InlineArray and Span

Guidance for Swift 6.2's low-level memory types: InlineArray for fixed-size inline storage without heap allocation, and Span for safe, zero-cost access to contiguous memory. These replace common uses of UnsafeBufferPointer and hand-tuned tuple storage with compiler-checked alternatives.

When This Skill Activates

Use this skill when the user:

• Asks about InlineArray, fixed-size arrays, or stack-allocated collections
• Mentions Span, MutableSpan, RawSpan, or UTF8Span
• Wants to eliminate heap allocations in hot paths
• Is replacing UnsafeBufferPointer or UnsafePointer with safe alternatives
• Asks about value generics or let count: Int generic parameters
• Needs zero-copy access to collection storage
• Mentions inline storage, contiguous memory, or memory layout
• Is doing binary parsing, signal processing, or embedded Swift work
• Wants to avoid copy-on-write overhead for small fixed collections
• Asks about non-escapable types or lifetime dependencies in Swift

Decision Tree

What memory optimization do you need?
│
├─ A fixed-size collection that never grows/shrinks
│  │
│  ├─ Size known at compile time, stored on stack
│  │  └─ InlineArray<N, Element>
│  │     ├─ No heap allocation
│  │     ├─ No reference counting
│  │     └─ No copy-on-write (eager copies)
│  │
│  └─ Size may vary at runtime
│     └─ Array<Element> (standard library)
│
├─ Safe read access to contiguous memory
│  │
│  ├─ Read-only access to typed elements
│  │  └─ Span<Element>
│  │
│  ├─ Mutable access to typed elements
│  │  └─ MutableSpan<Element>
│  │
│  ├─ Read-only access to raw bytes
│  │  └─ RawSpan
│  │
│  ├─ Mutable access to raw bytes
│  │  └─ MutableRawSpan
│  │
│  ├─ Unicode text processing
│  │  └─ UTF8Span
│  │
│  └─ Initializing a new collection's storage
│     └─ OutputSpan
│
├─ Unsafe pointer access (legacy or interop)
│  └─ UnsafeBufferPointer / UnsafeMutableBufferPointer
│     └─ Prefer Span instead for new code
│
└─ Standard dynamic collection
   └─ Array<Element>
      ├─ Heap-allocated, copy-on-write
      └─ Grows/shrinks dynamically

API Availability

Top 5 Mistakes

InlineArray

Declaration

InlineArray uses Swift's value generics to encode the count in the type:

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

Initialization

// Explicit count
let a: InlineArray<4, Int> = [1, 2, 4, 8]

// Count inferred from literal
let b: InlineArray<_, Int> = [1, 2, 4, 8]  // count = 4

// Element type inferred from literal
let c: InlineArray<4, _> = [1, 2, 4, 8]    // Element = Int

// Both inferred
let d: InlineArray = [1, 2, 4, 8]           // InlineArray<4, Int>

Memory Layout

Elements are stored contiguously with no overhead. Size equals count * MemoryLayout<Element>.stride:

MemoryLayout<InlineArray<0, UInt16>>.size       // 0
MemoryLayout<InlineArray<0, UInt16>>.stride      // 1
MemoryLayout<InlineArray<3, UInt16>>.size        // 6  (2 bytes x 3)
MemoryLayout<InlineArray<3, UInt16>>.stride       // 6
MemoryLayout<InlineArray<3, UInt16>>.alignment   // 2  (same as UInt16)

Basic Usage

var array: InlineArray<3, Int> = [1, 2, 3]

// Subscript access
array[0] = 4

// Iterate via indices
for i in array.indices {
    print(array[i])
}

// Copies are eager (no copy-on-write)
var copy = array
copy[0] = 99
// array[0] is still 4

InlineArray vs Array

Span Family

Span (Read-Only)

Provides safe, direct access to contiguous typed memory. Non-escapable -- cannot outlive the source.

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

// Access elements
let first = span[0]
let count = span.count

// Iterate
for element in span {
    print(element)
}

MutableSpan

Mutable access to contiguous storage:

var array = [1, 2, 3, 4]
array.withMutableSpan { span in
    for i in span.indices {
        span[i] *= 2
    }
}
// array is now [2, 4, 6, 8]

RawSpan

Untyped byte-level access for binary data:

let data = Data([0xFF, 0x00, 0xAB, 0xCD])
let rawSpan = data.span  // Span<UInt8> for byte-level access

UTF8Span

Specialized for safe and efficient Unicode processing of UTF-8 encoded text.

