swift-custom-dump/Sources/CustomDump/Conformances/KeyPath.swift

import Foundation

extension AnyKeyPath: CustomDumpStringConvertible {
  public var customDumpDescription: String {
    // NB: We can't currently rely on SE-0369 due to this crasher:
    // https://github.com/apple/swift/issues/64865
    //
    // #if swift(>=5.8)
    //   if #available(macOS 13.3, iOS 16.4, watchOS 9.4, tvOS 16.4, *) {
    //     return self.debugDescription
    //   }
    // #endif
    #if os(iOS) || os(macOS) || os(tvOS) || os(watchOS)
      keyPathToNameLock.lock()
      defer { keyPathToNameLock.unlock() }

      guard let name = keyPathToName[self] else {
        func reflectName() -> String {
          var namedKeyPaths = Reflection.allNamedKeyPaths(forUnderlyingTypeOf: Self.rootType)
          while !namedKeyPaths.isEmpty {
            let (name, keyPath) = namedKeyPaths.removeFirst()
            if keyPath == self {
              return #"\\#(typeName(Self.rootType)).\#(name)"#
            }
            let valueType = type(of: keyPath).valueType
            let valueNamedKeyPaths = Reflection.allNamedKeyPaths(forUnderlyingTypeOf: valueType)
            for (valueName, valueKeyPath) in valueNamedKeyPaths {
              if let appendedKeyPath = keyPath.appending(path: valueKeyPath) {
                namedKeyPaths.append(("\(name).\(valueName)", appendedKeyPath))
              }
            }
          }
          return """
            \(typeName(Self.self))<\
            \(typeName(Self.rootType, genericsAbbreviated: false)), \
            \(typeName(Self.valueType, genericsAbbreviated: false))>
            """
        }
        let name = reflectName()
        keyPathToName[self] = name
        return name
      }
      return name
    #else
      return """
        \(typeName(Self.self))<\
        \(typeName(Self.rootType, genericsAbbreviated: false)), \
        \(typeName(Self.valueType, genericsAbbreviated: false))>
        """
    #endif
  }
}

#if os(iOS) || os(macOS) || os(tvOS) || os(watchOS)
  private var keyPathToNameLock = NSRecursiveLock()
  private var keyPathToName: [AnyKeyPath: String] = [:]

  // The source code below was extracted from the "KeyPath Reflection" branch of Apple's
  // "Swift Evolution Staging" package:
  //
  // https://github.com/apple/swift-evolution-staging/tree/reflection

  //===----------------------------------------------------------------------===//
  //
  // This source file is part of the Swift.org open source project
  //
  // Copyright (c) 2020 Apple Inc. and the Swift project authors
  // Licensed under Apache License v2.0 with Runtime Library Exception
  //
  // See https://swift.org/LICENSE.txt for license information
  // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
  //
  //===----------------------------------------------------------------------===//

  private protocol RelativePointer {
    associatedtype Pointee

    var offset: Int32 { get }

    func address(from ptr: UnsafeRawPointer) -> UnsafePointer<Pointee>
    func pointee(from ptr: UnsafeRawPointer) -> Pointee?
  }

  extension RelativePointer {
    fileprivate func address(from ptr: UnsafeRawPointer) -> UnsafePointer<Pointee> {
      let newPtr = UnsafeRawPointer(
        bitPattern: UInt(bitPattern: ptr) &+ UInt(bitPattern: Int(offset)))!
      return newPtr.assumingMemoryBound(to: Pointee.self)
    }
  }

  private struct RelativeDirectPointer<Pointee>: RelativePointer {
    let offset: Int32

    func pointee(from ptr: UnsafeRawPointer) -> Pointee? {
      guard offset != 0 else {
        return nil
      }

      return address(from: ptr).pointee
    }
  }

  extension UnsafeRawPointer {
    fileprivate func relativeDirect<T>(as type: T.Type) -> UnsafePointer<T> {
      let relativePointer = RelativeDirectPointer<T>(
        offset: load(as: Int32.self)
      )
      return relativePointer.address(from: self)
    }
  }

