Lua is nice, but it’s a bit of a weird comparison for Swift. Swift is compiled and lua is interpreted. Swift is statically typed. Swift uses ref counting to Lua’s runtime GC. And Swift has non nullable types by default, classes, enums, ADTs, match expressions and so on.

Lua is nice. But it’s chalk and cheese comparing the languages. I can’t think of many cases where you’re trying to decide between lua and Swift.

In this case, the question is whether to use Swift for a backend server on a Linux machine, or something else. That's the environment. I'd prefer Lua, simply because its a very proven language for that application.

Sure, Swift has sexy features that make it appealing to developers. But the ecosystem for server infrastructure on Lua is formidable.