ObjCConvertible.swift

import Foundation

protocol ObjCConvertible {}
extension String: ObjCConvertible {}
extension Int: ObjCConvertible {}
extension Double: ObjCConvertible {}
extension Bool: ObjCConvertible {}

func objCMethod(_ dict: [String: AnyObject]) {
    print(dict)
}

func passToObjC(_ dict: [String: ObjCConvertible]) {
    objCMethod(dict as [String: AnyObject])
}

passToObjC(["a": "b", "c": 1])

Sugar for building [String: AnyObject] dictionaries from Swift, mostly for passing to ObjC.

This works great for things that are actually _ObjectiveCBridgeable! Compiler magic does the boxing the right way. But anything else (custom structs/enums, other non-ObjC stuff) will get passed along also, and blow up when you try to touch them from ObjC.
Be careful with this, and only ever let things you know to be safely representable in ObjC conform to ObjCConvertible.