stdlib/public/core/StringUTF8.swift

//===--- StringUTF8.swift - A UTF8 view of _StringCore --------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 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
//
//===----------------------------------------------------------------------===//
//
//  _StringCore currently has three representations: Native ASCII,
//  Native UTF-16, and Opaque Cocoa.  Expose each of these as UTF-8 in a
//  way that will hopefully be efficient to traverse
//
//===----------------------------------------------------------------------===//

extension String {
  /// A view of a string's contents as a collection of UTF-8 code units.
  ///
  /// You can access a string's view of UTF-8 code units by using its `utf8`
  /// property. A string's UTF-8 view encodes the string's Unicode scalar
  /// values as 8-bit integers.
  ///
  ///     let flowers = "Flowers 💐"
  ///     for v in flowers.utf8 {
  ///         print(v)
  ///     }
  ///     // 70
  ///     // 108
  ///     // 111
  ///     // 119
  ///     // 101
  ///     // 114
  ///     // 115
  ///     // 32
  ///     // 240
  ///     // 159
  ///     // 146
  ///     // 144
  ///
  /// A string's Unicode scalar values can be up to 21 bits in length. To
  /// represent those scalar values using 8-bit integers, more than one UTF-8
  /// code unit is often required.
  ///
  ///     let flowermoji = "💐"
  ///     for v in flowermoji.unicodeScalars {
  ///         print(v, v.value)
  ///     }
  ///     // 💐 128144
  ///
  ///     for v in flowermoji.utf8 {
  ///         print(v)
  ///     }
  ///     // 240
  ///     // 159
  ///     // 146
  ///     // 144
  ///
  /// In the encoded representation of a Unicode scalar value, each UTF-8 code
  /// unit after the first is called a *continuation byte*.
  ///
  /// UTF8View Elements Match Encoded C Strings
  /// =========================================
  ///
  /// Swift streamlines interoperation with C string APIs by letting you pass a
  /// `String` instance to a function as an `Int8` or `UInt8` pointer. When you
  /// call a C function using a `String`, Swift automatically creates a buffer
  /// of UTF-8 code units and passes a pointer to that buffer. The code units
  /// of that buffer match the code units in the string's `utf8` view.
  ///
  /// The following example uses the C `strncmp` function to compare the
  /// beginning of two Swift strings. The `strncmp` function takes two
  /// `const char*` pointers and an integer specifying the number of characters
  /// to compare. Because the strings are identical up to the 14th character,
  /// comparing only those characters results in a return value of `0`.
  ///
  ///     let s1 = "They call me 'Bell'"
  ///     let s2 = "They call me 'Stacey'"
  ///
  ///     print(strncmp(s1, s2, 14))
  ///     // Prints "0"
  ///     print(String(s1.utf8.prefix(14)))
  ///     // Prints "They call me '"
  ///
  /// Extending the compared character count to 15 includes the differing
  /// characters, so a nonzero result is returned.
  ///
  ///     print(strncmp(s1, s2, 15))
  ///     // Prints "-17"
  ///     print(String(s1.utf8.prefix(15)))
  ///     // Prints "They call me 'B"
  @_fixed_layout // FIXME(sil-serialize-all)
  public struct UTF8View
    : BidirectionalCollection,
      CustomStringConvertible, 
      CustomDebugStringConvertible {

    /// Underlying UTF-16-compatible representation 
    @_versioned
    internal let _core: _StringCore

    /// Distances to `(startIndex, endIndex)` from the endpoints of _core,
    /// measured in UTF-8 code units.
    ///
    /// Note: this is *only* here to support legacy Swift3-style slicing where
    /// `s.utf8[i..<j]` produces a `String.UTF8View`, and should be removed when
    /// those semantics are no longer supported.
    @_versioned
    internal let _legacyOffsets: (start: Int8, end: Int8)

