Swift 5.5 Released: Async/Await, Actors and Structured Concurrency

What happened

Swift 5.5 was released on September 20, 2021, according to the official announcement on the Swift blog by Ted Kremenek of the Swift Core Team. He described it as a massive release, driven by extensive discussion, review and iteration from the Swift community, that centred on newly introduced language capabilities for concurrency.

Async/await

The centrepiece was async/await, introduced through Swift Evolution proposal SE-0296, which lets developers write asynchronous code that reads like synchronous code instead of relying on nested completion handlers. The release also added async sequences via SE-0298, continuations for interfacing async tasks with synchronous code via SE-0300, and concurrency interoperability with Objective-C via SE-0297.

Structured concurrency and actors

Swift 5.5 introduced structured concurrency through SE-0304, giving a disciplined model for spawning and managing concurrent tasks with clear lifetimes, along with async let bindings via SE-0317 for running child tasks concurrently. It also added actors via SE-0306, a reference type that protects its mutable state from data races by serialising access, plus global actors via SE-0316 and improved control over actor isolation via SE-0313.

Other additions

Beyond concurrency, Swift 5.5 included a range of Evolution proposals such as Package Collections, extending property wrappers to function and closure parameters, Codable synthesis for enums with associated values, interchangeable use of CGFloat and Double, effectful read-only properties, and task-local values. An updated edition of The Swift Programming Language for Swift 5.5 was published alongside the release.

Why it mattered

Swift 5.5 was one of the most consequential Swift releases, bringing a complete, first-class concurrency model into the language. Async/await, structured concurrency and actors reshaped how developers write asynchronous and parallel Swift code, laying the groundwork that later releases, culminating in Swift 6's compile-time data-race safety, would build upon.

Related on Skillo

See also: Swift 5.9's macros and C++ interop.

Sources

• Swift 5.5 Released (official Swift blog)

Published date reflects the original event date (2021-09-20). This article is original Skillo editorial written from the sources above; facts were verified in September 2026.