C# would probably be the closest thing feature-wise (and, often, syntax-wise) if you need first-class Linux support, and a lot of knowledge is directly transfer

C# would probably be the closest thing feature-wise (and, often, syntax-wise) if you need first-class Linux support, and a lot of knowledge is directly transferable. Not a Java copy people claim it to be but something more low-level :) With that said, Swift enjoys better FFI by virtue of using LLVM and lower base memory footprint since it does not need to maintain data structures used by GC. Performance however will vary case by case, ARC generally has way higher upfront cost than GC, especially in multi-threaded scenarios but lends itself to an overall more deterministic memory usage, which is important on memory-constrained systems.