Add initial design document for Swift protocol buffers. (#1442)

* Add initial design doc for Swift protocol buffers.
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 # Protocol Buffers in Swift
 ## Objective
 This document describes the user-facing API and internal implementation of
 proto2 and proto3 messages in Apple’s Swift programming language.
 One of the key goals of protobufs is to provide idiomatic APIs for each
 language. In that vein, **interoperability with Objective-C is a non-goal of
 this proposal.** Protobuf users who need to pass messages between Objective-C
 and Swift code in the same application should use the existing Objective-C proto
 library. The goal of the effort described here is to provide an API for protobuf
 messages that uses features specific to Swift—optional types, algebraic
 enumerated types, value types, and so forth—in a natural way that will delight,
 rather than surprise, users of the language.
 ## Naming
 *   By convention, both typical protobuf message names and Swift structs/classes
    are `UpperCamelCase`, so for most messages, the name of a message can be the
    same as the name of its generated type. (However, see the discussion below
    about prefixes under [Packages](#packages).)
 *   Enum cases in protobufs typically are `UPPERCASE_WITH_UNDERSCORES`, whereas
    in Swift they are `lowerCamelCase` (as of the Swift 3 API design
    guidelines). We will transform the names to match Swift convention, using
    a whitelist similar to the Objective-C compiler plugin to handle commonly
    used acronyms.
 *   Typical fields in proto messages are `lowercase_with_underscores`, while in
    Swift they are `lowerCamelCase`. We will transform the names to match
    Swift convention by removing the underscores and uppercasing the subsequent
    letter.
 ## Swift reserved words
 Swift has a large set of reserved words—some always reserved and some
 contextually reserved (that is, they can be used as identifiers in contexts
 where they would not be confused). As of Swift 2.2, the set of always-reserved
 words is:
 ```
 _, #available, #column, #else, #elseif, #endif, #file, #function, #if, #line,
 #selector, as, associatedtype, break, case, catch, class, continue, default,
 defer, deinit, do, dynamicType, else, enum, extension, fallthrough, false, for,
 func, guard, if, import, in, init, inout, internal, is, let, nil, operator,
 private, protocol, public, repeat, rethrows, return, self, Self, static,
 struct, subscript, super, switch, throw, throws, true, try, typealias, var,
 where, while
 ```
 The set of contextually reserved words is:
 ```
 associativity, convenience, dynamic, didSet, final, get, infix, indirect,
 lazy, left, mutating, none, nonmutating, optional, override, postfix,
 precedence, prefix, Protocol, required, right, set, Type, unowned, weak,
 willSet
 ```
 It is possible to use any reserved word as an identifier by escaping it with
 backticks (for example, ``let `class` = 5``). Other name-mangling schemes would
 require us to transform the names themselves (for example, by appending an
 underscore), which requires us to then ensure that the new name does not collide
 with something else in the same namespace.
 While the backtick feature may not be widely known by all Swift developers, a
 small amount of user education can address this and it seems like the best
 approach. We can unconditionally surround all property names with backticks to
 simplify generation.
 Some remapping will still be required, though, to avoid collisions between
 generated properties and the names of methods and properties defined in the base
 protocol/implementation of messages.
 # Features of Protocol Buffers
 This section describes how the features of the protocol buffer syntaxes (proto2
 and proto3) map to features in Swift—what the code generated from a proto will
 look like, and how it will be implemented in the underlying library.
 ## Packages
 Modules are the main form of namespacing in Swift, but they are not declared
 using syntactic constructs like namespaces in C++ or packages in Java. Instead,
 they are tied to build targets in Xcode (or, in the future with open-source
 Swift, declarations in a Swift Package Manager manifest). They also do not
 easily support nesting submodules (Clang module maps support this, but pure
 Swift does not yet provide a way to define submodules).
 We will generate types with fully-qualified underscore-delimited names. For
 example, a message `Baz` in package `foo.bar` would generate a struct named
 `Foo_Bar_Baz`. For each fully-qualified proto message, there will be exactly one
 unique type symbol emitted in the generated binary.
 Users are likely to balk at the ugliness of underscore-delimited names for every
 generated type. To improve upon this situation, we will add a new string file
 level option, `swift_package_typealias`, that can be added to `.proto` files.
 When present, this will cause `typealias`es to be added to the generated Swift
 messages that replace the package name prefix with the provided string. For
 example, the following `.proto` file:
 ```protobuf
 option swift_package_typealias = "FBP";
 package foo.bar;
 message Baz {
  // Message fields
 }
 ```
 would generate the following Swift source:
 ```swift
 public struct Foo_Bar_Baz {
  // Message fields and other methods
 }
 typealias FBPBaz = Foo_Bar_Baz
 ```
 It should be noted that this type alias is recorded in the generated
 `.swiftmodule` so that code importing the module can refer to it, but it does
 not cause a new symbol to be generated in the compiled binary (i.e., we do not
 risk compiled size bloat by adding `typealias`es for every type).
