xcode - 'Expressions are not allowed at the top level' if the module is not main.swift

Question

Is there anything special with a main.swift file?

I have created a command line based project in XCode. If I put an expression println("Hello, World!"); in a new swift file says test.swift, I will get the error message: Expressions are not allowed at the top level

However this expression is placed at top level in the main.swift that is created by XCode in the new project. No such exception is flagged by XCode.

Answer 1

Apparently yes, as per this answer. However, there are no citations as to this behaviour.

Update This is documented on the Swift blog:

... earlier we said top-level code isn’t allowed in most of your app’s source files. The exception is a special file named “main.swift”, which behaves much like a playground file, but is built with your app’s source code. The “main.swift” file can contain top-level code, and the order-dependent rules apply as well. In effect, the first line of code to run in “main.swift” is implicitly defined as the main entrypoint for the program. This allows the minimal Swift program to be a single line — as long as that line is in “main.swift”.