Swift 6 Released: Compile-Time Data-Race Safety and Typed Throws

What happened

Swift 6 reached general availability on September 17, 2024, according to the official announcement on the Swift blog by Holly Borla of the Swift Core Team. Described as a major release that expands Swift to more platforms and domains, it emphasised that Swift is great for more than apps, including libraries, internet-scale services, and performance-critical and secure code.

Data-race safety

The headline feature was a new opt-in language mode that extends Swift's memory-safety guarantees to prevent data races in concurrent code by diagnosing potential data races as compiler errors. These checks were previously available only as warnings via a strict-concurrency flag in Swift 5.10; improved Sendable inference and new compiler analysis for transferring mutable state between actors reduced false positives. Swift 6 also added a new Synchronization library with low-level atomic operations and a mutex API.

Typed throws and ownership

Swift 6 enabled functions to specify the type of error they throw as part of their signature, for example throws(ParseError), which is useful in generic code and resource-constrained environments. Typed throws generalises over throwing and non-throwing functions. The release also extended the non-copyable types introduced in Swift 5.9 to work with the generics system, so generic code can operate on both copyable and non-copyable types, and added signed and unsigned 128-bit integer types across all platforms.

Embedded Swift and Swift Testing

The release included a preview of Embedded Swift, a language subset and compilation mode for embedded software such as microcontrollers, targeting bare-metal ARM and RISC-V. Swift 6 also introduced Swift Testing, a new testing library designed from the ground up for Swift with expressive macros like @Test and #expect, parameterised tests, and detailed failure output, included directly in the toolchain. Foundation was unified across platforms with a portable Swift implementation, and platform support expanded with a fully static Linux SDK and official Debian and Fedora support.

Why it mattered

Swift 6 marked the start of making data-race safety dramatically easier by turning a whole class of concurrency bugs into compile-time errors. Combined with typed throws, Embedded Swift and the built-in Swift Testing library, it broadened Swift's reach from apps into systems, embedded and server programming while strengthening its safety story.

Related on Skillo

See also: Swift 5.9 and its macro system.

Sources

• Announcing Swift 6 (official Swift blog)

Published date reflects the original event date (2024-09-17). This article is original Skillo editorial written from the sources above; facts were verified in September 2026.