internal func _applyMapping(_ u_strTo: _U_StrToX) -> String {
  // Allocate 16 code units on the stack.
  var fixedArray = _FixedArray16<UInt16>(allZeros: ())
  let count: Int = fixedArray.withUnsafeMutableBufferPointer { bufPtr in
    return _scalar.withUTF16CodeUnits { utf16 in
      var err = __swift_stdlib_U_ZERO_ERROR
      let correctSize = u_strTo(
        bufPtr.baseAddress._unsafelyUnwrappedUnchecked,
        Int32(bufPtr.count),
        utf16.baseAddress._unsafelyUnwrappedUnchecked,
        Int32(utf16.count),
        "",
        &err)
      guard err.isSuccess else {
        fatalError("Unexpected error case-converting Unicode scalar.")
      }
      return Int(correctSize)
    }
  }
  if _fastPath(count <= 16) {
    fixedArray.count = count
    return fixedArray.withUnsafeBufferPointer {
      return String._uncheckedFromUTF16($0)
    }
  }
  // Allocate `count` code units on the heap.
  var array = Array<UInt16>(repeating: 0, count: count)
  array.withUnsafeMutableBufferPointer { bufPtr in
    _scalar.withUTF16CodeUnits { utf16 in
      var err = __swift_stdlib_U_ZERO_ERROR
      let correctSize = u_strTo(
        bufPtr.baseAddress._unsafelyUnwrappedUnchecked,
        Int32(bufPtr.count),
        utf16.baseAddress._unsafelyUnwrappedUnchecked,
        Int32(utf16.count),
        "",
        &err)
      guard err.isSuccess else {
        fatalError("Unexpected error case-converting Unicode scalar.")
      }
      _internalInvariant(count == correctSize, "inconsistent ICU behavior")
    }
  }
  return array.withUnsafeBufferPointer {
    return String._uncheckedFromUTF16($0)
  }
}