stdlib/public/core/UnsafeBufferPointer.swift.gyb

//===--- UnsafeBufferPointer.swift.gyb ------------------------*- swift -*-===//
//
// 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
//
//===----------------------------------------------------------------------===//


% for mutable in (True, False):
%  Self = 'UnsafeMutableBufferPointer' if mutable else 'UnsafeBufferPointer'
%  Mutable = 'Mutable' if mutable else ''
/// A nonowning collection interface to a buffer of ${Mutable.lower()}
/// elements stored contiguously in memory.
///
/// You can use an `${Self}` instance in low level operations to eliminate
/// uniqueness checks and, in release mode, bounds checks. Bounds checks are
/// always performed in debug mode.
///
/// ${Self} Semantics
/// =================
///
/// An `${Self}` instance is a view into memory and does not own the memory
/// that it references. Copying a value of type `${Self}` does not copy the
/// instances stored in the underlying memory. However, initializing another
/// collection with an `${Self}` instance copies the instances out of the
/// referenced memory and into the new collection.
// FIXME: rdar://18157434 - until this is fixed, this has to be fixed layout
// to avoid a hang in Foundation, which has the following setup:
// struct A { struct B { let x: UnsafeMutableBufferPointer<...> } let b: B }
@_fixed_layout // unsafe-performance
public struct Unsafe${Mutable}BufferPointer<Element> {

  @usableFromInline
  let _position: Unsafe${Mutable}Pointer<Element>?

  /// The number of elements in the buffer.
  ///
  /// If the `baseAddress` of this buffer is `nil`, the count is zero. However,
  /// a buffer can have a `count` of zero even with a non-`nil` base address.
  public let count: Int
}

%if not mutable:
extension UnsafeBufferPointer {
  /// An iterator for the elements in the buffer referenced by an
  /// `UnsafeBufferPointer` or `UnsafeMutableBufferPointer` instance.
  @_fixed_layout // unsafe-performance
  public struct Iterator {
    @usableFromInline
    internal var _position, _end: UnsafePointer<Element>?

    @inlinable // unsafe-performance
    public init(_position: UnsafePointer<Element>?, _end: UnsafePointer<Element>?) {
        self._position = _position
        self._end = _end
    }
  }
}

extension UnsafeBufferPointer.Iterator: IteratorProtocol {
  /// Advances to the next element and returns it, or `nil` if no next element
  /// exists.
  ///
  /// Once `nil` has been returned, all subsequent calls return `nil`.
  @inlinable // unsafe-performance
  public mutating func next() -> Element? {
    guard let start = _position else {
      return nil
    }
    _internalInvariant(_end != nil, "inconsistent _position, _end pointers")

    if start == _end._unsafelyUnwrappedUnchecked { return nil }

    let result = start.pointee
    _position  = start + 1
    return result
  }
}
%else:
extension UnsafeMutableBufferPointer {
  public typealias Iterator = UnsafeBufferPointer<Element>.Iterator
}
%end

extension Unsafe${Mutable}BufferPointer: Sequence {
  /// Returns an iterator over the elements of this buffer.
  ///
  /// - Returns: An iterator over the elements of this buffer.
  @inlinable // unsafe-performance
  public func makeIterator() -> Iterator {
    guard let start = _position else {
      return Iterator(_position: nil, _end: nil)
    }
    return Iterator(_position: start, _end: start + count)
  }

  /// Initializes the memory at `destination.baseAddress` with elements of `self`,
  /// stopping when either `self` or `destination` is exhausted.
  ///
  /// - Returns: an iterator over any remaining elements of `self` and the
  ///   number of elements initialized.
  @inlinable // unsafe-performance
  public func _copyContents(
    initializing destination: UnsafeMutableBufferPointer<Element>
  ) -> (Iterator, UnsafeMutableBufferPointer<Element>.Index) {
    guard !isEmpty && !destination.isEmpty else { return (makeIterator(), 0) }
    let s = self.baseAddress._unsafelyUnwrappedUnchecked
    let d = destination.baseAddress._unsafelyUnwrappedUnchecked
    let n = Swift.min(destination.count, self.count)
    d.initialize(from: s, count: n)
    return (Iterator(_position: s + n, _end: s + count), n)
  }
}

extension Unsafe${Mutable}BufferPointer: ${Mutable}Collection, RandomAccessCollection {
  public typealias Index = Int
  public typealias Indices = Range<Int>

