trim null values out of a dictionary in Swift 4

Trim.swift

//Uses only O(n) complexity.

extension Dictionary where Key == String, Value == Any? {

    var trimmingNullValues: [String: Any] {
        var copy = self
        forEach { (key, value) in
            if value == nil {
                copy.removeValue(forKey: key)
            }
        }
        return copy as [Key: ImplicitlyUnwrappedOptional<Value>]
    }
}

//Usage: ["ok": nil, "now": "k", "foo": nil].trimmingNullValues // = ["now": "k"]
//If you dictionary is mutable you could do this and prevent the inefficient copying:

extension Dictionary where Key == String, Value == Any? {
    mutating func trimmingNullValues() -> [String: Any] {
        forEach { (key, value) in
            if value == nil {
                removeValue(forKey: key)
            }
        }
        return self as [Key: ImplicitlyUnwrappedOptional<Value>]
    }
}

// Usage: 
// var dict: [String: Any?] = ["ok": nil, "now": "k", "foo": nil]
// dict.trimmingNullValues() // = ["now": "k"]