 Other strategies to handle packages that were considered and rejected can be
 found in [Appendix A](#appendix-a-rejected-strategies-to-handle-packages).
 ## Messages
 Proto messages are natural value types and we will generate messages as structs
 instead of classes. Users will benefit from Swift’s built-in behavior with
 regard to mutability. We will define a `ProtoMessage` protocol that defines the
 common methods and properties for all messages (such as serialization) and also
 lets users treat messages polymorphically. Any shared method implementations
 that do not differ between individual messages can be implemented in a protocol
 extension.
 The backing storage itself for fields of a message will be managed by a
 `ProtoFieldStorage` type that uses an internal dictionary keyed by field number,
 and whose values are the value of the field with that number (up-cast to Swift’s
 `Any` type). This class will provide type-safe getters and setters so that
 generated messages can manipulate this storage, and core serialization logic
 will live here as well. Furthermore, factoring the storage out into a separate
 type, rather than inlining the fields as stored properties in the message
 itself, lets us implement copy-on-write efficiently to support passing around
 large messages. (Furthermore, because the messages themselves are value types,
 inlining fields is not possible if the fields are submessages of the same type,
 or a type that eventually includes a submessage of the same type.)
 ### Required fields (proto2 only)
 Required fields in proto2 messages seem like they could be naturally represented
 by non-optional properties in Swift, but this presents some problems/concerns.
 Serialization APIs permit partial serialization, which allows required fields to
 remain unset. Furthermore, other language APIs still provide `has*` and `clear*`
 methods for required fields, and knowing whether a property has a value when the
 message is in memory is still useful.
 For example, an e-mail draft message may have the “to” address required on the
 wire, but when the user constructs it in memory, it doesn’t make sense to force
 a value until they provide one. We only want to force a value to be present when
 the message is serialized to the wire. Using non-optional properties prevents
 this use case, and makes client usage awkward because the user would be forced
 to select a sentinel or placeholder value for any required fields at the time
 the message was created.
 ### Default values
 In proto2, fields can have a default value specified that may be a value other
 than the default value for its corresponding language type (for example, a
 default value of 5 instead of 0 for an integer). When reading a field that is
 not explicitly set, the user expects to get that value. This makes Swift
 optionals (i.e., `Foo?`) unsuitable for fields in general. Unfortunately, we
 cannot implement our own “enhanced optional” type without severely complicating
 usage (Swift’s use of type inference and its lack of implicit conversions would
 require manual unwrapping of every property value).
 Instead, we can use **implicitly unwrapped optionals.** For example, a property
 generated for a field of type `int32` would have Swift type `Int32!`. These
 properties would behave with the following characteristics, which mirror the
 nil-resettable properties used elsewhere in Apple’s SDKs (for example,
 `UIView.tintColor`):
 *   Assigning a non-nil value to a property sets the field to that value.
 *   Assigning nil to a property clears the field (its internal representation is
    nilled out).
 *   Reading the value of a property returns its value if it is set, or returns
    its default value if it is not set. Reading a property never returns nil.
 The final point in the list above implies that the optional cannot be checked to
 determine if the field is set to a value other than its default: it will never
 be nil. Instead, we must provide `has*` methods for each field to allow the user
 to check this. These methods will be public in proto2. In proto3, these methods
 will be private (if generated at all), since the user can test the returned
 value against the zero value for that type.
 ### Autocreation of nested messages
 For convenience, dotting into an unset field representing a nested message will
 return an instance of that message with default values. As in the Objective-C
 implementation, this does not actually cause the field to be set until the
 returned message is mutated. Fortunately, thanks to the way mutability of value
 types is implemented in Swift, the language automatically handles the
 reassignment-on-mutation for us. A static singleton instance containing default
 values can be associated with each message that can be returned when reading, so
 copies are only made by the Swift runtime when mutation occurs. For example,
 given the following proto:
 ```protobuf
 message Node {
  Node child = 1;
  string value = 2 [default = "foo"];
 }
 ```
 The following Swift code would act as commented, where setting deeply nested
 properties causes the copies and mutations to occur as the assignment statement
 is unwound:
 ```swift
 var node = Node()
 let s = node.child.child.value
 // 1. node.child returns the "default Node".
 // 2. Reading .child on the result of (1) returns the same default Node.