  private struct RelativeIndirectPointer<T>: RelativePointer {
    typealias Pointee = UnsafePointer<T>

    let offset: Int32

    func pointee(from ptr: UnsafeRawPointer) -> Pointee? {
      guard offset != 0 else {
        return nil
      }

      return address(from: ptr).pointee
    }
  }

  private struct RelativeIndirectablePointer<Pointee>: RelativePointer {
    let offset: Int32

    func address(from ptr: UnsafeRawPointer) -> UnsafePointer<Pointee> {
      UnsafePointer<Pointee>((ptr + Int(offset & ~1))._rawValue)
    }

    func pointee(from ptr: UnsafeRawPointer) -> Pointee? {
      guard offset != 0 else {
        return nil
      }

      if offset & 1 == 1 {
        let pointer = UnsafeRawPointer(address(from: ptr))
          .load(as: UnsafePointer<Pointee>.self)
        return pointer.pointee
      } else {
        return address(from: ptr).pointee
      }
    }
  }

  //===----------------------------------------------------------------------===//
  // Metadata Structures
  //===----------------------------------------------------------------------===//

  // MetadataKind is the discriminator value found at the start of all metadata
  // records to determine what kind is a metadata.
  private enum MetadataKind: Int {
    case `class` = 0
    case `struct` = 512
  }

  // Metadata refers to the runtime representation of a type in Swift. This
  // protocol is the generic version handed out by various methods to retrieve
  // metadata from types.
  private protocol Metadata {
    // The required backing pointer which points at the metadata record.
    var pointer: UnsafeRawPointer { get }

    // The discriminator which determines what kind of metadata this is.
    var kind: MetadataKind { get }
  }

  extension Metadata {
    // The type representation of the metadata.
    fileprivate var type: Any.Type {
      unsafeBitCast(pointer, to: Any.Type.self)
    }
  }

  // Given an arbitrary type of anything, produce the metadata that represents
  // said type.
  // FIXME: Right now this only supports structs and class types, but in the
  // future if we ever want to produce keypaths for tuples, enums, etc. handle
  // that here.
  private func getMetadata(for type: Any.Type) -> Metadata? {
    let pointer = unsafeBitCast(type, to: UnsafeRawPointer.self)
    let int = pointer.load(as: Int.self)

    guard let kind = MetadataKind(rawValue: int) else {
      // If the metadata kind is greater than 2047, then it's an ISA pointer
      // meaning we have some class metadata.
      guard int > 2047 else {
        return nil
      }

      return ClassMetadata(pointer: pointer)
    }

    switch kind {
    case .class:
      return ClassMetadata(pointer: pointer)
    case .struct:
      return StructMetadata(pointer: pointer)
    }
  }

  // Type Metadata

  // Type Metadata is a more specialized metadata in that only struct, class, and
  // enum types conform to. There's more detail about the type and its properties
  // in the context descriptors, the generic types that make up said type, etc.
  private protocol TypeMetadata: Metadata {}

  extension TypeMetadata {
    // The context descriptors describes more in detail about the type. Some of
    // this information includes number of properties, the property names, the
    // name of this type, generic requirements, etc.
    fileprivate var contextDescriptor: TypeContextDescriptor {
      switch self {
      case let structMetadata as StructMetadata:
        return structMetadata.descriptor
      case let classMetadata as ClassMetadata:
        return classMetadata.descriptor
      default:
        fatalError("TypeMetadata conformance we don't know about?")
      }
    }

    // An array of integers that represent the offset to a certain field. This
    // corresponds to the index of fields in the field descriptor.
    fileprivate var fieldOffsets: [Int] {
      switch self {
      case let structMetadata as StructMetadata:
        return structMetadata.fieldOffsets
      case let classMetadata as ClassMetadata:
        return classMetadata.fieldOffsets
      default:
        fatalError("TypeMetadata conformance we don't know about?")
      }
    }

