Xcode 6 beta3 で Swift Arrayの仕様がまともになったようだぜ

Array in Swift has been completely redesigned to have full value semantics like Dictionary and
String have always had in Swift. This resolves various mutability problems – now a 'let' array
is completely immutable, and a 'var' array is completely mutable – composes properly with
Dictionary and String, and solves other deeper problems. Value semantics may be surprising
if you are used to NSArray or C arrays: a copy of the array now produces a full and
independent copy of all of the elements using an efficient lazy copy implementation. This is a
major change for Array, and there are still some performance issues to be addressed. Please
see the Swift Programming Language for more information. (17192555)!