Since the type system is so strict, the compiler can often infer immutability even for mutable variables, and avoid copying in those cases. Swift’s compiler wil

Since the type system is so strict, the compiler can often infer immutability even for mutable variables, and avoid copying in those cases. Swift’s compiler will even warn you if you declared a variable mutable when it’s never mutated. It’s an optimization rather than a guiding principle, but a very powerful one. Swift now is quasi on par with C++ performance, also thanks to optimizations like this.