Swift - Why String doesn't conform to RawRepresentable?-CodePudding

I'm learning Swift and cannot realize why this code is correct:

enum Test1: String {
    case value
}

let test1 = Test1.value.rawValue

but this one is incorrect and shows me errors

struct MyStruct {
}

extension MyStruct: Equatable {
    static func == (lhs: MyStruct, rhs: MyStruct) -> Bool {
        return true
    }
}

enum Test2: MyStruct {
    case value
}

I browsed thru Swift.String sources and didn't find rawValue declaration. How does it work in Swift? Is String a built-in type that "automatically" conforms to RawRepresentable, but all other types have to explicitly declare its conformance?

CodePudding user response：

Notice that Test.value has type Test1, not String.

There is special treatment (implicit conformance to RawRepresentable), but it applies to string-valued enums, not String itself.

CodePudding user response：

Raw values can be strings, characters, or any of the integer or floating-point number types. Each raw value must be unique within its enumeration declaration.

https://docs.swift.org/swift-book/LanguageGuide/Enumerations.html

But that's only for using the colon-based shortcut syntax. It's no problem to manually conform.

enum Test2 {
  case value
}

extension Test2: RawRepresentable {
  init?(rawValue: MyStruct) {
    self = .value
  }
  
  var rawValue: MyStruct { .init() }
}