Move Array.popLast to RangeReplaceableCollection

Author: airspeedswift

stdlib/public/core/Arrays.swift.gyb

@@ -1518,24 +1518,6 @@ extension ${Self} : RangeReplaceableCollection, ArrayProtocol {
   }
 
 %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 !isEmpty else { return nil }
-    return _customRemoveLast().unsafelyUnwrapped
-    let pointer = (_buffer.firstElementAddress + newCount)
-    let element = pointer.move()
-    _buffer.count = newCount
-    return element
-  }
-
   @_inlineable
   public mutating func _customRemoveLast() -> Element? {
     let newCount = _getCount() - 1

stdlib/public/core/RangeReplaceableCollection.swift

@@ -798,6 +798,24 @@ extension RangeReplaceableCollection
 }
 
 extension RangeReplaceableCollection where Self : BidirectionalCollection {
+  /// Removes and returns the last element of the collection.
+  ///
+  /// Calling this method may invalidate all saved indices of this
+  /// collection. Do not rely on a previously stored index value after
+  /// altering a collection with any operation that can change its length.
+  ///
+  /// - Returns: The last element of the collection if the collection is not
+  /// empty; otherwise, `nil`.
+  ///
+  /// - Complexity: O(1)
+  @_inlineable
+  public mutating func popLast() -> Element? {
+    if isEmpty { return nil }
+    // duplicate of removeLast logic below, to avoid redundant precondition
+    if let result = _customRemoveLast() { return result }
+    return remove(at: index(before: endIndex))
+  }
+
   /// Removes and returns the last element of the collection.
   ///
   /// The collection must not be empty.
@@ -813,9 +831,8 @@ extension RangeReplaceableCollection where Self : BidirectionalCollection {
   @discardableResult
   public mutating func removeLast() -> Element {
     _precondition(!isEmpty, "Can't remove last element from an empty collection")
-    if let result = _customRemoveLast() {
-      return result
-    }
+    // NOTE if you change this implementation, change popLast above as well
+    if let result = _customRemoveLast() { return result }
     return remove(at: index(before: endIndex))
   }