Let CodingKey inherit CustomStringConvertible for better debugging

Author: inamiy

stdlib/public/core/Codable.swift

@@ -47,7 +47,7 @@ public typealias Codable = Encodable & Decodable
 //===----------------------------------------------------------------------===//
 
 /// A type that can be used as a key for encoding and decoding.
-public protocol CodingKey {
+public protocol CodingKey: CustomStringConvertible {
     /// The string to use in a named collection (e.g. a string-keyed dictionary).
     var stringValue: String { get }
 
@@ -67,6 +67,12 @@ public protocol CodingKey {
     init?(intValue: Int)
 }
 
+extension CodingKey {
+    public var description: String {
+        return stringValue
+    }
+}
+
 //===----------------------------------------------------------------------===//
 // Encoder & Decoder
 //===----------------------------------------------------------------------===//