    // The pointer to the beginning of this type's generic arguments.
    fileprivate var genericArgumentPointer: UnsafeRawPointer {
      switch self {
      case is StructMetadata:
        return pointer + MemoryLayout<_StructMetadata>.size
      case let classMetadata as ClassMetadata:
        let descriptor = classMetadata.descriptor

        guard !descriptor.typeFlags.classHasResilientSuperclass else {
          let memberOffset = descriptor.resilientBounds._immediateMembersOffset
          return pointer + memberOffset
        }

        let negativeSize = descriptor.negativeSize
        let positiveSize = descriptor.positiveSize
        let numImmediateMembers = descriptor.numImmediateMembers

        if descriptor.typeFlags.classAreImmediateMembersNegative {
          return pointer + MemoryLayout<Int>.size * -negativeSize
        } else {
          return pointer + MemoryLayout<Int>.size * (positiveSize - numImmediateMembers)
        }
      default:
        fatalError("TypeMetadata conformance we don't know about?")
      }
    }

    // Given a mangled name (preferrably one of the property type name's), return
    // the type as represented by the mangled name within this type's context.
    fileprivate func type(of mangledName: UnsafePointer<CChar>) -> Any.Type? {
      let type = _getTypeByMangledNameInContext(
        UnsafePointer<UInt8>(mangledName._rawValue),
        UInt(getSymbolicMangledNameLength(UnsafeRawPointer(mangledName))),
        genericContext: contextDescriptor.pointer,
        genericArguments: genericArgumentPointer
      )

      return type
    }
  }

  // Struct Metadata

  // Struct Metadata refers to types whom are implemented via a struct. Consider
  // the standard library type 'Int', it's implemented using a struct, so getting
  // the type metadata for that type will return an instance of struct metadata.
  private struct StructMetadata: TypeMetadata, LayoutWrapper {
    typealias Layout = _StructMetadata

    // The backing metadata pointer.
    let pointer: UnsafeRawPointer

    // The metadata discriminator.
    var kind: MetadataKind {
      .struct
    }

    // The context descriptor of this struct.
    var descriptor: StructDescriptor {
      layout.descriptor
    }

    // An array of integers with the offsets for each stored field in this struct.
    var fieldOffsets: [Int] {
      let fieldOffsetVectorOffset = descriptor.fieldOffsetVectorOffset
      let start = pointer + MemoryLayout<Int>.size * fieldOffsetVectorOffset
      let buffer = UnsafeBufferPointer<UInt32>(
        start: UnsafePointer<UInt32>(start._rawValue),
        count: descriptor.numFields
      )
      return Array(buffer).map { Int($0) }
    }
  }

  private struct _StructMetadata {
    let kind: Int
    let descriptor: StructDescriptor
  }

  // Class Metadata

  // Class Metadata refers to types whom are implemented via a class. Consider
  // the standard library type 'KeyPath', it's implemented using a class, so
  // getting the type metadata for that type will return an instance of class
  // metadata.
  private struct ClassMetadata: TypeMetadata, LayoutWrapper {
    typealias Layout = _ClassMetadata

    // The backing metadata pointer.
    let pointer: UnsafeRawPointer

    // The metadata discriminator.
    var kind: MetadataKind {
      .class
    }

    // The context descriptor of this class.
    var descriptor: ClassDescriptor {
      layout._descriptor
    }

    // An array of integers with the offsets for each stored field in this class.
    var fieldOffsets: [Int] {
      let fieldOffsetVectorOffset = descriptor.fieldOffsetVectorOffset
      let start = pointer + MemoryLayout<Int>.size * fieldOffsetVectorOffset
      let buffer = UnsafeBufferPointer<Int>(
        start: UnsafePointer<Int>(start._rawValue),
        count: descriptor.numFields
      )
      return Array(buffer)
    }

    // The required size of instances of this type.
    var instanceSize: Int {
      Int(layout._instanceSize)
    }

    // The alignment mask of the address point for instances of this type.
    var instanceAlignMask: Int {
      Int(layout._instanceAlignMask)
    }
  }

  private struct _ClassMetadata {
    let _kind: Int
    let _superclass: Any.Type?
    let _reserved: (Int, Int)
    let _rodata: Int
    let _flags: UInt32
    let _instanceAddressPoint: UInt32
    let _instanceSize: UInt32
    let _instanceAlignMask: UInt16
    let _runtimeReserved: UInt16
    let _classSize: UInt32
    let _classAddressPoint: UInt32
    let _descriptor: ClassDescriptor
  }

