Add userInfo protocols to standard library

Karl · May 18, 2020, 6:15pm

Actually it is. See the original Codable proposal for the rationale:

apple/swift-evolution/blob/main/proposals/0166-swift-archival-serialization.md#dynamic-context-based-behavior

# Swift Archival & Serialization

* Proposal: [SE-0166](0166-swift-archival-serialization.md)
* Authors: [Itai Ferber](https://github.com/itaiferber), [Michael LeHew](https://github.com/mlehew), [Tony Parker](https://github.com/parkera)
* Review Manager: [Doug Gregor](https://github.com/DougGregor)
* Status: **Implemented (Swift 4)**
* Decision Notes: [Rationale](https://forums.swift.org/t/accepted-se-0166-swift-archival-serialization/5780)
* Implementation: [apple/swift#9004](https://github.com/apple/swift/pull/9004)

## Introduction

Foundation's current archival and serialization APIs (`NSCoding`, `NSJSONSerialization`, `NSPropertyListSerialization`, etc.), while fitting for the dynamism of Objective-C, do not always map optimally into Swift. This document lays out the design of an updated API that improves the developer experience of performing archival and serialization in Swift.

Specifically:

* It aims to provide a solution for the archival of Swift `struct` and `enum` types
* It aims to provide a more type-safe solution for serializing to external formats, such as JSON and plist

## Motivation

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CodingUserInfoKey is a bit of a useless type, though. It just wraps a string - I guess we probably should have just used String directly. It has already been acknowledged as a mistake: Why is CodingUserInfoKey's rawValue initializer failable? - #2 by itaiferber