Language Features

Coverage of the Swift 6.3 language surface across Tiers 0–9.

Legend: ✅ Done · 🟡 Partial · ⬜ Todo

Tier 0 — Lexical & Literals

Complete — everything downstream depends on these.

Status Feature Notes
✅ Integer literals (dec/hex/oct/bin, _ separators)
✅ Floating-point literals (dec/hex, exponents)
✅ Boolean literals true/false
✅ nil literal
✅ String literals (escapes, \u{})
✅ Multiline string literals """
✅ Raw string literals #"..."#
✅ String interpolation \(expr)
✅ Extended string delimiters #"\n"#
✅ Regex literals /…/ and #/…/#
✅ Unicode identifiers
✅ Comments (line, block, nested, doc)
✅ Operators: arithmetic / comparison / logical / bitwise / range
✅ Wrapping operators &+ &- &* &<< &>>
✅ Overflow-trapping integer semantics
✅ Nil-coalescing ??
✅ Range operators ..< ... (+ one-sided)
✅ Identity operators === !==

Tier 1 — Core Imperative

Bindings & Expressions

Status Feature
✅ let / var declarations + type annotations
✅ Type inference for initializers
✅ Arithmetic / comparison / logical / bitwise eval
✅ Compound assignment += -= *= …
✅ Ternary a ? b : c
✅ Tuples + tuple decomposition let (a,b) = …
✅ Parenthesized / wildcard _ expressions
✅ Integer width conversions & Int(x) casts

Functions

Status Feature
✅ Function declarations, params, return
✅ Argument labels & parameter names
✅ Default parameter values
✅ Variadic parameters T...
✅ inout parameters (true lvalue aliasing)
✅ Nested functions + capture
✅ Function types as values / params / returns
✅ Multiple return values via tuples
✅ @discardableResult
✅ Functions that never return (-> Never)

Control Flow

Status Feature
✅ if / else / else if
✅ if as expression (Swift 5.9)
✅ guard / guard let
✅ while / repeat-while
✅ for-in over ranges / arrays / sequences
✅ for case / for … where
✅ switch + cases + default
✅ switch value / range / tuple patterns
✅ where clauses in cases
✅ fallthrough
✅ break / continue + labeled statements
✅ switch exhaustiveness / @unknown default

Tier 2 — Value & Nominal Types

Structs & Enums

Status Feature
✅ struct declaration + stored properties
✅ Value semantics (copy on assign/pass)
✅ Memberwise initializers (synthesized)
✅ Methods on structs
✅ mutating methods
✅ enum with simple cases
✅ Enum associated values
✅ Enum raw values + RawRepresentable
✅ indirect enums (recursive)
✅ Enum methods / computed props
✅ CaseIterable synthesis
✅ Nested types

Properties

Status Feature
✅ Stored properties (let/var)
✅ Computed properties (get/set)
✅ Read-only computed properties
✅ Property observers willSet/didSet
✅ lazy stored properties
✅ Type properties static/class
✅ Property wrappers @propertyWrapper
🟡 Projected values $wrapper
⬜ Global & local lazy variables

Optionals

Status Feature
✅ Optional type T?
✅ if let / guard let binding
✅ Shorthand if let x (Swift 5.7)
✅ Forced unwrap ! (trap on nil)
✅ Optional chaining ?.
✅ Nil-coalescing ??
✅ Implicitly unwrapped optionals T!
✅ Optional pattern case let x?

Subscripts

Status Feature
✅ Instance subscripts
✅ Type subscripts (static subscript)
✅ Subscript overloads / multi-param

Tier 3 — Reference Types & Memory (ARC)

Status Feature
✅ class declaration + reference semantics
✅ ARC retain/release (refcount, no cycle GC)
✅ Deterministic deinit at refcount 0
✅ Inheritance + method/property override
✅ final / override semantics
✅ Dynamic dispatch (vtables)
✅ super. calls
🟡 Designated / convenience initializers
🟡 Initializer delegation + 2-phase init
✅ required initializers
✅ Failable initializers init? / init!
✅ weak references (zeroing)
✅ unowned references
✅ unowned(unsafe)
✅ Identity === !==
✅ Type casting is / as? / as! / as
✅ Downcasting in class hierarchies