  /// The index of the first element in a nonempty buffer.
  ///
  /// The `startIndex` property of an `Unsafe${Mutable}BufferPointer` instance
  /// is always zero.
  @inlinable // unsafe-performance
  public var startIndex: Int { return 0 }

  /// The "past the end" position---that is, the position one greater than the
  /// last valid subscript argument.
  ///
  /// The `endIndex` property of an `Unsafe${Mutable}BufferPointer` instance is
  /// always identical to `count`.
  @inlinable // unsafe-performance
  public var endIndex: Int { return count }

  @inlinable // unsafe-performance
  public func index(after i: Int) -> Int {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    return i + 1
  }

  @inlinable // unsafe-performance
  public func formIndex(after i: inout Int) {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    i += 1
  }

  @inlinable // unsafe-performance
  public func index(before i: Int) -> Int {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    return i - 1
  }

  @inlinable // unsafe-performance
  public func formIndex(before i: inout Int) {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    i -= 1
  }

  @inlinable // unsafe-performance
  public func index(_ i: Int, offsetBy n: Int) -> Int {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    return i + n
  }

  @inlinable // unsafe-performance
  public func index(_ i: Int, offsetBy n: Int, limitedBy limit: Int) -> Int? {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    let l = limit - i
    if n > 0 ? l >= 0 && l < n : l <= 0 && n < l {
      return nil
    }
    return i + n
  }

  @inlinable // unsafe-performance
  public func distance(from start: Int, to end: Int) -> Int {
    // NOTE: this is a manual specialization of index movement for a Strideable
    // index that is required for UnsafeBufferPointer performance. The
    // optimizer is not capable of creating partial specializations yet.
    // NOTE: Range checks are not performed here, because it is done later by
    // the subscript function.
    return end - start
  }

  @inlinable // unsafe-performance
  public func _failEarlyRangeCheck(_ index: Int, bounds: Range<Int>) {
    // NOTE: In release mode, this method is a no-op for performance reasons.
    _debugPrecondition(index >= bounds.lowerBound)
    _debugPrecondition(index < bounds.upperBound)
  }

  @inlinable // unsafe-performance
  public func _failEarlyRangeCheck(_ range: Range<Int>, bounds: Range<Int>) {
    // NOTE: In release mode, this method is a no-op for performance reasons.
    _debugPrecondition(range.lowerBound >= bounds.lowerBound)
    _debugPrecondition(range.upperBound <= bounds.upperBound)
  }

  @inlinable // unsafe-performance
  public var indices: Indices {
    return startIndex..<endIndex
  }

  /// Accesses the element at the specified position.
  ///
%if Mutable:
  /// The following example uses the buffer pointer's subscript to access and
  /// modify the elements of a mutable buffer pointing to the contiguous
  /// contents of an array:
  ///
  ///     var numbers = [1, 2, 3, 4, 5]
  ///     numbers.withUnsafeMutableBufferPointer { buffer in
  ///         for i in stride(from: buffer.startIndex, to: buffer.endIndex - 1, by: 2) {
  ///             let x = buffer[i]
  ///             buffer[i + 1] = buffer[i]
  ///             buffer[i] = x
  ///         }
  ///     }
  ///     print(numbers)
  ///     // Prints "[2, 1, 4, 3, 5]"
%else:
  /// The following example uses the buffer pointer's subscript to access every
  /// other element of the buffer:
  ///
  ///     let numbers = [1, 2, 3, 4, 5]
  ///     let sum = numbers.withUnsafeBufferPointer { buffer -> Int in
  ///         var result = 0
  ///         for i in stride(from: buffer.startIndex, to: buffer.endIndex, by: 2) {
  ///             result += buffer[i]
  ///         }
  ///         return result
  ///     }
  ///     // 'sum' == 9
%end
  ///
  /// - Note: Bounds checks for `i` are performed only in debug mode.
  ///
  /// - Parameter i: The position of the element to access. `i` must be in the
  ///   range `0..<count`.
  @inlinable // unsafe-performance
  public subscript(i: Int) -> Element {
    get {
      _debugPrecondition(i >= 0)
      _debugPrecondition(i < endIndex)
      return _position._unsafelyUnwrappedUnchecked[i]
    }
%if Mutable:
    nonmutating _modify {
      _debugPrecondition(i >= 0)
      _debugPrecondition(i < endIndex)
      yield &_position._unsafelyUnwrappedUnchecked[i]
    }
%end
  }