    @_inlineable // FIXME(sil-serialize-all)
    @_versioned // FIXME(sil-serialize-all)
    internal init(_ _core: _StringCore,
      legacyOffsets: (Int, Int) = (0, 0)
    ) {
      self._core = _core
      self._legacyOffsets = (Int8(legacyOffsets.0), Int8(legacyOffsets.1))
    }

    public typealias Index = String.Index

    /// The position of the first code unit if the UTF-8 view is
    /// nonempty.
    ///
    /// If the UTF-8 view is empty, `startIndex` is equal to `endIndex`.
    @_inlineable // FIXME(sil-serialize-all)
    public var startIndex: Index {
      let r = _index(atEncodedOffset: _core.startIndex)
      if _legacyOffsets.start == 0 { return r }
      return index(r, offsetBy: numericCast(_legacyOffsets.start))
    }

    /// The "past the end" position---that is, the position one
    /// greater than the last valid subscript argument.
    ///
    /// In an empty UTF-8 view, `endIndex` is equal to `startIndex`.
    @_inlineable // FIXME(sil-serialize-all)
    public var endIndex: Index {
      _sanityCheck(_legacyOffsets.end >= -3 && _legacyOffsets.end <= 0,
        "out of bounds legacy end")

      var r = Index(encodedOffset: _core.endIndex)
      if _fastPath(_legacyOffsets.end == 0) {
        return r
      }
      switch _legacyOffsets.end {
      case -3: r = index(before: r); fallthrough
      case -2: r = index(before: r); fallthrough
      case -1: return index(before: r)
      default: Builtin.unreachable()
      }
    }

    @_inlineable // FIXME(sil-serialize-all)
    @_versioned
    internal func _index(atEncodedOffset n: Int) -> Index {
      if _fastPath(_core.isASCII) { return Index(encodedOffset: n) }
      if n == _core.endIndex { return endIndex }
      
      var p = UTF16.ForwardParser()
      var i = _core[n...].makeIterator()
      var buffer = Index._UTF8Buffer()
    Loop:
      while true {
        switch p.parseScalar(from: &i) {
        case .valid(let u16):
          let u8 = Unicode.UTF8.transcode(u16, from: Unicode.UTF16.self)
           ._unsafelyUnwrappedUnchecked
          if buffer.count + u8.count > buffer.capacity { break Loop }
          buffer.append(contentsOf: u8)
        case .error:
          let u8 = Unicode.UTF8.encodedReplacementCharacter
          if buffer.count + u8.count > buffer.capacity { break Loop }
          buffer.append(contentsOf: u8)
        case .emptyInput:
          break Loop
        }
      }
      return Index(encodedOffset: n, .utf8(buffer: buffer))
    }
  
    /// Returns the next consecutive position after `i`.
    ///
    /// - Precondition: The next position is representable.
    @_inlineable // FIXME(sil-serialize-all)
    @inline(__always)
    public func index(after i: Index) -> Index {
      if _fastPath(_core.isASCII) {
        precondition(i.encodedOffset < _core.count)
        return Index(encodedOffset: i.encodedOffset + 1)
      }

      var j = i
      
      // Ensure j's cache is utf8
      if _slowPath(j._cache.utf8 == nil) {
        j = _index(atEncodedOffset: j.encodedOffset)
        precondition(j != endIndex, "Index out of bounds")
      }
      
      let buffer = j._cache.utf8._unsafelyUnwrappedUnchecked
      
      var scalarLength16 = 1
      let b0 = buffer.first._unsafelyUnwrappedUnchecked
      var nextBuffer = buffer

      let leading1s = (~b0).leadingZeroBitCount
      if _fastPath(leading1s == 0) { // ASCII in buffer; just consume it
        nextBuffer.removeFirst()
      }
      else {
        // Number of bytes consumed in this scalar
        let n8 = j._transcodedOffset + 1
        // If we haven't reached a scalar boundary...
        if _fastPath(n8 < leading1s) {
          // Advance to the next position in this scalar
          return Index(
            encodedOffset: j.encodedOffset,
            transcodedOffset: n8, .utf8(buffer: buffer))
        }
        // We reached a scalar boundary; compute the underlying utf16's width
        // based on the number of utf8 code units
        scalarLength16 = n8 >> 2 + 1
        nextBuffer.removeFirst(n8)
      }

      if _fastPath(!nextBuffer.isEmpty) {        
        return Index(
          encodedOffset: j.encodedOffset + scalarLength16,
          .utf8(buffer: nextBuffer))
      }
      // If nothing left in the buffer, refill it.
      return _index(atEncodedOffset: j.encodedOffset + scalarLength16)
    }

