[stdlib] use UnsafeMutableBufferPointer.initialize(fromContentsOf:) in Array.replaceSubrange

Author: oxy

stdlib/public/core/ArrayBufferProtocol.swift

@@ -149,46 +149,33 @@ extension _ArrayBufferProtocol {
     elementsOf newValues: __owned C
   ) where C: Collection, C.Element == Element {
     _internalInvariant(startIndex == 0, "_SliceBuffer should override this function.")
-    let oldCount = self.count
-    let eraseCount = subrange.count
-
-    let growth = newCount - eraseCount
-    // This check will prevent storing a 0 count to the empty array singleton.
-    if growth != 0 {
-      self.count = oldCount + growth
-    }
 
     let elements = self.firstElementAddress
 
     // erase all the elements we're replacing to create a hole
     let holeStart = elements + subrange.lowerBound
-    let holeEnd = holeStart + newCount
-
-    // directly forwards to Builtin.destroyArray
-    // TODO: should we branch here or does the compiler branch implicitly?
+    let eraseCount = subrange.count
     holeStart.deinitialize(count: eraseCount)
 
-    // resize the hole to make it the correct size
-    // safe to always call, moveInitialize to the same location is a no-op
-    let tailStart = elements + subrange.upperBound
-    let tailCount = oldCount - subrange.upperBound
-    holeEnd.moveInitialize(from: tailStart, count: tailCount)
+    let growth = newCount - eraseCount
 
-    // place the values into the hole we created
-    if newCount > 0 {
-      let done: Void? = newValues.withContiguousStorageIfAvailable {
-        holeStart.initialize(from: $0.baseAddress!, count: newCount)
-      }
-      if done == nil {
-        var place = holeStart
-        var i = newValues.startIndex
-        while place < holeEnd {
-          place.initialize(to: newValues[i])
-          place += 1
-          newValues.formIndex(after: &i)
-        }
-        _expectEnd(of: newValues, is: i)
-      }
+    // This check will prevent storing a 0 count to the empty array singleton.
+    if growth != 0 {
+      let holeEnd = holeStart + newCount
+      let tailStart = elements + subrange.upperBound
+      let tailCount = self.count - subrange.upperBound
+      holeEnd.moveInitialize(from: tailStart, count: tailCount)
+      self.count += growth
     }
+
+    // place the values into the hole we created
+    let buf = UnsafeMutableBufferPointer(start: holeStart, count: newCount)
+    let res = buf.initialize(fromContentsOf: newValues)
+    // memory safety: if we did not fill the buffer, we now have
+    // uninitialized memory in the middle of our array
+    precondition(
+      res == buf.endIndex,
+      "invalid Collection: count differed in successive traversals"
+    )
   }
 }