 // 3. Reading .value on the result of (2) returns the default value "foo".
 node.child.child.value = "bar"
 // 4. Setting .value on the default Node causes a copy to be made and sets
 //    the property on that copy. Subsequently, the language updates the
 //    value of "node.child.child" to point to that copy.
 // 5. Updating "node.child.child" in (4) requires another copy, because
 //    "node.child" was also the instance of the default node. The copy is
 //    assigned back to "node.child".
 // 6. Setting "node.child" in (5) is a simple value reassignment, since
 //    "node" is a mutable var.
 ```
 In other words, the generated messages do not internally have to manage parental
 relationships to backfill the appropriate properties on mutation. Swift provides
 this for free.
 ## Scalar value fields
 Proto scalar value fields will map to Swift types in the following way:
 .proto Type | Swift Type
 ----------- | -------------------
 `double`    | `Double`
 `float`     | `Float`
 `int32`     | `Int32`
 `int64`     | `Int64`
 `uint32`    | `UInt32`
 `uint64`    | `UInt64`
 `sint32`    | `Int32`
 `sint64`    | `Int64`
 `fixed32`   | `UInt32`
 `fixed64`   | `UInt64`
 `sfixed32`  | `Int32`
 `sfixed64`  | `Int64`
 `bool`      | `Bool`
 `string`    | `String`
 `bytes`     | `Foundation.NSData`
 The proto spec defines a number of integral types that map to the same Swift
 type; for example, `intXX`, `sintXX`, and `sfixedXX` are all signed integers,
 and `uintXX` and `fixedXX` are both unsigned integers. No other language
 implementation distinguishes these further, so we do not do so either. The
 rationale is that the various types only serve to distinguish how the value is
 **encoded on the wire**; once loaded in memory, the user is not concerned about
 these variations.
 Swift’s lack of implicit conversions among types will make it slightly annoying
 to use these types in a context expecting an `Int`, or vice-versa, but since
 this is a data-interchange format with explicitly-sized fields, we should not
 hide that information from the user. Users will have to explicitly write
 `Int(message.myField)`, for example.
 ## Embedded message fields
 Embedded message fields can be represented using an optional variable of the
 generated message type. Thus, the message
 ```protobuf
 message Foo {
  Bar bar = 1;
 }
 ```
 would be represented in Swift as
 ```swift
 public struct Foo: ProtoMessage {
  public var bar: Bar! {
    get { ... }
    set { ... }
  }
 }
 ```
 If the user explicitly sets `bar` to nil, or if it was never set when read from
 the wire, retrieving the value of `bar` would return a default, statically
 allocated instance of `Bar` containing default values for its fields. This
 achieves the desired behavior for default values in the same way that scalar
 fields are designed, and also allows users to deep-drill into complex object
 graphs to get or set fields without checking for nil at each step.
 ## Enum fields
 The design and implementation of enum fields will differ somewhat drastically
 depending on whether the message being generated is a proto2 or proto3 message.
 ### proto2 enums
 For proto2, we do not need to be concerned about unknown enum values, so we can
 use the simple raw-value enum syntax provided by Swift. So the following enum in
 proto2:
 ```protobuf
 enum ContentType {
  TEXT = 0;
  IMAGE = 1;
 }
 ```
 would become this Swift enum:
 ```swift
 public enum ContentType: Int32, NilLiteralConvertible {
  case text = 0
  case image = 1
  public init(nilLiteral: ()) {
    self = .text
  }
 }
 ```
 See below for the discussion about `NilLiteralConvertible`.
 ### proto3 enums
 For proto3, we need to be able to preserve unknown enum values that may come
 across the wire so that they can be written back if unmodified. We can
 accomplish this in Swift by using a case with an associated value for unknowns.
 So the following enum in proto3:
 ```protobuf
 enum ContentType {
  TEXT = 0;
  IMAGE = 1;
 }
 ```
 would become this Swift enum:
 ```swift
 public enum ContentType: RawRepresentable, NilLiteralConvertible {
  case text
  case image
  case UNKNOWN_VALUE(Int32)
  public typealias RawValue = Int32
  public init(nilLiteral: ()) {
    self = .text
  }
  public init(rawValue: RawValue) {
    switch rawValue {
      case 0: self = .text
      case 1: self = .image
      default: self = .UNKNOWN_VALUE(rawValue)
  }
  public var rawValue: RawValue {
    switch self {
      case .text: return 0
      case .image: return 1
      case .UNKNOWN_VALUE(let value): return value
    }
  }
 }
 ```
 Note that the use of a parameterized case prevents us from inheriting from the
 raw `Int32` type; Swift does not allow an enum with a raw type to have cases
 with arguments. Instead, we must implement the raw value initializer and
 computed property manually. The `UNKNOWN_VALUE` case is explicitly chosen to be
 "ugly" so that it stands out and does not conflict with other possible case
 names.
