[stdlib] Move _copyBuffer to be an init on _ArrayBufferProtocol (#14222)

* Move _copyBuffer to be an init on _ArrayBufferProtocol

* Make init(copying:) inlineable, even though its inline(never), to allow specialization.

* Delete old array swap test, as swap on arrays is no longer valid.

* Delete the old static method

Author: airspeedswift

stdlib/public/core/ArrayBufferProtocol.swift

@@ -24,6 +24,8 @@ internal protocol _ArrayBufferProtocol
   /// Adopt the entire buffer, presenting it at the provided `startIndex`.
   init(_buffer: _ContiguousArrayBuffer<Element>, shiftedToStartIndex: Int)
 
+  init(copying buffer: Self)
+
   /// Copy the elements in `bounds` from this buffer into uninitialized
   /// memory starting at `target`.  Return a pointer "past the end" of the
   /// just-initialized memory.
@@ -124,14 +126,27 @@ internal protocol _ArrayBufferProtocol
   var endIndex: Int { get }
 }
 
-extension _ArrayBufferProtocol {
+extension _ArrayBufferProtocol where Indices == CountableRange<Int>{
 
   @_inlineable
   @_versioned
   internal var subscriptBaseAddress: UnsafeMutablePointer<Element> {
     return firstElementAddress
   }
 
+  // Make sure the compiler does not inline _copyBuffer to reduce code size.
+  @_inlineable
+  @inline(never)
+  @_versioned
+  internal init(copying buffer: Self) {
+    let newBuffer = _ContiguousArrayBuffer<Element>(
+      _uninitializedCount: buffer.count, minimumCapacity: buffer.count)
+    buffer._copyContents(
+      subRange: Range(buffer.indices),
+      initializing: newBuffer.firstElementAddress)
+    self = Self( _buffer: newBuffer, shiftedToStartIndex: buffer.startIndex)
+  }
+
   @_inlineable
   @_versioned
   internal mutating func replaceSubrange<C>(

stdlib/public/core/Arrays.swift.gyb

@@ -835,27 +835,12 @@ extension ${Self} {
     return Builtin.unsafeCastToNativeObject(_buffer.owner)
   }
 
-  // FIXME(ABI)#12 : move to an initializer on _Buffer.
-  // Make sure the compiler does not inline _copyBuffer to reduce code size.
-  @inline(never)
-  @_inlineable
-  @_versioned
-  static internal func _copyBuffer(_ buffer: inout _Buffer) {
-    let newBuffer = _ContiguousArrayBuffer<Element>(
-      _uninitializedCount: buffer.count, minimumCapacity: buffer.count)
-    buffer._copyContents(
-      subRange: buffer.indices,
-      initializing: newBuffer.firstElementAddress)
-    buffer = _Buffer(
-      _buffer: newBuffer, shiftedToStartIndex: buffer.startIndex)
-  }
-
   @_inlineable
   @_versioned
   @_semantics("array.make_mutable")
   internal mutating func _makeMutableAndUnique() {
     if _slowPath(!_buffer.isMutableAndUniquelyReferenced()) {
-      ${Self}._copyBuffer(&_buffer)
+      _buffer = _Buffer(copying: _buffer)
     }
   }
 
@@ -864,7 +849,7 @@ extension ${Self} {
   @_semantics("array.make_mutable")
   internal mutating func _makeMutableAndUniqueOrPinned() {
     if _slowPath(!_buffer.isMutableAndUniquelyReferencedOrPinned()) {
-      ${Self}._copyBuffer(&_buffer)
+      _buffer = _Buffer(copying: _buffer)
     }
   }
 
@@ -1543,8 +1528,8 @@ extension ${Self} : RangeReplaceableCollection, ArrayProtocol {
   @_inlineable
   public mutating func popLast() -> Element? {
     let newCount = _getCount() - 1
-    guard newCount >= 0 else { return nil }
-    _makeUniqueAndReserveCapacityIfNotUnique()
+    guard !isEmpty else { return nil }
+    return _customRemoveLast().unsafelyUnwrapped
     let pointer = (_buffer.firstElementAddress + newCount)
     let element = pointer.move()
     _buffer.count = newCount