

When to Use

✅ Use when:

• Large value types being passed read-only (avoid copies)
• Working with noncopyable types (~Copyable)
• Reducing ARC retain/release traffic
• Factory methods that consume builder objects
• Performance-critical code where copies show in profiling

❌ Don't use when:

• Simple types (Int, Bool, small structs)
• Compiler optimization is sufficient (most cases)
• Readability matters more than micro-optimization
• You're not certain about the performance impact

Quick Reference

Modifier	Ownership	Copies	Use Case
(default)	Compiler chooses	Implicit	Most cases
borrowing	Caller keeps	Explicit copy only	Read-only, large types
consuming	Caller transfers	None needed	Final use, factories
inout	Caller keeps, mutable	None	Modify in place

Default Behavior by Context

Context	Default	Reason
Function parameters	borrowing	Most params are read-only
Initializer parameters	consuming	Usually stored in properties
Property setters	consuming	Value is stored
Method self	borrowing	Methods read self

Patterns

Pattern 1: Read-Only Large Struct

struct LargeBuffer {
    var data: [UInt8]  // Could be megabytes
}

// ❌ Default may copy
func process(_ buffer: LargeBuffer) -> Int {
    buffer.data.count
}

// ✅ Explicit borrow — no copy
func process(_ buffer: borrowing LargeBuffer) -> Int {
    buffer.data.count
}

Pattern 2: Consuming Factory

struct Builder {
    var config: Configuration

    // Consumes self — builder invalid after call
    consuming func build() -> Product {
        Product(config: config)
    }
}

let builder = Builder(config: .default)
let product = builder.build()
// builder is now invalid — compiler error if used

Pattern 3: Explicit Copy in Borrowing

With borrowing, copies must be explicit:

func store(_ value: borrowing LargeValue) {
    // ❌ Error: Cannot implicitly copy borrowing parameter
    self.cached = value

    // ✅ Explicit copy
    self.cached = copy value
}

Pattern 4: Consume Operator

Transfer ownership explicitly:

let data = loadLargeData()
process(consume data)
// data is now invalid — compiler prevents use

Pattern 5: Noncopyable Type

For ~Copyable types, ownership modifiers are required:

struct FileHandle: ~Copyable {
    private let fd: Int32

    init(path: String) throws {
        fd = open(path, O_RDONLY)
        guard fd >= 0 else { throw POSIXError.errno }
    }

    borrowing func read(count: Int) -> Data {
        // Read without consuming handle
        var buffer = [UInt8](repeating: 0, count: count)
        _ = Darwin.read(fd, &buffer, count)
        return Data(buffer)
    }

    consuming func close() {
        Darwin.close(fd)
        // Handle consumed — can't use after close()
    }

    deinit {
        Darwin.close(fd)
    }
}

// Usage
let file = try FileHandle(path: "/tmp/data.txt")
let data = file.read(count: 1024)  // borrowing
file.close()  // consuming — file invalidated

Pattern 6: Reducing ARC Traffic

class ExpensiveObject { /* ... */ }

// ❌ Default: May retain/release
func inspect(_ obj: ExpensiveObject) -> String {
    obj.description
}

// ✅ Borrowing: No ARC traffic
func inspect(_ obj: borrowing ExpensiveObject) -> String {
    obj.description
}

Pattern 7: Consuming Method on Self

struct Transaction {
    var amount: Decimal
    var recipient: String

    // After commit, transaction is consumed
    consuming func commit() async throws {
        try await sendToServer(self)
        // self consumed — can't modify or reuse
    }
}

Common Mistakes

Mistake 1: Over-Optimizing Small Types

// ❌ Unnecessary — Int is trivially copyable
func add(_ a: borrowing Int, _ b: borrowing Int) -> Int {
    a + b
}

// ✅ Let compiler optimize
func add(_ a: Int, _ b: Int) -> Int {
    a + b
}

Mistake 2: Forgetting Explicit Copy

func cache(_ value: borrowing LargeValue) {
    // ❌ Compile error
    self.values.append(value)

    // ✅ Explicit copy required
    self.values.append(copy value)
}

Mistake 3: Consuming When Borrowing Suffices

// ❌ Consumes unnecessarily — caller loses access
func validate(_ data: consuming Data) -> Bool {
    data.count > 0
}

// ✅ Borrow for read-only
func validate(_ data: borrowing Data) -> Bool {
    data.count > 0
}

~Copyable Limitations

Know the constraints before adopting ~Copyable:

Limitation	Impact	Workaround
Can't store in Array, Dictionary, Set	Collections require Copyable	Use Optional<T> wrapper or manage manually
Can't use with most generics	<T> implicitly means <T: Copyable>	Use <T: ~Copyable> (requires library support)
Protocol conformance restricted	Most protocols require Copyable	Use ~Copyable protocol definitions
Can't capture in closures by default	Closures copy captured values	Use borrowing closure parameters
No existential support	any ~Copyable doesn't work	Use generics instead

Common compiler errors when adopting ownership modifiers:

// Error: "Cannot implicitly copy a borrowing parameter"
// Fix: Add explicit `copy` or change to consuming
func store(_ v: borrowing LargeValue) {
    self.cached = copy v  // ✅ Explicit copy
}

// Error: "Noncopyable type cannot be used with generic"
// Fix: Constrain generic to ~Copyable
func use<T: ~Copyable>(_ value: borrowing T) { }  // ✅

// Error: "Cannot consume a borrowing parameter"
// Fix: Change to consuming if you need ownership transfer
func takeOwnership(_ v: consuming FileHandle) { }  // ✅

// Error: "Missing 'consuming' or 'borrowing' modifier"
// Fix: ~Copyable types require explicit ownership on all methods
struct Token: ~Copyable {
    borrowing func peek() -> String { ... }   // ✅ Explicit
    consuming func redeem() { ... }           // ✅ Explicit
}

When NOT to use ~Copyable:

• If you need collection storage (arrays, dictionaries)
• If you need to work with existing generic APIs
• If the type needs broad protocol conformance
• Prefer consuming func on regular types as a lighter alternative for "use once" semantics

Performance Considerations

When Ownership Modifiers Help

• Large structs (arrays, dictionaries, custom value types)
• High-frequency function calls in tight loops
• Reference types where ARC traffic is measurable
• Noncopyable types (required, not optional)

When to Skip

• Default behavior is almost always optimal
• Small value types (primitives, small structs)
• Code where profiling shows no benefit
• API stability concerns (modifiers affect ABI)

Decision Tree

Need explicit ownership?
├─ Working with ~Copyable type?
│  └─ Yes → Required (borrowing/consuming)
├─ Large value type passed frequently?
│  ├─ Read-only? → borrowing
│  └─ Final use? → consuming
├─ ARC traffic visible in profiler?
│  ├─ Read-only? → borrowing
│  └─ Transferring ownership? → consuming
└─ Otherwise → Let compiler choose

Resources

Swift Evolution: SE-0377

WWDC: 2024-10170

Skills: axiom-swift-performance, axiom-swift-concurrency