    @_inlineable // FIXME(sil-serialize-all)
    public func index(before i: Index) -> Index {
      if _fastPath(_core.isASCII) {
        precondition(i.encodedOffset > 0)
        return Index(encodedOffset: i.encodedOffset - 1)
      }
      
      if i._transcodedOffset != 0 {
        _sanityCheck(i._cache.utf8 != nil)
        var r = i
        r._compoundOffset = r._compoundOffset &- 1
        return r
      }
      
      // Handle the scalar boundary the same way as the not-a-utf8-index case.
      
      // Parse a single scalar
      var p =  Unicode.UTF16.ReverseParser()
      var s = _core[..<i.encodedOffset].reversed().makeIterator()
      let u8: Unicode.UTF8.EncodedScalar
      switch p.parseScalar(from: &s) {
      case .valid(let u16):
        u8 = Unicode.UTF8.transcode(
          u16, from: Unicode.UTF16.self)._unsafelyUnwrappedUnchecked
      case .error:
        u8 = Unicode.UTF8.encodedReplacementCharacter
      case .emptyInput:
        _preconditionFailure("Index out of bounds")
      }
      return Index(
        encodedOffset: i.encodedOffset &- (u8.count < 4 ? 1 : 2),
        transcodedOffset: u8.count &- 1,
        .utf8(buffer: String.Index._UTF8Buffer(u8))
      )
    }
    
    @_inlineable // FIXME(sil-serialize-all)
    public func distance(from i: Index, to j: Index) -> Int {
      if _fastPath(_core.isASCII) {
        return j.encodedOffset - i.encodedOffset
      }
      return j >= i
        ? _forwardDistance(from: i, to: j) : -_forwardDistance(from: j, to: i)
    }

    @_inlineable // FIXME(sil-serialize-all)
    @_versioned
    @inline(__always)
    internal func _forwardDistance(from i: Index, to j: Index) -> Int {
      var r = j._transcodedOffset - i._transcodedOffset
      UTF8._transcode(
        _core[i.encodedOffset..<j.encodedOffset], from: UTF16.self) {
        r += $0.count
      }
      return r
    }
    
    /// Accesses the code unit at the given position.
    ///
    /// The following example uses the subscript to print the value of a
    /// string's first UTF-8 code unit.
    ///
    ///     let greeting = "Hello, friend!"
    ///     let i = greeting.utf8.startIndex
    ///     print("First character's UTF-8 code unit: \(greeting.utf8[i])")
    ///     // Prints "First character's UTF-8 code unit: 72"
    ///
    /// - Parameter position: A valid index of the view. `position`
    ///   must be less than the view's end index.
    @_inlineable // FIXME(sil-serialize-all)
    public subscript(position: Index) -> UTF8.CodeUnit {
      @inline(__always)
      get {
        if _fastPath(_core.asciiBuffer != nil), let ascii = _core.asciiBuffer {
          _precondition(position < endIndex, "Index out of bounds")
          return ascii[position.encodedOffset]
        }
        var j = position
        while true {
          if case .utf8(let buffer) = j._cache {
            _onFastPath()
            return buffer[
              buffer.index(buffer.startIndex, offsetBy: j._transcodedOffset)]
          }
          j = _index(atEncodedOffset: j.encodedOffset)
          precondition(j < endIndex, "Index out of bounds")
        }
      }
    }

    @_inlineable // FIXME(sil-serialize-all)
    public var description: String {
      return String(_core)
    }