  // Skip all debug and runtime checks

  @inlinable // unsafe-performance
  internal subscript(_unchecked i: Int) -> Element {
    get {
      _internalInvariant(i >= 0)
      _internalInvariant(i < endIndex)
      return _position._unsafelyUnwrappedUnchecked[i]
    }
%if Mutable:
    nonmutating _modify {
      _internalInvariant(i >= 0)
      _internalInvariant(i < endIndex)
      yield &_position._unsafelyUnwrappedUnchecked[i]
    }
%end
  }

  /// Accesses a contiguous subrange of the buffer's elements.
  ///
  /// The accessed slice uses the same indices for the same elements as the
  /// original buffer uses. Always use the slice's `startIndex` property
  /// instead of assuming that its indices start at a particular value.
  ///
  /// This example demonstrates getting a slice from a buffer of strings, finding
  /// the index of one of the strings in the slice, and then using that index
  /// in the original buffer.
  ///
%if Mutable:
  ///     var streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
  ///     streets.withUnsafeMutableBufferPointer { buffer in
  ///         let streetSlice = buffer[2..<buffer.endIndex]
  ///         print(Array(streetSlice))
  ///         // Prints "["Channing", "Douglas", "Evarts"]"
  ///         let index = streetSlice.firstIndex(of: "Evarts")    // 4
  ///         buffer[index!] = "Eustace"
  ///     }
  ///     print(streets.last!)
  ///     // Prints "Eustace"
%else:
  ///     let streets = ["Adams", "Bryant", "Channing", "Douglas", "Evarts"]
  ///     streets.withUnsafeBufferPointer { buffer in
  ///         let streetSlice = buffer[2..<buffer.endIndex]
  ///         print(Array(streetSlice))
  ///         // Prints "["Channing", "Douglas", "Evarts"]"
  ///         let index = streetSlice.firstIndex(of: "Evarts")    // 4
  ///         print(buffer[index!])
  ///         // Prints "Evarts"
  ///     }
%end
  ///
  /// - Note: Bounds checks for `bounds` are performed only in debug mode.
  ///
  /// - Parameter bounds: A range of the buffer's indices. The bounds of
  ///   the range must be valid indices of the buffer.
  @inlinable // unsafe-performance
  public subscript(bounds: Range<Int>)
    -> Slice<Unsafe${Mutable}BufferPointer<Element>>
  {
    get {
      _debugPrecondition(bounds.lowerBound >= startIndex)
      _debugPrecondition(bounds.upperBound <= endIndex)
      return Slice(
        base: self, bounds: bounds)
    }
%  if Mutable:
    nonmutating set {
      _debugPrecondition(bounds.lowerBound >= startIndex)
      _debugPrecondition(bounds.upperBound <= endIndex)
      _debugPrecondition(bounds.count == newValue.count)

      // FIXME: swift-3-indexing-model: tests.
      if !newValue.isEmpty {
        (_position! + bounds.lowerBound).assign(
          from: newValue.base._position! + newValue.startIndex,
          count: newValue.count)
      }
    }
%  end
  }
% if mutable:

  /// Exchanges the values at the specified indices of the buffer.
  ///
  /// Both parameters must be valid indices of the buffer, and not
  /// equal to `endIndex`. Passing the same index as both `i` and `j` has no
  /// effect.
  ///
  /// - Parameters:
  ///   - i: The index of the first value to swap.
  ///   - j: The index of the second value to swap.
  @inlinable // unsafe-performance
  public func swapAt(_ i: Int, _ j: Int) {
    guard i != j else { return }
    _debugPrecondition(i >= 0 && j >= 0)
    _debugPrecondition(i < endIndex && j < endIndex)
    let pi = (_position! + i)
    let pj = (_position! + j)
    let tmp = pi.move()
    pi.moveInitialize(from: pj, count: 1)
    pj.initialize(to: tmp)
  }
% end # mutable
}

extension Unsafe${Mutable}BufferPointer {
  /// Creates a new buffer pointer over the specified number of contiguous
  /// instances beginning at the given pointer.
  ///
  /// - Parameters:
  ///   - start: A pointer to the start of the buffer, or `nil`. If `start` is
  ///     `nil`, `count` must be zero. However, `count` may be zero even for a
  ///     non-`nil` `start`. The pointer passed as `start` must be aligned to
  ///     `MemoryLayout<Element>.alignment`.
  ///   - count: The number of instances in the buffer. `count` must not be
  ///     negative.
  @inlinable // unsafe-performance
  public init(start: Unsafe${Mutable}Pointer<Element>?, count: Int) {
    _precondition(
      count >= 0, "Unsafe${Mutable}BufferPointer with negative count")
    _precondition(
      count == 0 || start != nil,
      "Unsafe${Mutable}BufferPointer has a nil start and nonzero count")
    _position = start
    self.count = count
  }

