My preferred code style for colons in Swift (following good coding habits learned long ago).

CodeWithDifferingIntentsShouldLookDifferent.swift

import Foundation


class Example {
	static func method(label argName: String) -> Void {
		NSLog(argName)
	}
}
class SuperClass {}
class AType {
	init(_ aString: String) {
	}
}
extension String {
	func __conversion() -> AType {
		return AType(self)
	}
}



//: Code With Differing Intents Should Look Different

class SubClass : SuperClass { // A colon used in an inheritence specifier has spaces both before & after it.
	var someVar: AType = AType("123") // A colon used as a variable type specifier has a space only after it, not before.
	func method1(argName: AType) {} // Likewise, a colon used as an _argument_ variable type specifier also has a space only after it.
	func method2(label argName: AType) {} // Same when an argument label is present.
}
Example.method(label: "value") // A colon used as an argument label has a space only after it, not before.