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Available from Swift 6.0

Paul Hudson      @twostraws

SE-0426 introduces a new BitwiseCopyable protocol, which has the sole purpose of allowing the compiler to create more optimized code for conforming types.

Most of the time you don't need to do anything to enable BitwiseCopyable support. Swift will automatically apply it to most structs and enums you create as long as all the properties they contain are also bitwise copyable. That includes a huge collection of built-in types: all integers, all floating-point numbers, Bool, Duration, StaticString, and more.

Where things take a little more thinking is when you're building a library – if Swift were to automatically apply a conformance to BitwiseCopyable it could cause problems if your type changed in the future in a way that made it not support the protocol.

So, Swift disables the automatic inference for types you export with public or package visibility unless you explicitly mark those types with @frozen.

If you specifically need to disable BitwiseCopyable, you can do that by adding ~BitwiseCopyable to your type's inheritance list. For example, the standard library's CommandLine enum is both public and @frozen, so the Swift team explicitly opt out of it being bitwise copyable like this:

public enum CommandLine : ~BitwiseCopyable {

Important: Opting out of BitwiseCopyable must happen directly where your type is declared rather than in an extension.

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