  @inlinable // unsafe-performance
  public init(_empty: ()) {
    _position = nil
    count = 0
  }

%  if Mutable:

  /// Creates a mutable typed buffer pointer referencing the same memory as the 
  /// given immutable buffer pointer.
  ///
  /// - Parameter other: The immutable buffer pointer to convert.
  @inlinable // unsafe-performance
  public init(mutating other: UnsafeBufferPointer<Element>) {
    _position = UnsafeMutablePointer<Element>(mutating: other._position)
    count = other.count
  }

  @inlinable
  public mutating func _withUnsafeMutableBufferPointerIfSupported<R>(
    _ body: (inout UnsafeMutableBufferPointer<Element>) throws -> R
  ) rethrows -> R? {
    return try body(&self)
  }

  @inlinable
  public mutating func withContiguousMutableStorageIfAvailable<R>(
    _ body: (inout UnsafeMutableBufferPointer<Element>) throws -> R
  ) rethrows -> R? {
    let (oldBase, oldCount) = (self.baseAddress, self.count)
    defer { 
      _debugPrecondition((oldBase, oldCount) == (self.baseAddress, self.count),
      "UnsafeMutableBufferPointer.withUnsafeMutableBufferPointer: replacing the buffer is not allowed")
    } 
    return try body(&self)
  }

  @inlinable
  public func withContiguousStorageIfAvailable<R>(
    _ body: (UnsafeBufferPointer<Element>) throws -> R
  ) rethrows -> R? {
    return try body(UnsafeBufferPointer(self))
  }

%  else:

  /// Creates an immutable typed buffer pointer referencing the same memory as the 
  /// given mutable buffer pointer.
  ///
  /// - Parameter other: The mutable buffer pointer to convert.
  @inlinable // unsafe-performance
  public init(_ other: UnsafeMutableBufferPointer<Element>) {
    _position = UnsafePointer<Element>(other._position)
    count = other.count
  }

  @inlinable
  public func withContiguousStorageIfAvailable<R>(
    _ body: (UnsafeBufferPointer<Element>) throws -> R
  ) rethrows -> R? {
    return try body(self)
  }

%  end
  
%  if not Mutable:

  /// Creates a buffer over the same memory as the given buffer slice.
  ///
  /// The new buffer represents the same region of memory as `slice`, but is
  /// indexed starting at zero instead of sharing indices with the original
  /// buffer. For example:
  ///
  ///     let buffer = returnsABuffer()
  ///     let n = 5
  ///     let slice = buffer[n...]
  ///     let rebased = UnsafeBufferPointer(rebasing: slice)
  ///
  /// After rebasing `slice` as the `rebased` buffer, the following are true:
  ///
  /// - `rebased.startIndex == 0`
  /// - `rebased[0] == slice[n]`
  /// - `rebased[0] == buffer[n]`
  /// - `rebased.count == slice.count`
  ///
  /// - Parameter slice: The buffer slice to rebase.
  @inlinable // unsafe-performance
  public init(rebasing slice: Slice<UnsafeBufferPointer<Element>>) {
    let base = slice.base.baseAddress?.advanced(by: slice.startIndex)
    self.init(start: base, count: slice.count)
  }

%  end

  /// Creates a buffer over the same memory as the given buffer slice.
  ///
  /// The new buffer represents the same region of memory as `slice`, but is
  /// indexed starting at zero instead of sharing indices with the original
  /// buffer. For example:
  ///
  ///     let buffer = returnsABuffer()
  ///     let n = 5
  ///     let slice = buffer[n...]
  ///     let rebased = Unsafe${Mutable}BufferPointer(rebasing: slice)
  ///
  /// After rebasing `slice` as the `rebased` buffer, the following are true:
  ///
  /// - `rebased.startIndex == 0`
  /// - `rebased[0] == slice[n]`
  /// - `rebased[0] == buffer[n]`
  /// - `rebased.count == slice.count`
  ///
  /// - Parameter slice: The buffer slice to rebase.
  @inlinable // unsafe-performance
  public init(rebasing slice: Slice<UnsafeMutableBufferPointer<Element>>) {
    let base = slice.base.baseAddress?.advanced(by: slice.startIndex)
    self.init(start: base, count: slice.count)
  }

