modern-swift

Modern Swift

Focused reference for Swift language features that are commonly misused or forgotten but are not concurrency-related. Concurrency lives in the swift-concurrency skill; this skill deliberately does not duplicate it.

When to load which reference

Topic	File	Load when
@available, #available, deprecation, renames	references/attributes.md#availability	Marking API availability, deprecating, renaming, or gating runtime behavior on OS version
@discardableResult	references/attributes.md#discardableresult	Function returns a value callers may reasonably ignore
@frozen	references/attributes.md#frozen	Library author deciding whether an enum/struct layout is stable
@inlinable, @usableFromInline	references/attributes.md#inlinable	Performance-critical code in a resilient library
@backDeployed	references/attributes.md#backdeployed	Shipping a new API to clients running older OSes
@resultBuilder	references/attributes.md#resultbuilder	Building a DSL (SwiftUI-style)
@propertyWrapper	references/attributes.md#propertywrapper	Encapsulating get/set behavior on stored properties
#Preview	references/macros.md#preview	Writing SwiftUI previews
Custom macros	references/macros.md#custom-macros	Understanding or authoring Swift macros

Routing guardrails

• Concurrency topics (async, await, actor, @MainActor, Sendable, @Observable, @preconcurrency) belong to swift-concurrency. Do not answer them from this skill - defer.
• @frozen, @inlinable, @backDeployed are relevant only to resilient libraries (frameworks distributed as binaries with ABI stability). App targets rarely need them. If the user is writing an app, say so and suggest the simpler alternative.
• @backDeployed has real rules beyond "copy the body": public or @usableFromInline, body must follow @inlinable restrictions, methods on classes must be final, stored properties are excluded. See references/attributes.md#backdeployed.

Common mistakes

1. Using @frozen in an app target. It's a library-evolution feature and has no effect outside resilient libraries. Remove it.
2. @backDeployed on a stored property. Not supported. Only computed properties (no storage), functions, final methods, and subscripts.
3. @inlinable body referencing an internal symbol. The body is emitted into client binaries, so it can only reference public or @usableFromInline symbols.
4. Forgetting @discardableResult on a fluent API. Causes "result unused" warnings for legitimate call sites. Mark the method, don't tell the caller to add _ =.
5. Property wrapper with didSet clamping loop. Swift prevents the recursion, but it's still clearer to clamp in the setter of a computed wrappedValue backed by a private stored value. See the canonical pattern in references/attributes.md#propertywrapper.