stdlib: Don't request additional capacity in Array/ContiguousArray's remove.

Just copy the buffer if it's not unique.
This also implies that if there is a copy-on-write in remove, "shrink" the capacity of the new buffer to the required amount of elements (instead of copying the capacity of the original buffer).

Author: eeckstein

stdlib/public/core/Array.swift

@@ -1244,9 +1244,9 @@ extension Array: RangeReplaceableCollection {
 
   @inlinable
   public mutating func _customRemoveLast() -> Element? {
+    _makeMutableAndUnique()
     let newCount = _getCount() - 1
     _precondition(newCount >= 0, "Can't removeLast from an empty Array")
-    _makeUniqueAndReserveCapacityIfNotUnique()
     let pointer = (_buffer.firstElementAddress + newCount)
     let element = pointer.move()
     _buffer.count = newCount
@@ -1271,10 +1271,11 @@ extension Array: RangeReplaceableCollection {
   @inlinable
   @discardableResult
   public mutating func remove(at index: Int) -> Element {
-    _precondition(index < endIndex, "Index out of range")
-    _precondition(index >= startIndex, "Index out of range")
-    _makeUniqueAndReserveCapacityIfNotUnique()
-    let newCount = _getCount() - 1
+    _makeMutableAndUnique()
+    let currentCount = _getCount()
+    _precondition(index < currentCount, "Index out of range")
+    _precondition(index >= 0, "Index out of range")
+    let newCount = currentCount - 1
     let pointer = (_buffer.firstElementAddress + index)
     let result = pointer.move()
     pointer.moveInitialize(from: pointer + 1, count: newCount - index)

stdlib/public/core/ContiguousArray.swift

@@ -903,9 +903,10 @@ extension ContiguousArray: RangeReplaceableCollection {
   @inlinable
   @discardableResult
   public mutating func remove(at index: Int) -> Element {
-    _precondition(index < endIndex, "Index out of range")
-    _precondition(index >= startIndex, "Index out of range")
-    _makeUniqueAndReserveCapacityIfNotUnique()
+    _makeMutableAndUnique()
+    let currentCount = _getCount()
+    _precondition(index < currentCount, "Index out of range")
+    _precondition(index >= 0, "Index out of range")
     let newCount = _getCount() - 1
     let pointer = (_buffer.firstElementAddress + index)
     let result = pointer.move()

test/stdlib/Inputs/CommonArrayTests.gyb

@@ -106,12 +106,34 @@ ${Suite}.test("${ArrayType}/appendNonUnique")
   var x: ${ArrayType}<Int> = []
   x.reserveCapacity(10002)
   let capacity = x.capacity
-  for _ in 1...10000 {
+  for _ in 1...100 {
     let y = x
     x.append(1)
     expectTrue(x.capacity == capacity)
-    let z = x
+  }
+}
+
+%   if ArrayType != 'ArraySlice':
+${Suite}.test("${ArrayType}/removeNonUnique")
+  .code {
+  var x = ${ArrayType}<Int>(repeating: 27, count: 200)
+  x.reserveCapacity(10002)
+  for _ in 1...100 {
+    let y = x
     x.remove(at: 0)
+    expectTrue(x.capacity < 1000)
+  }
+}
+%   end
+
+${Suite}.test("${ArrayType}/mutateNonUnique")
+  .code {
+  var x = ${ArrayType}<Int>(repeating: 27, count: 200)
+  x.reserveCapacity(10002)
+  for _ in 1...100 {
+    let y = x
+    x[0] = 0
+    expectTrue(x.capacity < 1000)
   }
 }