  /// Deallocates the memory block previously allocated at this buffer pointer’s 
  /// base address. 
  ///
  /// This buffer pointer's `baseAddress` must be `nil` or a pointer to a memory 
  /// block previously returned by a Swift allocation method. If `baseAddress` is 
  /// `nil`, this function does nothing. Otherwise, the memory must not be initialized 
  /// or `Pointee` must be a trivial type. This buffer pointer's `count` must 
  /// be equal to the originally allocated size of the memory block.
  @inlinable // unsafe-performance
  public func deallocate() {
    _position?.deallocate()
  }

% if Mutable:

  /// Allocates uninitialized memory for the specified number of instances of
  /// type `Element`.
  ///
  /// The resulting buffer references a region of memory that is bound to
  /// `Element` and is `count * MemoryLayout<Element>.stride` bytes in size.
  /// 
  /// The following example allocates a buffer that can store four `Int` 
  /// instances and then initializes that memory with the elements of a range:
  /// 
  ///     let buffer = UnsafeMutableBufferPointer<Int>.allocate(capacity: 4)
  ///     _ = buffer.initialize(from: 1...4)
  ///     print(buffer[2])
  ///     // Prints "3"
  ///
  /// When you allocate memory, always remember to deallocate once you're
  /// finished.
  ///
  ///     buffer.deallocate()
  ///
  /// - Parameter count: The amount of memory to allocate, counted in instances
  ///   of `Element`. 
  @inlinable // unsafe-performance
  public static func allocate(capacity count: Int) 
    -> UnsafeMutableBufferPointer<Element> {
    let size = MemoryLayout<Element>.stride * count
    // For any alignment <= _minAllocationAlignment, force alignment = 0.
    // This forces the runtime's "aligned" allocation path so that
    // deallocation does not require the original alignment.
    //
    // The runtime guarantees:
    //
    // align == 0 || align > _minAllocationAlignment:
    //   Runtime uses "aligned allocation".
    //
    // 0 < align <= _minAllocationAlignment:
    //   Runtime may use either malloc or "aligned allocation".
    var align = Builtin.alignof(Element.self)
    if Int(align) <= _minAllocationAlignment() {
      align = (0)._builtinWordValue
    }
    let raw  = Builtin.allocRaw(size._builtinWordValue, align)
    Builtin.bindMemory(raw, count._builtinWordValue, Element.self)
    return UnsafeMutableBufferPointer(
      start: UnsafeMutablePointer(raw), count: count)
  }
  
  /// Initializes every element in this buffer's memory to a copy of the given value.
  ///
  /// The destination memory must be uninitialized or the buffer's `Element`
  /// must be a trivial type. After a call to `initialize(repeating:)`, the
  /// entire region of memory referenced by this buffer is initialized.
  ///
  /// - Parameters:
  ///   - repeatedValue: The instance to initialize this buffer's memory with.
  @inlinable // unsafe-performance
  public func initialize(repeating repeatedValue: Element) {
    guard let dstBase = _position else {
      return
    }

    dstBase.initialize(repeating: repeatedValue, count: count)
  }
  
  /// Assigns every element in this buffer's memory to a copy of the given value.
  ///
  /// The buffer’s memory must be initialized or the buffer's `Element`
  /// must be a trivial type. 
  ///
  /// - Parameters:
  ///   - repeatedValue: The instance to assign this buffer's memory to.
  ///
  /// Warning: All buffer elements must be initialized before calling this. 
  /// Assigning to part of the buffer must be done using the `assign(repeating:count:)`` 
  /// method on the buffer’s `baseAddress`. 
  @inlinable // unsafe-performance
  public func assign(repeating repeatedValue: Element) {
    guard let dstBase = _position else {
      return
    }