    @_inlineable // FIXME(sil-serialize-all)
    public var debugDescription: String {
      return "UTF8View(\(self.description.debugDescription))"
    }
  }

  /// A UTF-8 encoding of `self`.
  @_inlineable // FIXME(sil-serialize-all)
  public var utf8: UTF8View {
    get {
      return UTF8View(self._core)
    }
    set {
      self = String(describing: newValue)
    }
  }

  /// A contiguously stored null-terminated UTF-8 representation of the string.
  ///
  /// To access the underlying memory, invoke `withUnsafeBufferPointer` on the
  /// array.
  ///
  ///     let s = "Hello!"
  ///     let bytes = s.utf8CString
  ///     print(bytes)
  ///     // Prints "[72, 101, 108, 108, 111, 33, 0]"
  ///
  ///     bytes.withUnsafeBufferPointer { ptr in
  ///         print(strlen(ptr.baseAddress!))
  ///     }
  ///     // Prints "6"
  @_inlineable // FIXME(sil-serialize-all)
  public var utf8CString: ContiguousArray<CChar> {
    var result = ContiguousArray<CChar>()
    result.reserveCapacity(utf8.count + 1)
    for c in utf8 {
      result.append(CChar(bitPattern: c))
    }
    result.append(0)
    return result
  }

  @_inlineable // FIXME(sil-serialize-all)
  @_versioned // FIXME(sil-serialize-all)
  internal func _withUnsafeBufferPointerToUTF8<R>(
    _ body: (UnsafeBufferPointer<UTF8.CodeUnit>) throws -> R
  ) rethrows -> R {
    if let asciiBuffer = self._core.asciiBuffer {
      return try body(UnsafeBufferPointer(
        start: asciiBuffer.baseAddress,
        count: asciiBuffer.count))
    }
    var nullTerminatedUTF8 = ContiguousArray<UTF8.CodeUnit>()
    nullTerminatedUTF8.reserveCapacity(utf8.count + 1)
    nullTerminatedUTF8 += utf8
    nullTerminatedUTF8.append(0)
    return try nullTerminatedUTF8.withUnsafeBufferPointer(body)
  }

  /// Creates a string corresponding to the given sequence of UTF-8 code units.
  ///
  /// If `utf8` is an ill-formed UTF-8 code sequence, the result is `nil`.
  ///
  /// You can use this initializer to create a new string from a slice of
  /// another string's `utf8` view.
  ///
  ///     let picnicGuest = "Deserving porcupine"
  ///     if let i = picnicGuest.utf8.index(of: 32) {
  ///         let adjective = String(picnicGuest.utf8[..<i])
  ///         print(adjective)
  ///     }
  ///     // Prints "Optional(Deserving)"
  ///
  /// The `adjective` constant is created by calling this initializer with a
  /// slice of the `picnicGuest.utf8` view.
  ///
  /// - Parameter utf8: A UTF-8 code sequence.
  @_inlineable // FIXME(sil-serialize-all)
  @available(swift, deprecated: 3.2,
    message: "Failable initializer was removed in Swift 4. When upgrading to Swift 4, please use non-failable String.init(_:UTF8View)")
  @available(swift, obsoleted: 4.0,
    message: "Please use non-failable String.init(_:UTF8View) instead")
  public init?(_ utf8: UTF8View) {
    if utf8.startIndex._transcodedOffset != 0
    || utf8.endIndex._transcodedOffset != 0 {
      return nil
    }
    // Attempt to recover the whole string, the better to implement the actual
    // Swift 3.1 semantics, which are not as documented above!  Full Swift 3.1
    // semantics may be impossible to preserve in the case of string literals,
    // since we no longer have access to the length of the original string when
    // there is no owner and elements have been dropped from the end.
    if let nativeBuffer = utf8._core.nativeBuffer {
      let wholeString = String(_StringCore(nativeBuffer))
      let offset = (utf8._core._baseAddress! - nativeBuffer.start)
        &>> utf8._core.elementShift

      if Index(
        encodedOffset: utf8.startIndex.encodedOffset + offset
      ).samePosition(in: wholeString) == nil
      || Index(
        encodedOffset: utf8.endIndex.encodedOffset + offset
      ).samePosition(in: wholeString) == nil {
        return nil
      }
    }
    self = String(utf8._core)
  }