  //===----------------------------------------------------------------------===//
  // Context Descriptor Structures
  //===----------------------------------------------------------------------===//

  // A context descriptor describes in entity in Swift who declares some context
  // which other declarations can be declared within.
  private protocol ContextDescriptor {
    // The backing context descriptor pointer.
    var pointer: UnsafeRawPointer { get }
  }

  extension ContextDescriptor {
    // The base structural representation of a context descriptor.
    fileprivate var _contextDescriptor: _ContextDescriptor {
      pointer.load(as: _ContextDescriptor.self)
    }

    // Flags that describe this context which include what kind it is, whether
    // or not it's a generic context, whether or not it's unique, etc.
    fileprivate var flags: ContextDescriptorFlags {
      _contextDescriptor._flags
    }
  }

  private struct _ContextDescriptor {
    let _flags: ContextDescriptorFlags
    let _parent: RelativeIndirectablePointer<_ContextDescriptor>
  }

  // Flags that describe this context which include what kind it is, whether
  // or not it's a generic context, whether or not it's unique, etc.
  private struct ContextDescriptorFlags {
    // The backing integer representation of these flags.
    let bits: UInt32

    // The reserved bits for other flags that are interpretted differently by
    // conforming context descriptor types.
    var kindSpecificFlags: UInt16 {
      UInt16((bits >> 0x10) & 0xFFFF)
    }
  }

  // Type Context Descriptor

  // A type context descriptor is a refined context descriptor who describes a
  // type in Swift. This includes structs, classes, and enums. Protocols also
  // define a new type in Swift, but aren't considered type contexts.
  private protocol TypeContextDescriptor: ContextDescriptor {
    // The field descriptor that describes the stored representation of this type.
    var fields: FieldDescriptor { get }
  }

  extension TypeContextDescriptor {
    // The base structural representation of a type context descriptor.
    fileprivate var _typeDescriptor: _TypeContextDescriptor {
      pointer.load(as: _TypeContextDescriptor.self)
    }

    // The field descriptor that describes the stored representation of this type.
    fileprivate var fields: FieldDescriptor {
      let offset = pointer.advanced(by: MemoryLayout<Int32>.size * 4)
      let address = UnsafeRawPointer(_typeDescriptor._fields.address(from: offset))
      return FieldDescriptor(signedPointer: address)
    }

    // Certain flags specific to types in Swift, such as whether or not a class
    // has a resilient superclass.
    fileprivate var typeFlags: TypeContextDescriptorFlags {
      TypeContextDescriptorFlags(bits: flags.kindSpecificFlags)
    }
  }

  private struct _TypeContextDescriptor {
    let _base: _ContextDescriptor
    let _name: RelativeDirectPointer<CChar>
    let _accessor: RelativeDirectPointer<UnsafeRawPointer>
    let _fields: RelativeDirectPointer<_FieldDescriptor>
  }

  // Certain flags specific to types in Swift, such as whether or not a class
  // has a resilient superclass.
  private struct TypeContextDescriptorFlags {
    // The backing integer representation of these flags.
    let bits: UInt16

    // Whether or not the class's members are negative.
    var classAreImmediateMembersNegative: Bool {
      bits & 0x1000 != 0
    }

    // Whether or not the class has a resilient superclass.
    var classHasResilientSuperclass: Bool {
      bits & 0x2000 != 0
    }
  }

  // Struct Descriptor

  // A struct descriptor that describes some structure context.
  private struct StructDescriptor: TypeContextDescriptor, PointerAuthenticatedLayoutWrapper {
    typealias Layout = _StructDescriptor

    // The backing context descriptor pointer.
    let signedPointer: UnsafeRawPointer

    // The offset to the field offset vector found in the metadata.
    var fieldOffsetVectorOffset: Int {
      Int(layout._fieldOffsetVectorOffset)
    }