    dstBase.assign(repeating: repeatedValue, count: count)
  }
  
% end

  /// Executes the given closure while temporarily binding the memory referenced 
  /// by this buffer to the given type.
  ///
  /// Use this method when you have a buffer of memory bound to one type and
  /// you need to access that memory as a buffer of another type. Accessing
  /// memory as type `T` requires that the memory be bound to that type. A
  /// memory location may only be bound to one type at a time, so accessing
  /// the same memory as an unrelated type without first rebinding the memory
  /// is undefined.
  ///
  /// The entire region of memory referenced by this buffer must be initialized.
  /// 
  /// Because this buffer's memory is no longer bound to its `Element` type
  /// while the `body` closure executes, do not access memory using the
  /// original buffer from within `body`. Instead, use the `body` closure's
  /// buffer argument to access the values in memory as instances of type
  /// `T`.
  ///
  /// After executing `body`, this method rebinds memory back to the original
  /// `Element` type.
  ///
  /// - Note: Only use this method to rebind the buffer's memory to a type
  ///   with the same size and stride as the currently bound `Element` type.
  ///   To bind a region of memory to a type that is a different size, convert
  ///   the buffer to a raw buffer and use the `bindMemory(to:)` method.
  ///
  /// - Parameters:
  ///   - type: The type to temporarily bind the memory referenced by this
  ///     buffer. The type `T` must have the same size and be layout compatible
  ///     with the pointer's `Element` type.
  ///   - body: A closure that takes a ${Mutable.lower()} typed buffer to the
  ///     same memory as this buffer, only bound to type `T`. The buffer argument 
  ///     contains the same number of complete instances of `T` as the original  
  ///     buffer’s `count`. The closure's buffer argument is valid only for the 
  ///     duration of the closure's execution. If `body` has a return value, that 
  ///     value is also used as the return value for the `withMemoryRebound(to:_:)` 
  ///     method.
  /// - Returns: The return value, if any, of the `body` closure parameter.
  @inlinable // unsafe-performance
  public func withMemoryRebound<T, Result>(
    to type: T.Type, _ body: (${Self}<T>) throws -> Result
  ) rethrows -> Result {
    if let base = _position {
      _debugPrecondition(MemoryLayout<Element>.stride == MemoryLayout<T>.stride)
      Builtin.bindMemory(base._rawValue, count._builtinWordValue, T.self)
      defer {
        Builtin.bindMemory(base._rawValue, count._builtinWordValue, Element.self)
      }

      return try body(${Self}<T>(
        start: Unsafe${Mutable}Pointer<T>(base._rawValue), count: count))
    }
    else {
      return try body(${Self}<T>(start: nil, count: 0))
    }
  }

  /// A pointer to the first element of the buffer.
  ///
  /// If the `baseAddress` of this buffer is `nil`, the count is zero. However,
  /// a buffer can have a `count` of zero even with a non-`nil` base address.
  @inlinable // unsafe-performance
  public var baseAddress: Unsafe${Mutable}Pointer<Element>? {
    return _position
  }
}

extension Unsafe${Mutable}BufferPointer : CustomDebugStringConvertible {
  /// A textual representation of the buffer, suitable for debugging.
  public var debugDescription: String {
    return "Unsafe${Mutable}BufferPointer"
      + "(start: \(_position.map(String.init(describing:)) ?? "nil"), count: \(count))"
  }
}
%end


extension UnsafeMutableBufferPointer {
  /// Initializes the buffer's memory with the given elements.
  ///
  /// When calling the `initialize(from:)` method on a buffer `b`, the memory
  /// referenced by `b` must be uninitialized or the `Element` type must be a
  /// trivial type. After the call, the memory referenced by this buffer up
  /// to, but not including, the returned index is initialized. The buffer
  /// must contain sufficient memory to accommodate
  /// `source.underestimatedCount`.
  ///
  /// The returned index is the position of the element in the buffer one past
  /// the last element written. If `source` contains no elements, the returned
  /// index is equal to the buffer's `startIndex`. If `source` contains an
  /// equal or greater number of elements than the buffer can hold, the
  /// returned index is equal to the buffer's `endIndex`.
  ///
  /// - Parameter source: A sequence of elements with which to initializer the
  ///   buffer.
  /// - Returns: An iterator to any elements of `source` that didn't fit in the
  ///   buffer, and an index to the point in the buffer one past the last
  ///   element written.
  @inlinable // unsafe-performance
  public func initialize<S: Sequence>(from source: S) -> (S.Iterator, Index)
    where S.Element == Element {
    return source._copyContents(initializing: self)
  }
}

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