The compiler enters "script" mode if there is a file called
`main.swift`, or if there is only one file. Parsing files as a script
means that anything in the file is interpreted as top-level code, which
is incompatible with a valid @main-annotated struct, so an error is
emitted.
Unfortunately, it is intentional in many cases, and the diagnostic
didn't provide anything actionable to indicate that the explicit main
function is intentional and that the file being passed in is not
actually a top-level context.
The new note indicates that passing `-parse-as-library` to the compiler
invocation will fix it if the explicit main function is intentional.
When a type (class, enum, or struct) is annotated @main, it is required
to provide a function with the following signature:
static func main() -> ()
That function will be called when the executable the type is defined
within is launched.