  /// Creates a string corresponding to the given sequence of UTF-8 code units.
  @_inlineable // FIXME(sil-serialize-all)
  @available(swift, introduced: 4.0, message:
    "Please use failable String.init?(_:UTF8View) when in Swift 3.2 mode")
  public init(_ utf8: UTF8View) {
    self = String(utf8._core)
  }

  /// The index type for subscripting a string.
  public typealias UTF8Index = UTF8View.Index
}

extension String.UTF8View : _SwiftStringView {
  @_inlineable // FIXME(sil-serialize-all)
  @_versioned // FIXME(sil-serialize-all)
  internal var _persistentContent : String { return String(self._core) }
}

extension String.UTF8View {
  @_fixed_layout // FIXME(sil-serialize-all)
  public struct Iterator {
    internal typealias _OutputBuffer = UInt64
    @_versioned // FIXME(sil-serialize-all)
    internal let _source: _StringCore
    @_versioned // FIXME(sil-serialize-all)
    internal var _sourceIndex: Int
    @_versioned // FIXME(sil-serialize-all)
    internal var _buffer: _OutputBuffer
  }
  public func makeIterator() -> Iterator {
    return Iterator(_core)
  }
}

extension String.UTF8View.Iterator : IteratorProtocol {
  @_inlineable // FIXME(sil-serialize-all)
  @_versioned // FIXME(sil-serialize-all)
  internal init(_ source: _StringCore) {
    _source = source
    _sourceIndex = 0
    _buffer = 0
  }
  
  @_inlineable // FIXME(sil-serialize-all)
  public mutating func next() -> Unicode.UTF8.CodeUnit? {
    if _fastPath(_buffer != 0) {
      let r = UInt8(truncatingIfNeeded: _buffer) &- 1
      _buffer >>= 8
      return r
    }
    if _slowPath(_sourceIndex == _source.count) { return nil }

    defer { _fixLifetime(_source) }
    
    if _fastPath(_source._unmanagedASCII != nil),
    let ascii = _source._unmanagedASCII {
      let result = ascii[_sourceIndex]
      _sourceIndex += 1
      for i in 0 ..< _OutputBuffer.bitWidth>>3 {
        if _sourceIndex == _source.count { break }
        _buffer |= _OutputBuffer(ascii[_sourceIndex] &+ 1) &<< (i << 3)
        _sourceIndex += 1
      }
      return result
    }
    
    if _fastPath(_source._unmanagedUTF16 != nil),
    let utf16 = _source._unmanagedUTF16 {
      return _next(refillingFrom: utf16)
    }
    return _next(refillingFrom: _source)
  }

  @_inlineable // FIXME(sil-serialize-all)
  @_versioned // FIXME(sil-serialize-all)
  internal mutating func _next<Source: Collection>(
    refillingFrom source: Source
  ) -> Unicode.UTF8.CodeUnit?
  where Source.Element == Unicode.UTF16.CodeUnit,
  Source.Index == Int
  {
    _sanityCheck(_buffer == 0)
    var shift = 0

    // ASCII fastpath
    while _sourceIndex != _source.endIndex && shift < _OutputBuffer.bitWidth {
      let u = _source[_sourceIndex]
      if u >= 0x80 { break }
      _buffer |= _OutputBuffer(UInt8(truncatingIfNeeded: u &+ 1)) &<< shift
      _sourceIndex += 1
      shift = shift &+ 8
    }
    
