Merge pull request #38867 from lorentey/no-swaps

[stdlib] Stop swapping self in Array.withUnsafeMutableBufferPointer

Author: lorentey

stdlib/public/core/Array.swift

@@ -1604,31 +1604,16 @@ extension Array {
     _makeMutableAndUnique()
     let count = _buffer.mutableCount
 
-    // Ensure that body can't invalidate the storage or its bounds by
-    // moving self into a temporary working array.
-    // NOTE: The stack promotion optimization that keys of the
-    // "array.withUnsafeMutableBufferPointer" semantics annotation relies on the
-    // array buffer not being able to escape in the closure. It can do this
-    // because we swap the array buffer in self with an empty buffer here. Any
-    // escape via the address of self in the closure will therefore escape the
-    // empty array.
-
-    var work = Array()
-    (work, self) = (self, work)
-
     // Create an UnsafeBufferPointer over work that we can pass to body
-    let pointer = work._buffer.mutableFirstElementAddress
+    let pointer = _buffer.mutableFirstElementAddress
     var inoutBufferPointer = UnsafeMutableBufferPointer(
       start: pointer, count: count)
 
-    // Put the working array back before returning.
     defer {
       _precondition(
         inoutBufferPointer.baseAddress == pointer &&
         inoutBufferPointer.count == count,
         "Array withUnsafeMutableBufferPointer: replacing the buffer is not allowed")
-
-      (work, self) = (self, work)
       _endMutation()
     }