Safety Constraints

Non-Escapable: Cannot Return from Functions

Span has a lifetime dependency on the container it was derived from. It cannot be returned:

// ❌ Wrong -- Span cannot escape the function
func getSpan() -> Span<UInt8> {
    let array: [UInt8] = Array(repeating: 0, count: 128)
    return array.span  // Compiler error
}

// ✅ Right -- process within the same scope
func processData(_ array: [UInt8]) {
    let span = array.span
    // Use span here
    let sum = span.reduce(0, +)
}

Non-Escapable: Cannot Capture in Closures

// ❌ Wrong -- Span cannot be captured
func makeClosure() -> () -> Int {
    let array: [UInt8] = Array(repeating: 0, count: 128)
    let span = array.span
    return { span.count }  // Compiler error
}

// ✅ Right -- use Span within the local scope only
func countElements(_ array: [UInt8]) -> Int {
    let span = array.span
    return span.count
}

Invalidation on Mutation

Modifying the source container invalidates any existing span:

// ❌ Wrong -- span is invalidated after mutation
var array = [1, 2, 3]
let span = array.span
array.append(4)        // Invalidates span
// let x = span[0]     // Undefined behavior

// ✅ Right -- re-acquire span after mutation
var array = [1, 2, 3]
array.append(4)
let span = array.span  // Fresh span after mutation
let x = span[0]        // Safe

Performance Considerations

When InlineArray Wins

• Small, fixed-size data (e.g., color components, matrix rows, coordinate tuples)
• Hot loops where heap allocation / ARC overhead is measurable
• Embedded Swift or contexts with no allocator
• Struct fields that should be stored inline rather than behind a pointer

When Array Wins

• Collection size is unknown or variable
• Collection is large and frequently shared (COW avoids full copies)
• Collection is passed across module boundaries where dynamic sizing is needed

When Span Wins Over UnsafePointer

• All new code that needs contiguous memory access -- Span provides the same performance with compile-time safety
• Binary parsing where you read structured data from a byte buffer
• Algorithm implementations that operate on slices of memory

When UnsafePointer Is Still Needed

• C interop requiring raw pointer parameters
• Legacy APIs that predate Span adoption
• Interfacing with system calls that take pointer arguments

When to Use / When NOT to Use

Use InlineArray When

• You know the exact element count at compile time
• You need zero heap allocation (stack storage)
• The collection is small (< ~64 elements as a guideline)
• You modify in place but rarely copy the whole collection
• You are writing performance-critical or embedded code

Do NOT Use InlineArray When

• The collection size varies at runtime
• The collection is large and frequently copied (no COW means expensive copies)
• You need to append, insert, or remove elements
• You benefit from sharing storage across variables

Use Span When

• You need fast, safe, read-only access to contiguous memory
• You want to replace UnsafeBufferPointer with a safe alternative
• You are processing data in place without needing to escape the reference
• You are implementing high-performance algorithms on collection storage

Do NOT Use Span When

• You need to store the reference for later use (Span is non-escapable)
• You need to pass memory access across async boundaries
• You need mutable access (use MutableSpan instead)
• C interop requires actual UnsafePointer arguments

Review Checklist

InlineArray

• Count is a compile-time constant (value generic), not a runtime variable
• Collection size is genuinely fixed and will not need dynamic resizing
• No code attempts to append, insert, or remove elements
• Copies are intentional -- each assignment copies all elements eagerly
• Large InlineArray values are not copied in hot loops (pass by inout or reference)
• MemoryLayout is verified if precise byte layout matters (e.g., GPU buffers, file formats)

Span

• Span is never returned from a function or stored in a property
• Span is never captured in an escaping closure
• Source container is not mutated while a span is in use
• Span is re-acquired after any mutation to the source
• Correct span variant is used: Span (read), MutableSpan (write), RawSpan (bytes)
• UnsafeBufferPointer is only used where C interop requires it; prefer Span otherwise

General Performance

• Profile before optimizing -- confirm heap allocation or ARC is the actual bottleneck
• Small fixed collections use InlineArray instead of Array in measured hot paths
• Span is used instead of UnsafeBufferPointer for safe contiguous access
• No premature optimization -- Array is correct default for most code

References

• Swift Standard Library - InlineArray
• Swift Standard Library - Span
• Value Generics in Swift
• SE-0453: Vector (InlineArray)
• SE-0447: Span