 Using this approach, proto3 consumers must always have a default case or handle
 the `.UNKNOWN_VALUE` case to satisfy case exhaustion in a switch statement; the
 Swift compiler considers it an error if switch statements are not exhaustive.
 ### NilLiteralConvertible conformance
 This is required to clean up the usage of enum-typed properties in switch
 statements. Unlike other field types, enum properties cannot be
 implicitly-unwrapped optionals without requiring that uses in switch statements
 be explicitly unwrapped. For example, if we consider a message with the enum
 above, this usage will fail to compile:
 ```swift
 // Without NilLiteralConvertible conformance on ContentType
 public struct SomeMessage: ProtoMessage {
  public var contentType: ContentType! { ... }
 }
 // ERROR: no case named text or image
 switch someMessage.contentType {
  case .text: { ... }
  case .image: { ... }
 }
 ```
 Even though our implementation guarantees that `contentType` will never be nil,
 if it is an optional type, its cases would be `some` and `none`, not the cases
 of the underlying enum type. In order to use it in this context, the user must
 write `someMessage.contentType!` in their switch statement.
 Making the enum itself `NilLiteralConvertible` permits us to make the property
 non-optional, so the user can still set it to nil to clear it (i.e., reset it to
 its default value), while eliminating the need to explicitly unwrap it in a
 switch statement.
 ```swift
 // With NilLiteralConvertible conformance on ContentType
 public struct SomeMessage: ProtoMessage {
  // Note that the property type is no longer optional
  public var contentType: ContentType { ... }
 }
 // OK: Compiles and runs as expected
 switch someMessage.contentType {
  case .text: { ... }
  case .image: { ... }
 }
 // The enum can be reset to its default value this way
 someMessage.contentType = nil
 ```
 One minor oddity with this approach is that nil will be auto-converted to the
 default value of the enum in any context, not just field assignment. In other
 words, this is valid:
 ```swift
 func foo(contentType: ContentType) { ... }
 foo(nil) // Inside foo, contentType == .text
 ```
 That being said, the advantage of being able to simultaneously support
 nil-resettability and switch-without-unwrapping outweighs this side effect,
 especially if appropriately documented. It is our hope that a new form of
 resettable properties will be added to Swift that eliminates this inconsistency.
 Some community members have already drafted or sent proposals for review that
 would benefit our designs:
 *   [SE-0030: Property Behaviors]
    (https://github.com/apple/swift-evolution/blob/master/proposals/0030-property-behavior-decls.md)
 *   [Drafted: Resettable Properties]
    (https://github.com/patters/swift-evolution/blob/master/proposals/0000-resettable-properties.md)
 ### Enum aliases
 The `allow_alias` option in protobuf slightly complicates the use of Swift enums
 to represent that type, because raw values of cases in an enum must be unique.
 Swift lets us define static variables in an enum that alias actual cases. For
 example, the following protobuf enum:
 ```protobuf
 enum Foo {
  option allow_alias = true;
  BAR = 0;
  BAZ = 0;
 }
 ```
 will be represented in Swift as:
 ```swift
 public enum Foo: Int32, NilLiteralConvertible {
  case bar = 0
  static public let baz = bar
  // ... etc.
 }
 // Can still use .baz shorthand to reference the alias in contexts
 // where the type is inferred
 ```
 That is, we use the first name as the actual case and use static variables for
 the other aliases. One drawback to this approach is that the static aliases
 cannot be used as cases in a switch statement (the compiler emits the error
 *“Enum case ‘baz’ not found in type ‘Foo’”*). However, in our own code bases,
 there are only a few places where enum aliases are not mere renamings of an
 older value, but they also don’t appear to be the type of value that one would
 expect to switch on (for example, a group of named constants representing
 metrics rather than a set of options), so this restriction is not significant.
 This strategy also implies that changing the name of an enum and adding the old
 name as an alias below the new name will be a breaking change in the generated
 Swift code.
 ## Oneof types
 The `oneof` feature represents a “variant/union” data type that maps nicely to
 Swift enums with associated values (algebraic types). These fields can also be
 accessed independently though, and, specifically in the case of proto2, it’s
 reasonable to expect access to default values when accessing a field that is not
 explicitly set.
 Taking all this into account, we can represent a `oneof` in Swift with two sets
 of constructs:
 *   Properties in the message that correspond to the `oneof` fields.
 *   A nested enum named after the `oneof` and which provides the corresponding
    field values as case arguments.
