Swift 5 Exclusivity Enforcement

The Swift 5 release enables runtime checking of “Exclusive Access to Memory” by default in Release builds, further enhancing Swift’s capabilities as a safe language. In Swift 4, these runtime checks were only enabled in Debug builds. In this post, I’ll first explain what this change means for Swift developers before delving into why it is essential to Swift’s strategy for safety and performance.

→ swift.org/blog/swift-5-exclusivity/

Swift 5 turns on runtime checks for exclusive access to memory by default in release builds. This change boosts memory safety by stopping simultaneous changes to variables, which could lead to bugs or crashes. It was only in debug builds before.

This may affect some projects. Code that broke the rules but ran fine might now trap at runtime. The message will say "Simultaneous accesses to …, but modification requires exclusive access".

To fix, use local copies or change how you access variables. For example, copy a value before changing it in a closure. This also helps with ABI stability and future features like move-only types.