    // The number of stored properties this struct has.
    var numFields: Int {
      Int(layout._numFields)
    }
  }

  private struct _StructDescriptor {
    let _base: _TypeContextDescriptor
    let _numFields: UInt32
    let _fieldOffsetVectorOffset: UInt32
  }

  // Class Descriptor

  // A class descriptor that descibes some class context.
  private struct ClassDescriptor: TypeContextDescriptor, PointerAuthenticatedLayoutWrapper {
    typealias Layout = _ClassDescriptor

    // The backing context descriptor pointer.
    let signedPointer: UnsafeRawPointer

    // The offset to the field offset vector found in the metadata.
    var fieldOffsetVectorOffset: Int {
      Int(layout._fieldOffsetVectorOffset)
    }

    // The negative size of the metadata objects in this class.
    var negativeSize: Int {
      assert(!typeFlags.classHasResilientSuperclass)
      return Int(layout._negativeSizeOrResilientBounds)
    }

    // The number of stored properties this class defines.
    var numFields: Int {
      Int(layout._numFields)
    }

    // The total number of members this class defines (not including it's
    // superclass, if it has one).
    var numImmediateMembers: Int {
      Int(layout._numImmediateMembers)
    }

    // The positive size of the metadata objects in this class.
    var positiveSize: Int {
      assert(!typeFlags.classHasResilientSuperclass)
      return Int(layout._positiveSizeOrExtraFlags)
    }

    // The resilient bounds for this class.
    var resilientBounds: _StoredClassMetadataBounds {
      let addr = address(for: \._negativeSizeOrResilientBounds)
      let pointer = UnsafeRawPointer(addr)
      return pointer.relativeDirect(as: _StoredClassMetadataBounds.self).pointee
    }
  }

  private struct _ClassDescriptor {
    let _base: _TypeContextDescriptor
    let _superclassMangledName: RelativeDirectPointer<CChar>
    let _negativeSizeOrResilientBounds: Int32
    let _positiveSizeOrExtraFlags: Int32
    let _numImmediateMembers: UInt32
    let _numFields: UInt32
    let _fieldOffsetVectorOffset: UInt32
  }

  private struct _StoredClassMetadataBounds {
    let _immediateMembersOffset: Int
  }

  // Field Descriptor

  // A special descriptor that describes a type's fields.
  private struct FieldDescriptor: PointerAuthenticatedLayoutWrapper {
    typealias Layout = _FieldDescriptor

    // The backing field descriptor pointer.
    let signedPointer: UnsafeRawPointer

    // The number of fields this type has. This could mean different things
    // depending on what kind of type this is found under. For example, this is
    // the number of stored properties found within a struct, but for enums this
    // is the number of cases.
    var numFields: Int {
      Int(layout._numFields)
    }

    // An array of the field record information. Field record information contains
    // things like it's mangled type, whether or not its a var, indirect, etc.
    var records: [FieldRecord] {
      var result = [FieldRecord]()
      result.reserveCapacity(numFields)

      for i in 0..<numFields {
        let address = trailing + MemoryLayout<_FieldRecord>.size * i
        result.append(FieldRecord(signedPointer: address))
      }

      return result
    }
  }

  // A record that describes a single stored property or an enum case.
  private struct FieldRecord: PointerAuthenticatedLayoutWrapper {
    typealias Layout = _FieldRecord

    // The backing field record pointer.
    let signedPointer: UnsafeRawPointer

    // The flags that describe this field record.
    var flags: FieldRecordFlags {
      layout._flags
    }

    // The mangled type name that demangles to the field's type.
    var mangledTypeName: UnsafePointer<CChar> {
      address(for: \._mangledTypeName)
    }

    // The name of the stored property/enum case.
    var name: String {
      String(cString: address(for: \._fieldName))
    }
  }

  private struct _FieldDescriptor {
    let _mangledTypeName: RelativeDirectPointer<CChar>
    let _superclassMangledTypeName: RelativeDirectPointer<CChar>
    let _kind: UInt16
    let _recordSize: UInt16
    let _numFields: UInt32
  }

  private struct _FieldRecord {
    let _flags: FieldRecordFlags
    let _mangledTypeName: RelativeDirectPointer<CChar>
    let _fieldName: RelativeDirectPointer<CChar>
  }