Closures

Status Feature
✅ Closure expressions
✅ Trailing closures (+ multiple)
✅ Shorthand args $0 $1
✅ Capture by reference (upvalues)
🟡 Capture lists [weak self] [unowned]
🟡 @escaping closures
✅ @autoclosure
✅ Closures capturing inout

Tier 4 — Abstraction: Protocols, Generics, Extensions

Protocols

Status Feature
✅ Protocol declaration (methods/props/init)
✅ Conformance + witness tables
🟡 Protocol inheritance
✅ Protocol composition P & Q
✅ Default implementations (in extensions)
✅ Associated types
✅ Protocol as type / existential any P
✅ Self requirements
✅ Protocol witness for operators
⬜ Optional protocol requirements (@objc optional)
✅ Class-only protocols (AnyObject)
✅ Conditional conformance
🟡 Synthesized Equatable/Hashable/Comparable
🟡 Synthesized Codable

Generics

Status Feature
✅ Generic functions <T>
✅ Generic types (struct/class/enum)
✅ Type constraints <T: Protocol>
🟡 where clauses
✅ Associated-type constraints
✅ Generic subscripts
⬜ Monomorphization vs witness dispatch
✅ Contextual where on extensions
⬜ Primary associated types (some Collection<Int>)
⬜ Parameter packs / variadic generics each
⬜ Integer generic parameters (let N: Int)
⬜ ~Copyable / ~Escapable

Extensions

Status Feature
✅ Extend struct/class/enum/protocol
✅ Add methods/computed props/inits/subscripts
✅ Add protocol conformance via extension
✅ Conditional extensions (where)
✅ Extensions on generic types

Tier 5 — Error Handling & Resource Management

Status Feature
✅ Error protocol + custom errors
✅ throws functions
✅ throw statement
✅ do / catch (+ pattern catches)
✅ try / try? / try!
🟡 rethrows
🟡 Typed throws throws(E) (Swift 6)
✅ defer statements (LIFO on scope exit)
✅ Error propagation through call stack
✅ Result<Success, Failure>

Tier 6 — Advanced Types & Expressions

Status Feature
✅ Opaque types some P (return)
✅ Boxed/existential any P
✅ Metatypes T.self / .Type / .Protocol
✅ type(of:) dynamic type
✅ Key paths \Root.path
✅ Key-path expressions as functions
⬜ @dynamicMemberLookup
⬜ @dynamicCallable
⬜ #selector / #keyPath
⬜ Self type in return position
✅ Implicit member expr .foo
⬜ consume / borrow operators
⬜ discard self

Tier 7 — Concurrency

Runs on a single-threaded cooperative executor. Results and structure are faithful; preemptive scheduling order may differ from native Swift.

Status Feature
✅ async functions
✅ await expressions
✅ async let
✅ Task / Task.detached
✅ Task groups (withTaskGroup)
🟡 Task cancellation
✅ actor declarations + isolation
🟡 Actor reentrancy / serial executor
✅ @MainActor / global actors
🟡 nonisolated / isolated params
🟡 Sendable checking
✅ AsyncSequence / for await
⬜ Continuations (withCheckedContinuation)
⬜ Strict concurrency (Swift 6 mode)

Tier 8 — Macros & Metaprogramming

Status Feature
⬜ Freestanding macros #macro
⬜ Attached macros @Macro
⬜ Macro declarations
✅ Built-in #file/#line/#function/#column
🟡 #warning / #error
⬜ @freestanding / @attached roles
✅ @resultBuilder (DSL transform)
✅ Result-builder method synthesis

Tier 9 — Attributes, Access Control, Operators

Access Control

Status Feature
⬜ open public internal fileprivate private
⬜ package access level
⬜ Access on setters private(set)
🟡 Module boundaries / import

Custom Operators

Status Feature
✅ prefix/infix/postfix operator decls
✅ precedencegroup
✅ Operator method implementations
✅ Operator overloading

Compiler Directives

Status Feature
✅ Conditional compilation #if/#elseif/#else/#endif
✅ #if platform/arch/compiler/canImport/swift()
⬜ #sourceLocation line control
⬜ #available / #unavailable conditions