    var i = IndexingIterator(_elements: source, _position: _sourceIndex)
    var parser = Unicode.UTF16.ForwardParser()
  Loop:
    while true {
      let u8: UTF8.EncodedScalar
      switch parser.parseScalar(from: &i) {
      case .valid(let s):
        u8 = UTF8.transcode(s, from: UTF16.self)._unsafelyUnwrappedUnchecked
      case .error(_):
        u8 = UTF8.encodedReplacementCharacter
      case .emptyInput:
        break Loop
      }
      var newBuffer = _buffer
      for x in u8 {
        newBuffer |= _OutputBuffer(x &+ 1) &<< shift
        shift = shift &+ 8
      }
      guard _fastPath(shift <= _OutputBuffer.bitWidth) else { break Loop }
      _buffer = newBuffer
      _sourceIndex = i._position &- parser._buffer.count
    }
    guard _fastPath(_buffer != 0) else { return nil }
    let result = UInt8(truncatingIfNeeded: _buffer) &- 1
    _buffer >>= 8
    return result
  }
}

extension String.UTF8View {
  @_inlineable // FIXME(sil-serialize-all)
  public var count: Int {
    if _fastPath(_core.isASCII) { return _core.count }
    let b = _core._unmanagedUTF16
    if _fastPath(b != nil) {
      defer { _fixLifetime(_core) }
      return _count(fromUTF16: b!)
    }
    return _count(fromUTF16: self._core)
  }

  @_inlineable // FIXME(sil-serialize-all)
  @_versioned // FIXME(sil-serialize-all)
  internal func _count<Source: Sequence>(fromUTF16 source: Source) -> Int
  where Source.Element == Unicode.UTF16.CodeUnit
  {
    var result = 0
    var prev: Unicode.UTF16.CodeUnit = 0
    for u in source {
      switch u {
      case 0..<0x80: result += 1
      case 0x80..<0x800: result += 2
      case 0x800..<0xDC00: result += 3
      case 0xDC00..<0xE000: result += UTF16.isLeadSurrogate(prev) ? 1 : 3
      default: result += 3
      }
      prev = u
    }
    return result
  }
}

// Index conversions
extension String.UTF8View.Index {
  /// Creates an index in the given UTF-8 view that corresponds exactly to the
  /// specified `UTF16View` position.
  ///
  /// The following example finds the position of a space in a string's `utf16`
  /// view and then converts that position to an index in the string's
  /// `utf8` view.
  ///
  ///     let cafe = "Café 🍵"
  ///
  ///     let utf16Index = cafe.utf16.index(of: 32)!
  ///     let utf8Index = String.UTF8View.Index(utf16Index, within: cafe.utf8)!
  ///
  ///     print(Array(cafe.utf8[..<utf8Index]))
  ///     // Prints "[67, 97, 102, 195, 169]"
  ///
  /// If the position passed in `utf16Index` doesn't have an exact
  /// corresponding position in `utf8`, the result of the initializer is
  /// `nil`. For example, because UTF-8 and UTF-16 represent high Unicode code
  /// points differently, an attempt to convert the position of the trailing
  /// surrogate of a UTF-16 surrogate pair fails.
  ///
  /// The next example attempts to convert the indices of the two UTF-16 code
  /// points that represent the teacup emoji (`"🍵"`). The index of the lead
  /// surrogate is successfully converted to a position in `utf8`, but the
  /// index of the trailing surrogate is not.
  ///
  ///     let emojiHigh = cafe.utf16.index(after: utf16Index)
  ///     print(String.UTF8View.Index(emojiHigh, within: cafe.utf8))
  ///     // Prints "Optional(String.Index(...))"
  ///
  ///     let emojiLow = cafe.utf16.index(after: emojiHigh)
  ///     print(String.UTF8View.Index(emojiLow, within: cafe.utf8))
  ///     // Prints "nil"
  ///
  /// - Parameters:
  ///   - sourcePosition: A position in a `String` or one of its views.
  ///   - target: The `UTF8View` in which to find the new position.
  @_inlineable // FIXME(sil-serialize-all)
  public init?(_ sourcePosition: String.Index, within target: String.UTF8View) {
    switch sourcePosition._cache {
    case .utf8:
      self.init(encodedOffset: sourcePosition.encodedOffset,
        transcodedOffset:sourcePosition._transcodedOffset, sourcePosition._cache)

    default:
      guard String.UnicodeScalarView(target._core)._isOnUnicodeScalarBoundary(
        sourcePosition) else { return nil }
      self.init(encodedOffset: sourcePosition.encodedOffset)
    }
  }
}