  // The flags which describe a field record.
  private struct FieldRecordFlags {
    // The backing integer representation of these flags.
    let bits: UInt32

    // Whether or not this stored property is a var.
    var isVar: Bool {
      bits & 0x2 != 0
    }
  }

  //===----------------------------------------------------------------------===//
  // Misc. Utilities
  //===----------------------------------------------------------------------===//

  private protocol LayoutWrapper {
    associatedtype Layout
    var pointer: UnsafeRawPointer { get }
  }

  private protocol PointerAuthenticatedLayoutWrapper: LayoutWrapper {
    var signedPointer: UnsafeRawPointer { get }
  }

  extension PointerAuthenticatedLayoutWrapper {
    fileprivate var pointer: UnsafeRawPointer {
      signedPointer
    }
  }

  extension LayoutWrapper {
    fileprivate var layout: Layout {
      pointer.load(as: Layout.self)
    }

    fileprivate var trailing: UnsafeRawPointer {
      pointer + MemoryLayout<Layout>.size
    }

    fileprivate func address<T>(for field: KeyPath<Layout, T>) -> UnsafePointer<T> {
      let offset = MemoryLayout<Layout>.offset(of: field)!
      return UnsafePointer<T>((pointer + offset)._rawValue)
    }

    fileprivate func address<T: RelativePointer, U>(
      for field: KeyPath<Layout, T>
    ) -> UnsafePointer<U> where T.Pointee == U {
      let offset = MemoryLayout<Layout>.offset(of: field)!
      return layout[keyPath: field].address(from: pointer + offset)
    }
  }

  // This is a utility within KeyPath.swift in the standard library. If this
  // gets moved into there, then this goes away, but will have to rethink if this
  // goes into a different module.
  private func getSymbolicMangledNameLength(_ base: UnsafeRawPointer) -> Int {
    var end = base
    while let current = Optional(end.load(as: UInt8.self)), current != 0 {
      // Skip the current character
      end = end + 1

      // Skip over a symbolic reference
      if current >= 0x1 && current <= 0x17 {
        end += 4
      } else if current >= 0x18 && current <= 0x1F {
        end += MemoryLayout<Int>.size
      }
    }

    return end - base
  }

  @_silgen_name("swift_allocObject")
  internal func _allocObject(_: UnsafeMutableRawPointer, _: Int, _: Int) -> AnyObject?

  // This is a utility within KeyPath.swift in the standard library. If this
  // gets moved into there, then this goes away, but will have to rethink if this
  // goes into a different module.
  extension AnyKeyPath {
    fileprivate static func _create(
      capacityInBytes bytes: Int,
      initializedBy body: (UnsafeMutableRawBufferPointer) -> Void
    ) -> Self {
      assert(
        bytes > 0 && bytes % 4 == 0,
        "capacity must be multiple of 4 bytes")
      let metadata = getMetadata(for: self) as! ClassMetadata
      var size = metadata.instanceSize

      let tailStride = MemoryLayout<Int32>.stride
      let tailAlignMask = MemoryLayout<Int32>.alignment - 1

      size += tailAlignMask
      size &= ~tailAlignMask
      size += tailStride * (bytes / 4)

      let alignment = metadata.instanceAlignMask | tailAlignMask

      let object = _allocObject(
        UnsafeMutableRawPointer(mutating: metadata.pointer),
        size,
        alignment
      )

      guard object != nil else {
        fatalError("Allocating \(self) instance failed for keypath reflection")
      }

      // This memory layout of Int by 2 is the size of a heap object which object
      // points to. Tail members appear immediately afterwards.
      let base =
        unsafeBitCast(object, to: UnsafeMutableRawPointer.self) + MemoryLayout<Int>.size * 2

      // The first word is the kvc string pointer. Set it to 0 (nil).
      base.storeBytes(of: 0, as: Int.self)

