Add rich diagnostic; Restrict Codable & AllOfCodable to struct uses; Handle labels in enums

[mikhailmaslo]

Bug fixes

• Support of enum cases with labels

@OneOfCodable
enum Example {
  case withLabel(label: Int)
  case withUnderscoredLabel(_ label: Int)
}

• Support optionals in enum

@OneOfCodable
enum Example {
  case optionalOneOf(Int?)
}

Diagnostic

@codable, @AllOfCodable

can be only applied to a struct

@Codable
╰─ 🛑 '@Codable' macro can only be applied to a struct
enum NoApplicable {}

@Codable
╰─ 🛑 '@Codable' macro can only be applied to a struct
class NoApplicable {}

@OneOf

Require only one associated value

@OneOfCodable
enum Example {
    case a
         ┬
         ╰─ 🛑 '@OneOfCodable' macro requires each case to have one associated value
}

@OneOfCodable
enum InvalidEnum2 {
    case a(Int, Int)
         ┬──────────
         ╰─ 🛑 '@OneOfCodable' macro requires each case to have one associated value
}

Annotations

Duplicated declaration

@Codable
struct Example {
    @OmitCoding
    ╰─ 🛑 '@Omitcoding' attribute has been applied more than once. Redundant attribute applications have no effect on the generated code and may cause confusion.
    @OmitCoding
    let a: Int
}

Unsupported declarations

@Codable
struct Example {
    let a, b: Int
    ┬────────────
    ╰─ 🛑 '@Codable' macro is only applicable to declarations with an identifier followed by a type
}

@Codable
struct Example {
    let a: Int, b: Int
    ┬─────────────────
    ╰─ 🛑 '@Codable' macro is not applicable to compound declarations, declare each variable on a new line
}

@Codable
struct Example {
    @DefaultValue(IntZero)
    ╰─ 🛑 '@Codable' macro is only applicable to stored properties declared with an identifier followed by a type, example: `let variable: Int`
    var a: Int { 0 }
}