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)

InlineArray

Fixed-size, stack-allocated array using value generics. No heap allocation, no reference counting, no copy-on-write.

Declaration

@frozen struct InlineArray<let count: Int, Element> where Element: ~Copyable

The let count: Int is a value generic — the size is part of the type, checked at compile time. InlineArray<3, Int> and InlineArray<4, Int> are different types.

When to Use InlineArray

Use InlineArray Use Array

Size known at compile time Size changes at runtime

Hot path needing zero heap allocation Copy-on-write sharing is beneficial

Embedded in other value types Frequently copied between variables

Performance-critical inner loops General-purpose collection needs

Canonical Example

// Fixed-size, inline storage — no heap allocation var matrix: InlineArray<9, Float> = [1, 0, 0, 0, 1, 0, 0, 0, 1] matrix[4] = 2.0

// Type inference works for count, element, or both let rgb: InlineArray = [0.2, 0.4, 0.8] // InlineArray<3, Double>

// Eager copy on assignment (no COW) var copy = matrix copy[0] = 99 // matrix[0] still 1

Memory Layout

Elements are stored contiguously with no overhead:

MemoryLayout<InlineArray<3, UInt16>>.size // 6 (2 bytes × 3) MemoryLayout<InlineArray<3, UInt16>>.alignment // 2 (same as UInt16)

~Copyable Integration

InlineArray supports noncopyable elements — enables fixed-size collections of unique resources:

struct Sensor: ~Copyable { var id: Int } var sensors: InlineArray<4, Sensor> = ... // Valid: ~Copyable elements allowed

Span — Safe Contiguous Memory Access

Span replaces unsafe pointers with compile-time-enforced safe memory views. Zero runtime overhead.

The Span Family

Type Access Use Case

Span<Element>

Read-only elements Safe iteration, passing to algorithms

MutableSpan<Element>

Read-write elements In-place mutation without copies

RawSpan

Read-only bytes Binary parsing, protocol decoding

MutableRawSpan

Read-write bytes Binary serialization

OutputSpan

Write-only Initializing new collection storage

UTF8Span

Read-only UTF-8 Safe Unicode processing

Accessing Spans

Containers with contiguous storage expose .span and .mutableSpan :

let array = [1, 2, 3, 4] let span = array.span // Span<Int>

var mutable = [10, 20, 30] var ms = mutable.mutableSpan // MutableSpan<Int> ms[0] = 99

Lifetime Safety — Compile-Time Enforcement

Spans are non-escapable — the compiler guarantees they cannot outlive the container they borrow from:

// ❌ Cannot return span that depends on local variable func getSpan() -> Span<UInt8> { let array: [UInt8] = Array(repeating: 0, count: 128) return array.span // Compile error }

// ❌ Cannot capture span in closure let span = array.span let closure = { span.count } // Compile error

// ❌ Cannot access span after mutating original var array = [1, 2, 3] let span = array.span array.append(4) // span[0] // Compile error: container was modified

These constraints prevent use-after-free, dangling pointers, and overlapping mutation at compile time with zero runtime cost.

Span vs Unsafe Pointers

Span UnsafeBufferPointer

Memory safety Compile-time enforced Manual, error-prone

Lifetime tracking Automatic, non-escapable None — dangling pointers possible

Runtime overhead Zero Zero

Use-after-free Impossible Common source of crashes

Canonical Example — Binary Parsing

func parseHeader(_ data: borrowing [UInt8]) -> Header { var raw = data.span.rawSpan // RawSpan over the array's bytes let magic = raw.unsafeLoadUnaligned(as: UInt32.self) raw = raw.extracting(droppingFirst: 4) let version = raw.unsafeLoadUnaligned(as: UInt16.self) return Header(magic: magic, version: version) }

When to Use Span

• Replace UnsafeBufferPointer — same performance, compile-time safety

• Performance-critical algorithms — direct memory access without copying

• Binary parsing/serialization — RawSpan for byte-level access

• Passing data between functions — borrow the container, pass the span

• UTF-8 processing — UTF8Span for safe string byte access

Value Generics

Value generics allow integer values as generic parameters, making sizes part of the type system:

// let count: Int is a value generic parameter struct InlineArray<let count: Int, Element> { ... }

// Different counts = different types let a: InlineArray<3, Int> = [1, 2, 3] let b: InlineArray<4, Int> = [1, 2, 3, 4] // a = b // Compile error: different types

Currently limited to Int parameters. Enables stack-allocated, fixed-size abstractions where the compiler verifies size compatibility at compile time.

Decision Tree

Need explicit ownership? ├─ Working with ~Copyable type? │ └─ Yes → Required (borrowing/consuming) ├─ Fixed-size collection, no heap allocation? │ └─ Yes → InlineArray<let count, Element> ├─ Need safe pointer-like access to contiguous memory? │ ├─ Read-only? → Span<Element> │ ├─ Mutable? → MutableSpan<Element> │ └─ Raw bytes? → RawSpan / MutableRawSpan ├─ 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, SE-0453 (Span), SE-0451 (InlineArray), SE-0452 (value generics)

WWDC: 2024-10170, 2025-245, 2025-312

Docs: /swift/inlinearray, /swift/span

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