5.7 · Swift 6 language mode and upcoming features
The compiler version and the language it accepts are two different dials. Understanding the difference is what makes a Swift 6 migration a series of small steps instead of one big one.
Mental model. A Swift 6 compiler can build Swift 5 code. Language mode decides which rules it enforces, and each module chooses its own.
Two dials
Compiler version versus language mode, and the checks that change.
- What is the difference between the Swift 6 compiler and Swift 6 language mode?
The compiler version is the toolchain you build with (Xcode 16 ships Swift 6). The language mode (-swift-version, or the Swift Language Version build setting)… - What are upcoming features and experimental features, and how do they help a migration?
Many language changes are source-breaking, so they cannot simply be switched on for everyone. Instead each ships as an upcoming feature with its own name and…
Migrating
A staged plan for moving to Swift 6.
- How would you migrate a large codebase to Swift 6 without a freeze?
Move in stages that each ship. First update the toolchain and your dependencies while every module stays in mode 5. Then switch on strict concurrency checking…
More in Compiler & language internals
- 5.1 · From source to machine code, and how to look inside
- 5.2 · Devirtualisation, whole-module optimisation, inlining, exclusivity
- 5.3 · Specialisation, witness tables, and opaque vs existential in the compiled code
- 5.4 · Library evolution, module interfaces, and the ABI boundary
- 5.5 · How Swift macros work, and the roles they play
- 5.6 · Why it compiles slowly, and what rebuilds
- 5.8 · Symbol names, linkage, lazy globals and reflection metadata: what is in the binary
- 5.9 · Optimisation levels, closure contexts, ownership conventions and value layout
- 5.10 · What property wrappers, result builders, async and throws compile to
- 5.11 · Package access, SPI, internal imports, strict memory safety and pre-specialisation