      // Return an offseted base after the kvc string pointer.
      let newBase = base + MemoryLayout<Int>.size
      let newBytes = bytes - MemoryLayout<Int>.size

      body(UnsafeMutableRawBufferPointer(start: newBase, count: newBytes))

      return unsafeBitCast(object, to: self)
    }
  }

  // Helper struct to represent the keypath buffer header. This structure is also
  // found within KeyPath.swift, so if this gets moved there this goes away.
  private struct KeyPathBufferHeader {
    let bits: UInt32

    init(hasReferencePrefix: Bool, isTrivial: Bool, size: UInt32) {
      var bits = size

      if hasReferencePrefix {
        bits |= 0x4000_0000
      }

      if isTrivial {
        bits |= 0x8000_0000
      }

      self.bits = bits
    }
  }

  // This initializes the raw keypath buffer with the field offset information.
  private func instantiateKeyPathBuffer(
    _ metadata: TypeMetadata,
    _ leafIndex: Int,
    _ data: UnsafeMutableRawBufferPointer
  ) {
    let header = KeyPathBufferHeader(
      hasReferencePrefix: false,
      isTrivial: true,
      size: UInt32(MemoryLayout<UInt32>.size)
    )

    data.storeBytes(of: header, as: KeyPathBufferHeader.self)

    var componentBits = UInt32(metadata.fieldOffsets[leafIndex])
    componentBits |= metadata.kind == .struct ? 1 << 24 : 3 << 24

    data.storeBytes(
      of: componentBits,
      toByteOffset: MemoryLayout<Int>.size,
      as: UInt32.self
    )
  }

  // Returns a concrete type for which this keypath is going to be given a root
  // and leaf type.
  private func getKeyPathType(
    from root: TypeMetadata,
    for leaf: FieldRecord
  ) -> AnyKeyPath.Type {
    let leafType = root.type(of: leaf.mangledTypeName)!

    func openRoot<Root>(_: Root.Type) -> AnyKeyPath.Type {
      func openLeaf<Value>(_: Value.Type) -> AnyKeyPath.Type {
        if leaf.flags.isVar {
          return root.kind == .class
            ? ReferenceWritableKeyPath<Root, Value>.self
            : WritableKeyPath<Root, Value>.self
        }
        return KeyPath<Root, Value>.self
      }
      return _openExistential(leafType, do: openLeaf)
    }
    return _openExistential(root.type, do: openRoot)
  }

  // Given a root type and a leaf index, create a concrete keypath object at
  // runtime.
  private func createKeyPath(root: TypeMetadata, leaf: Int) -> AnyKeyPath {
    let field = root.contextDescriptor.fields.records[leaf]

    let keyPathTy = getKeyPathType(from: root, for: field)
    let size = MemoryLayout<Int>.size * 3
    let instance = keyPathTy._create(capacityInBytes: size) {
      instantiateKeyPathBuffer(root, leaf, $0)
    }

    let heapObj = UnsafeRawPointer(Unmanaged.passRetained(instance).autorelease().toOpaque())
    let keyPath = unsafeBitCast(heapObj, to: AnyKeyPath.self)
    return keyPath
  }

  private enum Reflection {
    /// Returns the collection of all named key paths of this type.
    ///
    /// - Parameter value: A value of any type to return the stored key paths of.
    /// - Returns: An array of tuples with both the name and partial key path
    ///            for this value.
    static func allNamedKeyPaths(
      forUnderlyingTypeOf type: Any.Type
    ) -> [(name: String, keyPath: AnyKeyPath)] {
      guard let metadata = getMetadata(for: type) as? TypeMetadata else {
        return []
      }

      var result = [(name: String, keyPath: AnyKeyPath)]()
      result.reserveCapacity(metadata.contextDescriptor.fields.numFields)

      for i in 0..<metadata.contextDescriptor.fields.numFields {
        let name = metadata.contextDescriptor.fields.records[i].name
        let keyPath = createKeyPath(root: metadata, leaf: i)
        result.append((name: name, keyPath: keyPath))
      }

      return result
    }
  }
#endif