// Reflection
extension String.UTF8View : CustomReflectable {
  /// Returns a mirror that reflects the UTF-8 view of a string.
  @_inlineable // FIXME(sil-serialize-all)
  public var customMirror: Mirror {
    return Mirror(self, unlabeledChildren: self)
  }
}

extension String.UTF8View : CustomPlaygroundQuickLookable {
  @_inlineable // FIXME(sil-serialize-all)
  @available(*, deprecated, message: "UTF8View.customPlaygroundQuickLook will be removed in a future Swift version")
  public var customPlaygroundQuickLook: PlaygroundQuickLook {
    return .text(description)
  }
}

// backward compatibility for index interchange.  
extension String.UTF8View {
  @_inlineable // FIXME(sil-serialize-all)
  @available(
    swift, obsoleted: 4.0,
    message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
  public func index(after i: Index?) -> Index {
    return index(after: i!)
  }
  @_inlineable // FIXME(sil-serialize-all)
  @available(
    swift, obsoleted: 4.0,
    message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
  public func index(_ i: Index?, offsetBy n: Int) -> Index {
    return index(i!, offsetBy: n)
  }
  @_inlineable // FIXME(sil-serialize-all)
  @available(
    swift, obsoleted: 4.0,
    message: "Any String view index conversion can fail in Swift 4; please unwrap the optional indices")
  public func distance(
    from i: Index?, to j: Index?) -> Int {
    return distance(from: i!, to: j!)
  }
  @_inlineable // FIXME(sil-serialize-all)
  @available(
    swift, obsoleted: 4.0,
    message: "Any String view index conversion can fail in Swift 4; please unwrap the optional index")
  public subscript(i: Index?) -> Unicode.UTF8.CodeUnit {
    return self[i!]
  }
}

//===--- Slicing Support --------------------------------------------------===//
/// In Swift 3.2, in the absence of type context,
///
///   someString.utf8[someString.utf8.startIndex..<someString.utf8.endIndex]
///
/// was deduced to be of type `String.UTF8View`.  Provide a more-specific
/// Swift-3-only `subscript` overload that continues to produce
/// `String.UTF8View`.
extension String.UTF8View {
  public typealias SubSequence = Substring.UTF8View
  
  @_inlineable // FIXME(sil-serialize-all)
  @available(swift, introduced: 4)
  public subscript(r: Range<Index>) -> String.UTF8View.SubSequence {
    return String.UTF8View.SubSequence(self, _bounds: r)
  }

  @_inlineable // FIXME(sil-serialize-all)
  @available(swift, obsoleted: 4)
  public subscript(r: Range<Index>) -> String.UTF8View {
    if r.upperBound._transcodedOffset == 0 {
      return String.UTF8View(
        _core[r.lowerBound.encodedOffset..<r.upperBound.encodedOffset],
        legacyOffsets: (r.lowerBound._transcodedOffset, 0))
    }

    let b0 = r.upperBound._cache.utf8!.first!
    let scalarLength8 = (~b0).leadingZeroBitCount
    let scalarLength16 = scalarLength8 == 4 ? 2 : 1
    let coreEnd = r.upperBound.encodedOffset + scalarLength16
    return String.UTF8View(
      _core[r.lowerBound.encodedOffset..<coreEnd],
      legacyOffsets: (
        r.lowerBound._transcodedOffset,
        r.upperBound._transcodedOffset - scalarLength8))
  }

  @_inlineable // FIXME(sil-serialize-all)
  @available(swift, obsoleted: 4)
  public subscript(bounds: ClosedRange<Index>) -> String.UTF8View {
    return self[bounds.relative(to: self)]
  }
}