 This approach fulfills the needs of proto consumers by providing a
 Swift-idiomatic way of simultaneously checking which field is set and accessing
 its value, providing individual properties to access the default values
 (important for proto2), and safely allows a field to be moved into a `oneof`
 without breaking clients.
 Consider the following proto:
 ```protobuf
 message MyMessage {
  oneof record {
    string name = 1 [default = "unnamed"];
    int32 id_number = 2 [default = 0];
  }
 }
 ```
 In Swift, we would generate an enum, a property for that enum, and properties
 for the fields themselves:
 ```swift
 public struct MyMessage: ProtoMessage {
  public enum Record: NilLiteralConvertible {
    case name(String)
    case idNumber(Int32)
    case NOT_SET
    public init(nilLiteral: ()) { self = .NOT_SET }
  }
  // This is the "Swifty" way of accessing the value
  public var record: Record { ... }
  // Direct access to the underlying fields
  public var name: String! { ... }
  public var idNumber: Int32! { ... }
 }
 ```
 This makes both usage patterns possible:
 ```swift
 // Usage 1: Case-based dispatch
 switch message.record {
  case .name(let name):
    // Do something with name if it was explicitly set
  case .idNumber(let id):
    // Do something with id_number if it was explicitly set
  case .NOT_SET:
    // Do something if it’s not set
 }
 // Usage 2: Direct access for default value fallback
 // Sets the label text to the name if it was explicitly set, or to
 // "unnamed" (the default value for the field) if id_number was set
 // instead
 let myLabel = UILabel()
 myLabel.text = message.name
 ```
 As with proto enums, the generated `oneof` enum conforms to
 `NilLiteralConvertible` to avoid switch statement issues. Setting the property
 to nil will clear it (i.e., reset it to `NOT_SET`).
 ## Unknown Fields (proto2 only)
 To be written.
 ## Extensions (proto2 only)
 To be written.
 ## Reflection and Descriptors
 We will not include reflection or descriptors in the first version of the Swift
 library. The use cases for reflection on mobile are not as strong and the static
 data to represent the descriptors would add bloat when we wish to keep the code
 size small.
 In the future, we will investigate whether they can be included as extensions
 which might be able to be excluded from a build and/or automatically dead
 stripped by the compiler if they are not used.
 ## Appendix A: Rejected strategies to handle packages
 ### Each package is its own Swift module
 Each proto package could be declared as its own Swift module, replacing dots
 with underscores (e.g., package `foo.bar` becomes module `Foo_Bar`). Then, users
 would simply import modules containing whatever proto modules they want to use
 and refer to the generated types by their short names.
 **This solution is simply not possible, however.** Swift modules cannot
 circularly reference each other, but there is no restriction against proto
 packages doing so. Circular imports are forbidden (e.g., `foo.proto` importing
 `bar.proto` importing `foo.proto`), but nothing prevents package `foo` from
 using a type in package `bar` which uses a different type in package `foo`, as
 long as there is no import cycle. If these packages were generated as Swift
 modules, then `Foo` would contain an `import Bar` statement and `Bar` would
 contain an `import Foo` statement, and there is no way to compile this.
 ### Ad hoc namespacing with structs
 We can “fake” namespaces in Swift by declaring empty structs with private
 initializers. Since modules are constructed based on compiler arguments, not by
 syntactic constructs, and because there is no pure Swift way to define
 submodules (even though Clang module maps support this), there is no
 source-drive way to group generated code into namespaces aside from this
 approach.
 Types can be added to those intermediate package structs using Swift extensions.
 For example, a message `Baz` in package `foo.bar` could be represented in Swift
 as follows:
 ```swift
 public struct Foo {
  private init() {}
 }
 public extension Foo {
  public struct Bar {
    private init() {}
  }
 }
 public extension Foo.Bar {
  public struct Baz {
    // Message fields and other methods
  }
 }
 let baz = Foo.Bar.Baz()
 ```
 Each of these constructs would actually be defined in a separate file; Swift
 lets us keep them separate and add multiple structs to a single “namespace”
 through extensions.
 Unfortunately, these intermediate structs generate symbols of their own
 (metatype information in the data segment). This becomes problematic if multiple
 build targets contain Swift sources generated from different messages in the
 same package. At link time, these symbols would collide, resulting in multiple
 definition errors.
 This approach also has the disadvantage that there is no automatic “short” way
 to refer to the generated messages at the deepest nesting levels; since this use
 of structs is a hack around the lack of namespaces, there is no equivalent to
 import (Java) or using (C++) to simplify this. Users would have to declare type
 aliases to make this cleaner, or we would have to generate them for users.