Swift 1.2 will include the @inline attribute, with never and __always as parameters. For more info, see here.
As stated before, you rarely need to declare a function explicitly as @inline(__always) because Swift is fairly smart as to when to inline a function. Not having a function inlined, however, can be necessary in some code.
Swift-5 added the @inlinable attribute, which helps ensuring that library/framework stuff are inlineable for those that link to your library. Make sure you read up about it, as there may be a couple of gotchas that might make it unusable. It’s also only for functions/methods declared public, as it’s meant for libraries wanting to expose inline stuff.