Add custom implementations of removeLast and remove(at:) to Array (#14273)

Author: airspeedswift

stdlib/public/core/Arrays.swift.gyb

@@ -1493,17 +1493,6 @@ extension ${Self} : RangeReplaceableCollection, ArrayProtocol {
   }
 
 %if Self == 'ArraySlice':
-  /// Removes and returns the last element of the array.
-  ///
-  /// The array must not be empty. This example removes the last number from an
-  /// array of `Double` values.
-  ///
-  ///     var measurements: [Double] = [1.1, 1.5, 2.9, 1.2, 1.5, 1.3, 1.2]
-  ///     let removed = measurements.removeLast()
-  ///     print(measurements)
-  ///     // Prints "[1.1, 1.5, 2.9, 1.2, 1.5, 1.3]"
-  ///
-  /// - Returns: The element that was removed.
   @_inlineable
   public mutating func _customRemoveLast() -> Element? {
     _precondition(count > 0, "Can't removeLast from an empty ${Self}")
@@ -1519,31 +1508,58 @@ extension ${Self} : RangeReplaceableCollection, ArrayProtocol {
     self.replaceSubrange((i &- 1)..<i, with: EmptyCollection())
     return result
   }
-%end
-
-  /// Inserts a new element at the specified position.
-  ///
-  /// The new element is inserted before the element currently at the specified
-  /// index. If you pass the array's `endIndex` property as the `index`
-  /// parameter, the new element is appended to the array.
+  
+  /// Removes and returns the element at the specified position.
   ///
-  ///     var numbers = [1, 2, 3, 4, 5]
-  ///     numbers.insert(100, at: 3)
-  ///     numbers.insert(200, at: numbers.endIndex)
+  /// All the elements following the specified position are moved up to
+  /// close the gap.
   ///
-  ///     print(numbers)
-  ///     // Prints "[1, 2, 3, 100, 4, 5, 200]"
+  ///     var measurements: [Double] = [1.1, 1.5, 2.9, 1.2, 1.5, 1.3, 1.2]
+  ///     let removed = measurements.remove(at: 2)
+  ///     print(measurements)
+  ///     // Prints "[1.1, 1.5, 1.2, 1.5, 1.3, 1.2]"
   ///
-  /// - Parameter newElement: The new element to insert into the array.
-  /// - Parameter i: The position at which to insert the new element.
-  ///   `index` must be a valid index of the array or equal to its `endIndex`
-  ///   property.
+  /// - Parameter index: The position of the element to remove. `index` must
+  ///   be a valid index of the array.
+  /// - Returns: The element at the specified index.
   ///
   /// - Complexity: O(*n*), where *n* is the length of the array.
   @_inlineable
-  public mutating func insert(_ newElement: Element, at i: Int) {
-    _checkIndex(i)
-    self.replaceSubrange(i..<i, with: CollectionOfOne(newElement))
+  @discardableResult
+  public mutating func remove(at index: Int) -> Element {
+    let result = self[index]
+    self.replaceSubrange(index..<(index + 1), with: EmptyCollection())
+    return result
+  }
+
+%else:
+  /// Removes and returns the last element of the array.
+  ///
+  /// - Returns: The last element of the array if the array is not empty;
+  ///   otherwise, `nil`.
+  ///
+  /// - Complexity: O(*n*) if the array is bridged, where *n* is the length
+  ///   of the array; otherwise, O(1).
+  @_inlineable
+  public mutating func popLast() -> Element? {
+    let newCount = _getCount() - 1
+    guard newCount >= 0 else { return nil }
+    _makeUniqueAndReserveCapacityIfNotUnique()
+    let pointer = (_buffer.firstElementAddress + newCount)
+    let element = pointer.move()
+    _buffer.count = newCount
+    return element
+  }
+
+  @_inlineable
+  public mutating func _customRemoveLast() -> Element? {
+    let newCount = _getCount() - 1
+    _precondition(newCount >= 0, "Can't removeLast from an empty ${Self}")
+    _makeUniqueAndReserveCapacityIfNotUnique()
+    let pointer = (_buffer.firstElementAddress + newCount)
+    let element = pointer.move()
+    _buffer.count = newCount
+    return element
   }
 
   /// Removes and returns the element at the specified position.
@@ -1564,10 +1580,42 @@ extension ${Self} : RangeReplaceableCollection, ArrayProtocol {
   @_inlineable
   @discardableResult
   public mutating func remove(at index: Int) -> Element {
-    let result = self[index]
-    self.replaceSubrange(index..<(index + 1), with: EmptyCollection())
+    _precondition(index < endIndex, "Index out of range")
+    _precondition(index >= startIndex, "Index out of range")
+    _makeUniqueAndReserveCapacityIfNotUnique()
+    let newCount = _getCount() - 1
+    let pointer = (_buffer.firstElementAddress + index)
+    let result = pointer.move()
+    pointer.moveInitialize(from: pointer + 1, count: newCount - index)
+    _buffer.count = newCount
     return result
   }
+%end
+
+  /// Inserts a new element at the specified position.
+  ///
+  /// The new element is inserted before the element currently at the specified
+  /// index. If you pass the array's `endIndex` property as the `index`
+  /// parameter, the new element is appended to the array.
+  ///
+  ///     var numbers = [1, 2, 3, 4, 5]
+  ///     numbers.insert(100, at: 3)
+  ///     numbers.insert(200, at: numbers.endIndex)
+  ///
+  ///     print(numbers)
+  ///     // Prints "[1, 2, 3, 100, 4, 5, 200]"
+  ///
+  /// - Parameter newElement: The new element to insert into the array.
+  /// - Parameter i: The position at which to insert the new element.
+  ///   `index` must be a valid index of the array or equal to its `endIndex`
+  ///   property.
+  ///
+  /// - Complexity: O(*n*), where *n* is the length of the array.
+  @_inlineable
+  public mutating func insert(_ newElement: Element, at i: Int) {
+    _checkIndex(i)
+    self.replaceSubrange(i..<i, with: CollectionOfOne(newElement))
+  }
 
   /// Removes all elements from the array.
   ///
@@ -2358,35 +2406,6 @@ extension Array {
 }
 #endif
 
-extension Array {
-  /// Removes and returns the last element of the array.
-  ///
-  /// - Returns: The last element of the array if the array is not empty;
-  ///   otherwise, `nil`.
-  ///
-  /// - Complexity: O(*n*) if the array is bridged, where *n* is the length
-  ///   of the array; otherwise, O(1).
-  @_inlineable
-  public mutating func popLast() -> Element? {
-    guard !isEmpty else { return nil }
-    return removeLast()
-  }
-}
-
-extension ContiguousArray {
-  /// Removes and returns the last element of the array.
-  ///
-  /// - Returns: The last element of the array if the array is not empty;
-  ///   otherwise, `nil`.
-  ///
-  /// - Complexity: O(1)
-  @_inlineable
-  public mutating func popLast() -> Element? {
-    guard !isEmpty else { return nil }
-    return removeLast()
-  }
-}
-
 extension ArraySlice {
   @_inlineable
   public // @testable